The morphology and evolution of the adult head of Adephaga (Insecta: Coleoptera) 1

The adult heads of representatives of different adephagan families – aquatic, semiaquatic and terrestrial – were examined and compared. External and internal structures were described and documented in detail for the genera Trachypachus (Tra-chypachidae), Haliplus (Haliplidae), Amphizoa (Amphizoidae) and the recently discovered Aspidytes (Aspidytidae). A list of characters of potential phylogenetic relevance was compiled and the data matrix combined with the large data set of thoracic and abdominal features for different life stages. The cladistic analysis of this comprehensive data matrix of 138 characters for 16 taxa covering all adephagan families led to one most parsimonous tree. The monophyly of the Geadephaga (Trachypachidae + Carabidae) is strongly supported. The Gyrinidae are the sistergroup of all remaining adephagan beetles. The Meruidae are sister to the Dytiscoidea and both together form the sistergroup of the Haliplidae. The sistergroup relationship of Aspidytidae and Amphizoidae is con ﬁ rmed. The placement of Meruidae is impeded by the lack of larval characters. It may change when information on structural features of immature stages becomes available. The Trachypachidae, a small relict family with its greatest diversity and distribution in the early Mesozoic, probably come close to the last common ancestor of the Adephaga in the structural features of the adult head. They share structural similarities with the aquatic Dytiscoidea and the terrestrial Carabidae. It is hypothesized that the common ancestor of Adephaga had a relatively unspecialised head morphology and was a predator, possibly with a preference for a riparian habitat. Adaptations for an aquatic environment evolved at least two times and possibly even three times independently. Within these lineages a great diversity of different life styles developed such as the highly specialised surface gliding Gyrinidae, the hygropetric Aspidytidae, the strongly miniaturised Meruidae or the algophagous Haliplidae.


Introduction
The order Coleoptera is composed of the very small suborders Archostemata (ca.30 spp.) and Myxophaga (ca. 100 spp.), the extremely diverse Polyphaga (ca.330.000 spp.), and the Adephaga (ca.30.000), the latter are mainly characterized by predacious habits as adults and larvae.The discussion of the phylogeny and evolution of adephagan beetles has been greatly stimulated by the recent discovery of two new 1 This study is dedicated to the late Prof.Dr. Robert E. Roughly, whose untimely death came as a great loss to the community of adephagan workers.hygropetric families, Aspidytidae (RIBERA et al. 2002a;BALKE et al. 2003BALKE et al. , 2005) ) and Meruidae (SPANGLER & STEINER 2005;BEUTEL et al. 2006;BALKE et al. 2008).The position of these relict families with only 2 and 1 species, respectively, is not unambiguously resolved (BALKE et al. 2003(BALKE et al. , 2008)).Likewise, the placement of the terrestrial relict family Trachypachidae (6 species in 2 genera), which has long been recognised as a key taxon (e.g., BELL 1983;ROUGHLEY 1981;BEUTEL 1993), is still controversial.Trachypachidae share apomorphic characters with two large lineages, the aquatic Dytiscoidea (e.g., subcubital setal binding patch, fused metacoxae), and the terrestrial Carabidae (e.g., protibial antenna-cleaning organ) (e.g., BEUTEL 1993;BEUTEL et al. 2006).A sistergroup relationship with the entire Hydradephaga was suggested by ROUGHLEY (1981) and a sistergroup relationship with Dytiscoidea by BELL (1983) and BEUTEL (1993BEUTEL ( , 1997)).However, in more recent years a clade Geadephaga comprising Trachypachidae and Caraboidea has gained strong support in several studies, e.g., BEUTEL & HAAS (1996) and SHULL et al. (2001).Apparently the placement of the small family is important for the reconstruction of the adephagan groundplan and crucial for the interpretation of evolutionary changes between aquatic and terrestrial habitats.
The present study was focussed on head structures of adults, a complex character system which has been proven as phylogenetically informative in studies on other groups of insects (e.g., BEUTEL & VILHELMSEN 2007;BEUTEL & BAUM 2008).Numerous publications on the larval head anatomy of adephagans are available (e.g., ARNDT 1993;ARNDT & BEUTEL 1994;BEUTEL 1991BEUTEL , 1992aBEUTEL , 1993;;ALARIE et al. 2004), but very few detailed treatments of adult head structures have been published.The only studies covering both external features and internal soft parts are those of KORSCHELT (1923KORSCHELT ( , 1924) ) on Dytiscus, HONOMICHL (1975) on Gyrinus, BEUTEL (1986aBEUTEL ( , 1989a) ) on Hygrobia and Spanglerogyrus, respectively, and BELKACEME (1991) on Noterus.No detailed data are available for the Aspidytidae and Meruidae, and surprisingly also not for the phylogenetically critical families Trachypachidae, Haliplidae, and Amphizoidae.Consequently, the primary purpose of this study was to provide detailed descriptions of external and internal head structures of representatives of these families.The obtained cha r acters of the head were included in a comprehensive data matrix from BEUTEL et al. (2006) and analysed cladistically, and an evolutionary scenario for adult head structures was developed.To evaluate the variability of the chosen characters within highly diverse taxa such as Dytiscidae or Carabidae, further representatives of these groups were examined (indet.sp. of Nebria, Elaphrus, Loricera, Notiophilus, Brachinus, Bembidion and Trechus), and Systolo soma breve Solier, 1849 as a second trachypachid species.The variability of the observed character states is minimal or absent as in Trachypachidae.The additional specimens were not included in the analysis.

Morphological techniques
The specimens were stored in ethanol (70%).The external morphology was studied under a binocular microscope (Leica MZ 125) and documented with line drawings.For the detailed morphological description of particular adephagan families, the heads of Trachypachus, Aspidytes, Amphizoa and Haliplus were critical point dried and scanning electron micrographs were made with an FEI Philips XL 30 ESEM with Scandium software.Furthermore the mouthparts of all representatives were removed and compared under a binocular microscope and for specifi c details examined with the SEM.To minimize backscatter, improve backround contrast and enable scanning each specimen in different viewing angles a special specimen holder was used (POHL 2010).
For studying the internal structures specimens of all listed taxa were dissected and drawn in successive stages.Detailed features of the musculature and endoskeleton were studied with serial cross-sections.The heads of Trachypachus and Haliplus were embedded in Araldite, cut at 1.5 μm with a Microm microtome (HM 360), and stained with toluidine blue and pyronin G (red).For Aspidytes a microtome series in Historesin cut at 3 μm and stained with methylene blue and acid fuchsine was already available.For comparative analysis and documentation, selected sections were photographed on a Zeiss Axioplan microscope with AnalySIS ® imaging software.
The line drawings were digitised and all images were edited and arranged for publication with Adobe ® Photoshop ® CS2 and/or Adobe ® Illustrator ® CS2.
The characters examined comprise the skeletomuscular system, the cranial parts of the digestive tract, the brain and other elements of the nervous system, and glands.

Cladistic analysis
The observed features of external and internal morphology were coded as defi ned, comparable character states.To complete and evaluate the matrix of characters of the head, data were taken from literature for the following taxa: Priacma serrata LeConte (HÖRNSCHEMEYER et al. 2002) The data set of 58 head characters of Adephaga was combined with the comprehensive data set in BEUTEL et al. (2006).The data matrix (16 taxa, 138 characters -Tab. 1) was generated in Winclada (NIXON 1999) and analysed with NONA (GOLOBOFF 1995) (Ratchet search/Island Hopper, 1000 replicates, all characters equally weighted) and PAUP 4.0b10 (SWOFFORD 2001) (branch and bound search [computed via stepwise, minimal trees only, addition sequence furthest]).Bremer support values were calculated with AutoDecay 5.0 (ERIKSSON 2003).The bootstrap analysis was run with 1000 replicates.The head is prognathous and almost as broad as long (about 1 mm).Its colouration is dark brown to black without a metallic sheen.The surface is almost glabrous without granulation, specifi c sculpture or pubescence.The compound eyes are laterally protruding.

List of abbreviations
The clypeus is almost three times as long as the labrum.
The clypeofrontal suture is a continuous furrow and forms an obtuse angle medially.The distinct anterior tentorial pits lie within the clypeofrontal suture close to its lateral margins.The globular protuberances articulating with the secondary mandibular joints are located at the posterolateral edges of the clypeus (Fig. 5A).A long seta originates close to the lateral clypeal margin.Longitudinal strengthening ridges, the supraocular ridges, extend from the clypeus along the dorsal margins of the compound eyes and reach their posterior border.Frontal and coronal sutures are absent.A low circular ridge on the caudal third of the head almost reaches the gular sutures ventrally (Fig. 5F).Up to this ridge the head is retracted into the thorax.
In lateral view the head appears distinctly wedgeshaped with a widened, almost globular posterior part.On the ventral side a transverse constriction is recognisable between submentum and gula, which are placed at a distinct angle to each other.The fusion line is marked by a pair of conspicuous posterior tentorial pits.The gula is narrow in relation to the width of the entire head.The median gular apodeme is recognisable externally in the anterior part of the gula.The gular sutures are anteriorly continuous with the hypostomal sutures which reach the hypostomal grooves.The fairly extensive hypostomata are not fully covered by the maxillary bases and form an acute angle with the genae.The maxillae are inserted in the hypostomal grooves.The caudal occipital foramen is surrounded by a wide postoccipital ridge except for the ventral gular part.The postgular ridge is formed by the hind margin of the gula.

Internal skeletal structures
Figs. 2, 3, 5 A distinct internal transverse epistomal ridge corresponds to the external clypeofrontal suture.The longitudinal gular ridges -externally marked by the gular sutures -are thin high internal walls with a strengthened dorsal edge.They are posteriorly continuous with the postoccipital ridge.The anterior edges are fused with the posterior tentorial arms arising from the posterior tentorial pits.At their cranial third the gular ridges are connected by the tentorial bridge -a thin, sclerotised, slightly arched bar with a short anteriorly directed median process.The gular ridges slope conspicuously in the caudal third before reaching the postoccipital ridge.At their lowest part a pair of slender, medially projecting apodemes serves as insertion area of M. profurcatentorialis (M.58).The mid-gular apodeme is a strongly developed, triangular, median process on the cranial third of the gula.Labial muscles originate on its lateral faces and the cranial edge.The tentorium is well developed.The anterior arms arise from the epistomal ridge.The dorsal arms are attached to the dorsal head capsule by fi brillae (Fig. 5E).The central body of the tentorium connects the anterior, dorsal and posterior arm (incl.gular ridge).The laminatentoria, a pair of medially projecting, nearly horizontal plate-like processes, arise from the central body.The plates almost meet medially but are not fused (Fig. 5D).They provide a wide area of origin for the large stipital retractor.Further endoskeletal structures are the circumantennal ridges and the circumocular ridges, enclosing the antennal bases and the compound eyes, respectively (Fig. 5B).The internal ridges of the mentum will be described in section 4.1.7.(Labium) and the suspensorium in section 4.1.8.(Hypopharynx).

Antennae
Figs. 1, 4B,C, 5B Skeletal features.The antennae are inserted laterally in a wide groove between the eyes and mandibles.The articulation area of the antenna is enclosed by a circumantennal ridge with an inconspicuous anteroventral process, which corresponds to a small furrow on the scapus.
The antenna is 11-segmented and fi liform.The scapus is bipartite and the largest antennomere.The proximal articulatory part is globular and separated from the distal cylindrical part by a deep constriction.The longitudinal axes of both parts form a distinct angle.Because of this acentric attachment of the globular part its anterior base appears distinctly prominent (Fig. 4C).All the following antennomeres are centrically attached to each other and widening distally.The pedicellus is about half the size of the scapus and shorter than the fl agellomeres.The apical segment is slightly longer than the preceding ones.
The antennomeres are not pubescent.An apical circle of setae is present on all antennomeres except the scapus.A second, basal circle is present on the third and following fl agellomeres.In addition several single setae are scattered on the scapus and the apical segment.The tip of the apical antennomere bears a sensory fi eld of fi ne, very short sensilla.The anterior labral margin is folded inwards and forms a triangular plate projecting into the membranous epipharynx.It is posteriorly enclosed by a Vshaped ridge converging into a broad median bar (Fig. 4F).The ventral side of this bar is covered with minute microtrichiae.Tormae are present at the caudolateral labral angles.They are fi rmly connected to the dorsal suspensorial arms (see section 4.1.8.[Epipharynx]).Thus the epipharynx is supported by the median bar and the tormae and the following suspensorial arms.

Mandibles
Figs. 1, 3, 4K -N, 5A, 6 Skeletal features.The mandibles are distinctly protruding beyond the anterior and lateral labral margin.The length/width ratio is 1.6.The dorsal and ventral sides are fl attened.The lateral side is concave at the base with pronounced rims enclosing the concavity dorsally, ventrally, and posteriorly.
immediately above the condyle is the insertion point of the abductor tendon (M.12).The dorsal secondary joint is a triangular, concave socket on the mandible articulating with a protuberance of the clypeus.Numerous sensilla inserted in deep, round pores are scattered on the distal dorsal and lateral surfaces (Fig. 6C).Laterally a large elliptic pore (diameter 4 μm × 28 μm) is present.Viewed with SEM the pore appears bipartite with a less deep and smaller anterior part and an extremely deep and larger posterior part (Fig. 6N).Apparently the latter completely penetrates the mandibular wall.In Systolosoma this pore is associated with a long curved seta.It is likely that the socket of the setae in Trachypachus is combined with a glandular duct associated with glandular tissue posterad the mandibular base within the head capsule.
The mandibles are nearly symmetrical, but differ in some details.The left mandible is slightly longer The well developed apical incisor is acute and slightly bent downwards, whereas the subapical tooth is markedly smaller and blunt.In mesal view the distal cutting edge between them is shaped like a reversed J.A third, mesal tooth on the inner margin is separated from the subapical incisor by a deep emargination.A mola is absent.The mesal side of the mandible between the mesal tooth and the basal margin is broadened, forming a nearly trapezoid, concave area (Fig. 6D,E).On the ventral side a fringe of hairs is inserted along a minute ridge parallel to the inner margin (Fig. 6G -K).
The primary and secondary mandibular joints at the laterobasal corners are connected by the protruding basal rim (Fig. 6L,M).The axis through them is nearly vertical, resulting in almost horizontal movements of the mandibles.The ventral primary joint is a globular condyle of the mandible articulating with a shallow socket on the hypostoma.A round, fl at bulge  than the right, which appears more compact.The distal cutting edge of the right mandible is shorter and concave (Fig. 6A,B), and its subapical incisor fi ts with a minute notch of the straight and longer cutting edge of the left mandible (Fig. 6G).The mesal mandibular regions also differ slightly in their proportions.
Musculature (see also Fig. 5B -F   General features.The anterior epipharynx is internally subdivided by a median labral bar that is posteriorly continuous with a median bulge (Fig. 5A).Epipharyngeal lobes are not developed and sensory appendages are also lacking.The surface is smooth except for the area below the bar, which is covered with microtrichia.The hypopharynx is subdivided into an anterior and a posterior part.The anterior hypopharynx is retractile into the posterior hypopharynx and covers the prementum dorsally.A pair of membranous lobes overlaps the anterior margin of the prementum and encloses the median triangular premental sclerite (Fig. 3D).The hypopharyngeal surface is entirely smooth.The preoral cavity between the dorsal epipharyngeal bulge and the convex posterior hypopharynx is distinctly narrowed and appears x-shaped in cross-section (Fig. 5B).The ana tomical mouth opening is ventrally strengthened by a transverse sclerite, the suspensorial cross-bar.It is anteriorly connected to a pair of dorsal and ventral arms.The dorsal suspensorial arms are continuous with the tormae (see section 4.1.4.) and support the epipharynx laterally.The ventral suspensorial arms similarly strengthen the lateral edges of the hypopharynx (see section 4.1.7.).They are fi rmly connected to the mental ridges.Skeletal features.The submentum is fi rmly connected to the head capsule and posteriorly completely fused with the gula.The anterior margin is connected to the mentum.A transverse row of six setae is present parallel to the anterior margin.The submentum and mentum are more than twice as wide as the gula.Two large lateral lobes of the mentum enclose a deep emargination with two paramedian cusps.The internal mental ridges (Fig. 5B) are recognisable externally.The upper posterior corners of the triangular ridges are fi rmly connected to the ventral suspensorial arms.The surface of the mentum bears no setae.The prementum is connected to the anterior mental margin and is inserted in its median emargination.A pair of setae arises from the anterior margin, which is folded inwards and forms a triangular sclerite extending onto the surface of the hypopharynx.The ventral wall of the prementum forms a blunt longitudinal keel (Fig. 5A).The palpigera are attached to the prementum and subdivided into an external cylindrical part and an internal arcuate process.
The palps fi t into the groove between the premental median keel and the mental lateral lobes.They are 3-segmented and distinctly shorter than the maxillary palp.Palpomere 1 is markedly shortened.Palpomere 2 bears mesally two small setae and palpomere 3 a large terminal undivided sensory fi eld.The anterior precerebral pharynx lies in the upper region of the head.The posterior postcerebral pharynx declines slightly towards the foramen occipitale.The pharyngeal wall is equipped with ring muscles and longitudinal muscles (Fig. 5C,D).The lumen of the anterior pharynx is oval in cross-section, whereas the lumen of the posterior pharynx is strongly narrowed by folds.The folds provide space for the strongly developed longitudinal muscles and the edges serve as attachment areas for dilators.

Musculature. M. frontobuccalis anterior (M.45):
( The frontal and coronal sutures are absent.Two postocular ridges parallel to the posterior margins of the compound eyes are present on the lateral side of the head.The anterior ridge is curved and continuous with the circumocular ridge ventrally and dorsally.The longer posterior ridge is almost straight laterally.It reaches the ventrolateral head capsule, where it is bent anteriorly towards the cardo.A further, short, longitudinal ridge extends from the hypostoma posteriorly (Fig. 12K).
The gula is broad and fused with the submentum anteriorly.The posterior tentorial pits are concealed by a pair of small submental lobes.The few pores on the gular surface bear small, hook-shaped sensilla.The hypostoma, which is entirely concealed by the maxillary base, forms an acute angle with the gena with a strengthened edge.The deep socket of the primary mandibular joint is present anterolaterally on the hypostoma (Fig. 11A).
On the posteriormost gula the internal gular apodeme is recognisable.The postoccipital ridge is widened laterally, where it is connected to the gular ridges.A pair of small apodemes is present dorsally.The thoracic M. praephragmapostoccipitalis medialis (M.55) originates on these apodemes.

Internal skeletal structures
Figs. 8,9,11 The epistomal ridge is largely absent, but a short internal ridge connects the well developed circumantennal ridges with the posterior clypeal angles.The dorsal margins of the gular ridges are strengthened and their anterior margins are fused with the posterior tentorial arms.Their bases are anteriorly continuous with the hypostomal ridges.The gular ridges are posteriorly fused with the postoccipital ridge.Above the middle gular region the tentorial bridge connects the gular ridges.The bridge is distinctly curved anteriorly and bears a median process.Immediately behind the bridge a pair of small apodemes arises on the mesal sides of the gular ridges.Two muscles originate on their front or back sides, respectively.The median gular apodeme is small and triangular, and placed on the posterior gula close to the postgular ridge.The central element of the well developed tentorium connects the dorsal and anterior arm to the posterior arm which is fused with the gular ridge.The teriorly it splits into paired connectives leading to the prothoracic ganglia.
Nerves (Figs. 2A, 3A, 5C,D).The optic neuropils originate on the anterior cerebral lobes, close to the nervi antennalis and the nervi frontalis, which belong to the deutocerebrum and tritocerebrum, respectively.The nervi mandibulares originate on the tritocerebral connectives close to the tritocerebral commissure and lie above the laminatentoria.The nervi maxillares and nervi labiales arise anteriorly on the suboesophageal ganglion.
Frontal ganglion and neuroendocrine glands (Fig. 5).The frontal ganglion is of rhombic shape and lies above the anteriormost region of the pharynx close to the anatomical mouth opening (Fig. 5C).Two pairs of nerves and two single nerves originate from the ganglion's body.The nervus procurrens arises anteromedially.A pair of anterolateral nerves innervates the epipharyngeal glands in the median bulge and around the tormae.The nervi frontales connecting the ganglion to the tritocerebrum arise laterally.The nervus recurrens originates posteromedially.It is connected to the ganglion ventriculare posterad of the tentorial bridge and sends out fi ne nerves to the corpora cardiaca and corpora allata (Fig. 5F).These neuroendocrine glands are laterally adjacent to the posterior pharynx.The head is prognathous and slightly broader than long.The large kidney-shaped compound eyes are largely integrated into the contour of the head capsule but protrude to some extend anteriorly.The colouration is light brown.The surface displays an irregular pattern of distinct pores with sensilla.The diameter of the pores, their distribution and the shape of the sensilla varies on different head regions.The central area of the dorsal head capsule is glabrous without any pores, sensilla or setae (Fig. 12E).The retracted posteriormost part of the head, a broad collar around the postoccipital ridge, is also smooth.Generally the pores increase in diameter from anterior to posterior and in density around the compound eyes, and towards the anterior margin of the head and the posterior collar.

Head morphology of Haliplus
The ventral head capsule is largely devoid of pores.The clypeal region is short.Its pores bear broadened and fl attened, short setae.The clypeofrontal suture is interrupted and the clypeus confl uent with nates mesally on the central tentorial body.The laminatentoria are medially fused, forming a pair of adjacent vertical plates (Fig. 11D,E).The voluminous stipital retractor (M.tentoriostipitalis, M.18a) originates on the lateral faces of these plates, whereas the hypopharynx-retractor (M.tentoriobuccalis anterior, M.48) passes through the median space between them (Fig. 11E).
The internal ridges of the mentum are described in section 4.2.7.(Labium) and the suspensorium in section 4.2.8.(Hypopharynx).

Antennae
Figs. 7A,C, 10B,C, 12F Skeletal features.The fi liform, 11-segmented antenna is inserted dorsolaterally above the anterior margin of the compound eye.The insertion area is distinctly separated from the mandibular base.The surface is glabrous except for one seta on the distal scapus (Fig. 12F).Several sensory pores with paddle-shaped sensilla are scattered over the surface of the scapus and pedicellus (Fig. 12F).The bipartite scapus is the widest antennomere.The proximal part is globular and articulates with the head capsule, whereas the distal part is almost cylindrical but strongly shortened.The parts are separated by a deep constriction.A small, anterior process on the antennal circular ridge articulates with a basal emargination on the scapus.The pedicellus is distinctly smaller than the scapus with a proximal globular articulatory part and a cylindrical, but shortened distal part.The fi rst fl agellomere is as long as the scapus but more slender.The remaining fl agello meres are similar to the pedicellus in length and barrel-shaped.anterior arm arises from the circumantennal ridge.The dorsal arm is shortened and does not reach the head capsule (Fig. 11D).The laminatentorium origi- The left and the right mandible are moderately asymmetric, with the left mandible appearing less compact.Its distal cutting edge is longer and straight, whereas the cutting edge of the right mandible is emarginated close to the subapical incisor.The right mesal tooth is reduced to a rounded shallow bump.The mesal margin of the left mandible is concave, whereas it is slightly convex on the right.The anterior margin is folded inwards and forms an extensive sclerite on the ventral side (Fig. 10F).Some short and thick setae are inserted ventrally, close to the anterior edge (Fig. 12C).A broad, heart-shaped, dense fi eld of long microtrichia is integrated into the ventral sclerite (Fig. 12C).An internal median bar is present between the anterior half of the labrum.The posterolateral angles form a pair of tormae, which are connected to the dorsal suspensorial arms (see section 4.2.8.[Hypopharynx]) by strong ligament-like structures.

Mandibles
Figs. 7, 9A, 10K -N, 11, 12A,D Skeletal features.The mandibles are slightly protruding laterally but not beyond the anterior labral margin.They are longer than wide with a length/ width ratio of 1.6.The ventral side is fl at whereas the dorsal side is moderately convex.The surface is smooth except for the lateral side, which is covered with pores with styliform sensilla (Fig. 12A).
The distal part of the mandibles is conspicuously darker than the remaining regions.The apical tooth is slightly bent downwards.The distal cutting edge between the subapical and the apical incisor is formed like a reversed letter J.A smaller tooth is present in the middle region of the inner margin.The ventral edge between the mesal and the apical tooth is deeply emarginated.On the ventral side a dense brush of microtrichia is inserted on a distinct ridge parallel to the mesal margin.The fringe reaches the emargination distally.The long microtrichia overtop the mesal edge.A mola is absent.The lateral margin is convex and rounded.The abductor tendon is attached to a bulge on the lateral base which is anteriorly delimited by a constriction.The small condyle of the primary mandibular joint is located submarginally on the ventrobasal margin of the mandible.The dorsobasal margin is emarginated close to the lateral bulge thus forming a concavity, the socket of the secondary joint.The rim surrounding the emargination is strengthened.The axis through both joints is almost vertical.connected.The lateral margin of the mediostipes is rim-like and prominent (Fig. 12G).Its mesal margin is rigidly fused to the lacinia.On the ventral side of the hook-shaped lacinia a distinct furrow is formed by a pair of prominent rims (Fig. 12G).The bipartite, palp-like galea, which is connected to the anterior margin of the mediostipes, fi ts in this furrow.On the dorsal side of the lacinia a central, less sclerotised area is re cognisable (Fig. 10G).A row of strong setae is inserted above the mesal margin and a few setae below it.The palpifer is composed of a palp-socket with an enlarged dorsal plate.ture.Two pairs of long setae are inserted at the anterior angles.A pair of lateral submental lobes reaches the hypostomal grooves anteriorly and covers the posterior tentorial pits.The lateral lobes of the anteriorly attached mentum enclose a broad median emargination with two paramedian cusps.The lobes are narrowing anteriorly forming rounded tips.The surface of the lobes is smooth without any setae or pores, whereas the central mentum is covered with pores with styliform sensilla.A pair of long setae is inserted close to the lateral lobes.The longitudinal triangular internal mental ridges (Fig. 11B) are visible externally and originate at the rounded tip of the lateral lobes.The upper posterior angles of the mental ridges are continuous with the ventral suspensorial arms.
The small prementum fi ts into the median emargination of the mentum.Between the palpal bases the prementum forms a blunt ventral keel.A row of deep setiferous pores is arranged along the anterior edge and a pair of long setae inserts paramedially.The anterior margin of the prementum is broadly folded inwards and forms a bilobed sclerite on the dorsal side (Fig. 9D), which is posteriorly connected to the membranous hypopharynx.The dorsal sclerite is covered with irregularly arranged short bristles.
The palpiger consists of an anterior external cylindrical part and a posterior internal process.The internal process is longer than the cylindrical part and its posterior end is curved inwards.The 3-segmented palps are distinctly shorter than the maxillary palps.Palpomere 2 is enlarged, whereas palpomere 3 is shortened, similar to palpomeres 3 and 4 of the maxillary palp.The surface of the palps is glabrous.wide.Palpomere 2 is somewhat longer and cylindrical.Palpomere 3 is strongly enlarged and at least four times longer than wide, whereas the apical palpomere is greatly shortened.It is cone-shaped, hardly longer than wide, with an undivided sensory fi eld at the tip.The pores which are densely scattered over the basistipes and scarcely on the cardo, palpifer and the palp bear paddle-shaped sensilla.

Musculature
Musculature (see also Fig. 8).M. craniocardinalis externus (M.15): (O) posteroventral area of the head capsule, between M.12 and M.17 Skeletal features.The submentum is distinctly wider than the gula and fused to it without a recognisable su- The hypopharynx forms a distinct dorsal bulge.A pair of membranous, comb-shaped lobes is inserted at the base of the prementum and laterally adjacent to the hypopharynx (Fig. 9D).It is possible that these structures represent modifi ed paraglossae.
The suspensorium consists of a transverse crossbar and a pair of ventral and dorsal arms.The dorsal  or ring muscles and a longitudinal muscle layer (Fig. 11D).The ventral transverse muscles between the suspensorial cross-bar and the laminatentorium are multilayered and large, corresponding to the dorsal Mm.III.The pharyngeal lumen is narrowed by folds, thus providing space for the longitudinal muscle bundles of the precerebral pharynx.The extent of the pharyngeal musculature and the lumen of the pharynx decrease from anterior to posterior.The anterior part of the oesophagus is devoid of ring musculature.The lumen is not narrowed by folds.present.The anterior and posterior parts of the well developed tentorium are connected by a central tentorial body.The anterior arms arise from the antennal ridges (Fig. 16B).The distinctly broader dorsal arms reach the dorsal head capsule but are not fi rmly connected to it.Plate-like laminatentoria arise from the mesal side of the central element.They are not fused medially.Further endoskeletal structures are the mental ridges and the suspensorium (see sections 4.3.7.
The frontal ganglion lies above Mm.III and between M.44 and M.45.It is approximately triangular.The nervus recurrens connects it to the ganglion ventriculare, which lies above the postcerebral pharynx.
The corpora cardiaca and allata are almost globular.They are located posterad the cerebrum and enclose the postcerebral pharynx.They are connected to the ventricular ganglion.The head is prognathous, broadly oval.The colouration is dark brown.Numerous fi ne pores are spread over the nearly glabrous surface.The kidney-shaped compound eyes are completely integrated in the contour of the head capsule.The clypeus is longer than the labrum and the clypeofrontal suture is broadly interrupted medially.The condyles of the secondary mandibular joint are present at the posterior angles on the ventral side.Several short setae are inserted on the lateral clypeal margins.The anterior tentorial pits are not visible externally.The frontal or coronal sutures are absent.
The broad gula is fused with the submentum anteriorly.A pair of inconspicuous posterior tentorial pits indicates the border between both sclerites.The hypostomata are almost fully covered by the maxillary bases and form an acute angle with the genae.The postoccipital ridge forms a pair of paramedian dorsal apodemes which serve as area of origin of a thoracic muscle (Fig. 14B).The gula forms a narrow postgular ridge.

Internal skeletal structures
Figs. 14,15 The lateral remnants of the clypeofrontal suture (Fig. 13A) correspond internally with short, low ridges, which are connected to the circumantennal ridges.The gular ridges are high with strengthened dorsal edges.
Their anterior margins are fused with the posterior tentorial arms which are continuous with the hypostomal ridges.The tentorial bridge connects the gular ridges at their mid-length.The bridge is distinctly curved anteriorly and bears a median process.A pair of mesally projecting apodemes arises from the gular ridges posteriorly of the bridge.A mid-gular apodeme is not  Skeletal features.The antennae are inserted laterally between the compound eyes and the mandibles in deep grooves.In dorsal view the insertion areas are concealed by the lateral margins of the frons.The 11-segmented antennae are fi liform and completely glabrous.The antennomeres are scarcely longer than wide except for the scapus, the fi rst and the last fl agellomere, which are almost twice as long as their diameter (Fig. 13A).The scapus is bipartite with a proximal spherical articulatory part and a distal strongly shortened cylindrical part, separated by a constriction (Fig. 16C; see also RIBERA et al. 2002a: fi g. 1C).The proximal part is acentrically attached to the distal part, i.e. their longitudinal axes form a distinct angle.A groove on the spherical part articulates with an anteroventral process on the circumanten-  whereas the dorsal side is convex.The exposed parts of the dorsal surface are covered with sensilla inserted in small grooves (Fig. 17A,C).The subapical incisor is blunt; the apical incisor is more acute and slightly bent downwards.So the dorsal cutting edge between these teeth is formed like a reversed letter J.The mesal tooth is small and closely located to the subapical incisor.The ventral edge between the apical and the mesal tooth is deeply emarginated.A mola is absent.Three fringes of microtrichia (Figs.16M,N, 17B,D,E) are present on the ventral side; one along the mesal edge, reaching the apical tooth, the second immediately inserted on the mesal margin, and a third, very short one behind the apical incisor.The hairs of the fi rst fringe are longer than that of the second one, but do not overtop the mesal edge.
The mandibular joints are subapically located, i.e. medially shifted from the lateral angles.The abductor tendon is attached to a conspicuous bulge on the laterobasal angle.The ventral primary joint is more anteriorly positioned compared to the dorsal secondary joint, which results in a slightly inclined axis of movement.The mandibular condyle of the primary joint is small (Fig. 17B).The socket of the secondary joint is formed by a deep emargination with a strengthened mesal edge (Fig. 17A).
The left and the right mandible are nearly symmetric but differ in some details.The dorsal cutting edge of the left mandible is longer and straight whereas the right cutting edge is emarginated close to the subapical incisor.The right subapical tooth is more prominent and rounded.The asymmetry of the distal parts ensures a close fi t of both mandibles in the fl exed position.

Labium
Figs. 13B, 14, 15C, 16D Skeletal features.The submentum is very short and completely fused with the gula; only marked by the posterior tentorial pits.A transverse row of setae is arranged in a broad arch.The submentum is ante riorly connected to the mentum.The glabrous mentum forms a pair of anteriorly narrowing, lateral lobes.The median emargination between the lobes bears a pair of short paramedian cusps.The externally visible mental ridges originate internally along the mesal margin of the lateral lobes.The posterior angles of the trian gular walls (Fig. 14B) are connected to the ventral suspensorial arms by a membrane.The prementum is inserted between the mental lobes.The widely inwards folded anterior margin forms an extended sclerite on the dorsal side which is connected to the hypopharynx.On the ventral side two converging rows of fi ne, long setae margin a median longitudinal blunt keel between the palpigers (Fig. 16D).The latter are composed of an external cylindrical part and an arcuate internal process.Palpomere 1 of the 3-segmented palp is short (similar to the maxillary palp), whereas palpomere 2 is distinctly enlarged with a nearly globular distal part.Palpomere 3 is of similar length but more slender.An undivided sensory fi eld is present on its apex.The glabrous palp is due to the enlarged palpomere 2 slightly longer than the maxillary palp.The elongate cardo is arranged in a transverse position.The dorsal side of the cardo bears an internal bifurcate process with a shorter mesal branch and a longer lateral one.The anterolateral margin is connected to the basistipes.Several short setae are inserted on the lateral side of the cardo and the basistipes.Basi-and mediostipes are completely separated from each other.The ventral mediostipes bears the galea and is continuous with the mesal lacinia.The galea is bipartite, palp-like and laterally adjacent to the lacinia.The lacinia is hook-shaped.Its base is widened and equipped with a fi eld of microtrichia.A row of bristles is inserted along its mesal margin.An oval semimembranous fi eld is present on the mesal base of the dorsal side.The anterior part of this area is densely set with fi ne microtrichia (Fig. 16G).The basistipes is anteriorly attached to the palpifer.The distal socket of the palpifer bears the maxillary palp; the elongated dorsal side forms a plate between the lacinia and the basistipes (Fig. 16G).The 4-segmented palp protrudes to some extent beyond the lacinia.All palpomeres are basally enclosed by the preceding ones and short, but distad slightly increasing in length.Palpomere 1 is only half the size of the other ones.Palpomere 2 is cup-shaped with a slender base and a widened apex.An undivided sensory fi eld is present at the tip of palpomere 4. The palp is entirely glabrous.

Musculature (see also Figs. 14B, 15A). M. craniocardinalis externus (M.15): (O) posteroventral area
of the head capsule, between M.12 and M.17    The surface of the hypopharynx is glabrous and fl at but ascending towards the anatomical mouth and thus narrowing the preoral cavitiy posteriorly.The cibarium appears oval in cross-section even close to the mouth opening.
The suspensorium consists of a transverse crossbar below the anatomical mouth and a pair of dorsal and ventral suspensorial arms.The dorsal arms are connected to the tormae by distinct membranous strands.The ventral arms are connected to the mental ridges by a membrane.The prognathous head is somewhat longer than broad.The relatively small compound eyes are moderately The ventral hypostomata are lobe-like extended and form an acute angle with the lateral genae.The edge between them is strengthened (Fig. 18C).The broad gula is fused with the submentum and the mentum (see section 4.4.7. [Labium]).The submento-gular border is not visible except for the conspicuous posterior tentorial pits.The median internal gular apodeme is visible on the anterior gula.The postoccipital ridge and postgular ridge are well developed.A pair of dorsal paramedian apodemes and a pair of lateral extensions serve as areas of origin for cervical muscles.

Internal skeletal structures
Figs. 19,20 The circumantennal ridge forms a prominent external socket.The epistomal ridge corresponding with the clypeofrontal suture is low.A short lateral ridge connects the epistomal ridge and the antennal ring is the origin of the anterior tentorial arm.The arm is approximately triangular due to its widened base and the strongly narrowed posterior part (Fig. 19C).The constantly broad, dorsal tentorial arm reaches the head capsule but is not rigidly fused to it.The anterior and dorsal arms are connected to a central tentorial element which is continuous with the gular ridge posteriorly.The platelike apodemes of the central elements, the laminatentoria, are not fused medially.The laminatentorial plate is composed of an anterior horizontal wall and a posterior almost vertical wall (Fig. 19A).The entire ventral and anterior face serves as area of origin of the large stipesretractor (M.18a).The posterior tentorial arms are completely fused to the anterior margins of the gular ridges.
The anterior bases are connected to the low hypostomal ridges which correspond to the hypostomal sutures.The gular ridges are high and their dorsal margins are bent outwards.They are continuous with the postoccipital ridge posteriorly.A pair of short mesal processes is present above the submento-gular border but obviously no muscle originates on it (Fig. 19A).A further pair of apodemes arises above the posterior gula close to the postoccipital ridge.The anterior and posterior faces serve as attachment areas for muscles.The tentorial bridge originates far posteriorly on the gular ridges closely to the apodemes.The bridge is deeply arched anterad and its median, almost straight part reaches the mid-gular region.The gular apodeme arises medially on the anterior gula.It is a vertical triangular plate and its lateral faces are the area of origin of labial muscles.The mental ridges and the suspensorium are described in sections 4.  Skeletal features.The mandibles do not protrude beyond the anterior labral margin but are distinctly protruding laterally beyond the clypeal margin.The length/width ratio is about 0.8, i.e. the mandibles are broader than long.The dorsal side is convex whereas the ventral side is fl at.The lateral and the anterior margin form a right angle, thus the mandibular shape appears square.The exposed parts of the surface are covered with pores bearing styliform or hooked processes.True setae are absent.Both incisors are placed apically and thus the distal cutting edge between them is almost vertical.The dorsal incisor is more rounded and the ventral more acute.The cutting edge is distinctly bent on the left and notched on the right side.The third, mesal tooth is large and prominent on the right but small on the left mandible.The mesal edge between the tooth and the base forms another small cusp on the left mandible whereas it is convex on the right one.A mola is absent.Two fringes of microtrichia are present on the ventral side (Fig. 21H).The fi rst close to the mesal margin reaches the ventral incisor anteriorly; the hairs do not overtop the margin.The second fringe behind the incisor is very short.
The abductor tendon is attached to a rounded bulge at the posterolateral angle of the mandible.The mandibular joints are located subapically.The axis of rotation is not vertical but inclined.The ventral primary joint is a hemispherical condyle.The dorsal secondary joint is a deep basal emargination mesad of the bulge surrounded by a crescent-shaped concave socket.
The left and the right mandible are asymmetrical in some details, e.g., the formation of the dorsal cutting edge, the mesal tooth or the mesal margin (see above).The articulation socket -the circumantennal ridge -is protruding.The 11-segmented, fi liform antenna is glabrous but covered with pores containing sensory processes like those on the head capsule (Fig. 21B).The scapus is bipartite.A notch of the basal margin of the globular basal part articulates with an anteroventral process on the circumantennal ridge (Fig. 20B,C).The distal part of the scapus is barrel-shaped and about 1.5 times as long as wide.

Musculature. M. craniomandibularis internus
The two scapal subunits are separated by a deep constriction and their longitudinal axes form an obtuse angle.Due to this acentric attachment the anterobasal portion of the distal part appears distinctly prominent (Figs.20C, 21B).The pedicellus is also barrel-shaped but only about half the size of the distal scapus.The fl agellomeres are nearly cylindrical but slightly narrowing basally.They are as wide as the pedicellus but conspicuously longer.Skeletal features.The labrum is about half as long as the clypeus.The anterior half of the dorsal surface is densely covered with pores similar to those of the head capsule (Fig. 21C).The anterior margin is broadly bent inwards thus forming a semicircular plate on the ventral side.This area is densely covered with microtrichia (Fig. 21C).The anterior edge forms a shallow median bulge and a pair of lateral lobes.Some minute setae originate on the median region along the edge.A pair of tormae arises laterally from the posterior margin (Fig. 20E).The tormae are connected to the dorsal suspensorial arms by ligamental strands.

Musculature
Musculature.Absent.gated, approximately oval plate.The maxillary palp is 4-segmented.Palpomere 1 is small and curved; palpomere 2 and 3 are somewhat calyx-shaped and about 1.5 × the size of the fi rst one.The apical segment is elongated (about three times as long as wide) with two small round sensory fi elds at the tip (Fig. 21N).No setae are inserted on galea, palpifer and the palp.
Musculature (see also Fig. 19B).M. craniocardinalis externus (M.15): (O) ventral wall of the head capsule, between M.12 and M.17 The cardo and the anteriorly attached basistipes form a right angle.The elongate lateral basistipes bears a pair of setae and is completely separated from the ventral mediostipes.The latter is narrowed posteriorly and its anterior region is continuous with the mesal lacinia.The lacinial base is widened and forms a mesal elongate bulge whereas the distal part is hook-shaped.A row of irregularly arranged setae is inserted on both sides of the mesal edge of the hook.Close to the margin of the basal bulge a dense fringe of hairs is present on the dorsal side whereas the ventral face is scattered with single setae (Fig. 21L).The moveable galea is placed between the lacinia and the maxillary palp and distally sickle-shaped.The lateral and ventral side of the palp-bearing palpifer is somewhat longer than wide, whereas the dorsal side forms a distinctly elon-     General features.The epipharynx is distinctly convex, whereas the hypopharynx is almost fl at and abruptly rising posteriorly.Epipharyngeal lobes are present close to the labral margin with large sclerotised appendages bearing styliform sensory processes (Fig. 21E).The surface of the lobes is densely covered with minute microtrichiae (Fig. 21E).Posterad of it a group of small sclerotised cusps is present (Fig. 21F).
The hypopharynx is attached to the dorsal margin of the prementum.A pair of membranous, fl attened, comb-shaped lobes originates at the ventral base of the prementum and lies laterally adjacent to the hypopharynx and prementum.They possibly represent derivatives of the paraglossae (Fig. 19D).Epipharynx and hypopharynx are entirely lacking true setae.
The suspensorium is well developed.The transverse cross-bar supports the anatomical mouth opening ventrally (Fig. 19A,C).A pair of dorsal suspensorial arms is connected to the labral tormae by ligamental strands (Fig. 20E).The ventral suspensorial arms are fi rmly attached to the internal mental ridges.The suspensorial arms originate at the lateral ends of the cross-bar.General features.The ventral suspensorial cross-bar and the dorsal frontal ganglion mark the anatomical mouth opening.Furthermore the muscles M.44 and M.45 insert immediately anterad and posterad of the mouth.The precerebral pharynx is gradually palpopalpalis quartus, M.27) cannot be identifi ed and described without cross-section series.).The prementum is inserted in the mental emargination.The ventral side forms a median blunt keel which is anteriorly covered with pores.The premental anterior margin is continuous with a triangular sclerite on the dorsal side which is covered with short microtrichiae.The palpigers fi t in the concavities between the keel and the lateral mental lobes.The external cylindrical part of each palpiger is two times longer than wide and bears the 3-segmented labial palp.The posterior internal process is paddle-shaped with a curved shaft.The widened paddle-part is area of origin of two labial muscles.Similar to the maxillary palp the labial palpomere 1 is shortened and palpomere 3 is elongated and bears two small round sensory fi elds apically.True setae are absent on the entire labium.longer than the cerebrum.It is posteriorly continuous with the connectives to the prothorax.

List of characters and data matrix
The use of the terms Geadephaga and Hydradephaga does not imply the monophyletic origin of the aquatic and terrestrial families, respectively.Presumably plesiomorphic states are coded as 0 in most cases.However, this convention is irrelevant when a cladistic approach is used (a posteriori polarity determination)., 13A).

Results of the cladistic analysis
The However, it has to be pointed out here that the basal branches of the adephagan tree we obtained are also not strongly supported statistically (see Fig. 22).Synapomorphies of Trachypachidae and Carabidae are the supraocular ridge, supraocular setae, a circular ridge on the posterior head region, the position of the anterior tentorial pits, two long setae on the clypeus, the shape of the gular ridges, apical whorls of setae on the postscapal antennomeres, and the specifi c shape of the secondary mandibular joint.Except for the clypeal and specifi cally arranged antennal setae all other apomorphic conditions have evolved independently in one of the other adephagan families.The more elongate shape of the head in terrestrial Adephaga is probably plesiomorphic compared to the condition found in the more advanced aquatic lineages.
In our study, the highly specialised Gyrinidae were placed as the sister taxon of the remaining suborder, as already suggested by BEUTEL & ROUGHLEY (1988) and BEUTEL & HAAS (1996).The monophyly of Adephaga excl.Gyrinidae is supported by several features of larvae and the thorax and abdomen of adults (see, e.g., BEUTEL 1997).However, there is only one potential apomorphy of the adult head, and the bootstrap value is rather low (0.66).A bifurcate M. tentoriopraementalis and the reduction of the hypopharynx were suggested as synapomorphic head features of Geadephaga, Haliplidae and Dytiscoidea by BEUTEL & ROUGHLEY (1988).The latter feature cannot be upheld, as hypopharyngeal structures comparable to those of Spanglerogyrus (BEUTEL 1989a) do also occur in Geadephaga, Haliplidae, and Dytiscoidea (see below).

Discussion
The results of our cladistic analysis support a clade Geadephaga comprising the terrestrial Trachypachidae and Carabidae (KAVANAUGH 1986;BEUTEL & HAAS 1996;SHULL et al. 2001;RIBERA et al. 2002b;MADDISON et al. 2009), whereas the "Hydradephaga" (all aquatic and semiaquatic families) turned out as non-monophyletic, as already suggested in earlier morphology-based studies (e.g., BEUTEL & ROUGHLEY 1988;BEUTEL 1993).The obtained branching pattern implies that an aquatic lifestyle has evolved at least two times independently within Adephaga (see also LAWRENCE & NEWTON 1982;EVANS 1985;KAVANAUGH 1986;BEUTEL & ROUGHLEY 1988;BEUTEL & HAAS 1996).This is in contrast to several older contributions (e.g., BURMEISTER 1976; ROUGHLEY 1981) and also recent molecular analyses, which support the monophyletic origin of Hydradephaga and a singular adaptive transition to aquatic lifestyle within Adephaga (SHULL et al. 2001; RIBERA et.al. and rounded head, eyes integrated in the contours of the head capsule, and largely concealed mouthparts evolved at least two times independently, namely in Gyrinidae and the remaining aquatic Adephaga, or possibly three times, in Gyrinidae, Haliplidae and Dytiscoidea (see above).Due to the great diversity of microhabitats (e.g., Gyrinidae on the water surface, Haliplidae among algae, Aspidytidae and Meruidae on rocks with a thin water fi lm) considerably different adaptations have evolved in different aquatic or semiaquatic lineages.The highest degree of specialisation is doubtlessly reached in Gyrinidae, with subdivided compound eyes, an extremely modifi ed antenna, one or three regular rows of long labral setae, and other unusual features (see, e.g., HONOMICHL 1975;BEUTEL 1989a,b).Mouthparts of Haliplidae are apparently adapted to their main diet of algae, but to a much lesser degree than those of the larvae (SEEGER 1971;BEUTEL 1986b).Whether the comparatively elongate head of Meru is a plesiomorphy within the dytiscoid (or hydradephagan) complex is presently uncertain.Autapomorphic conditions are the extremely developed accessory tentorial bridge (formed by fusion of the laminatentoria) and the raspberry compound eyes, with comparatively few conspicuous corneal lenses.The latter condition is very likely a result of miniaturisation as a similar condition is found in the very small Gehringia olympiaca Darlington (LINDROTH 1961) and other very small insects.Features of the antenna, notably the presence of sensorial fi elds on specifi c antennomeres, suggest noterid affi nities of Meruidae (see SPANGLER & STEINER 2005;BEUTEL et al. 2006).A detailed investigation of Phreatodytes, which is very likely the sistergroup of the remaining Noteridae, would be highly desirable.
Aspidytidae and Amphizoidae were placed as sistergroups as suggested based on molecular data (RIBERA et al. 2002b;BALKE et al. 2003BALKE et al. , 2005BALKE et al. , 2008)).Nevertheless, the adult head structures differ very distinctly in both groups.Interestingly the hygropetric Aspidytes has a streamlined head characteristic of the advanced aquatic groups, whereas the head of the fully aquatic Amphizoa is likely plesiomorphic in its general shape.This is possibly correlated with the specifi c habits of the beetles, which do not actively swim but mainly crawl among branches and leaf litter stuck in small creeks or rivers (R. Beutel, pers.obs.).Amphizoidae are characterised by a fused mentum and submentum, an undivided galea, and a specifi c cuticular surface structure as autapomorphies.A fairly elongate and non-streamlined head is also preserved in Hygrobiidae (Hygrobia), even though the beetles are excellent swimmers.Autapomorphies of this group such as the bifurcate apical galeomere, the rounded and deeply excavated hypopharynx, and the extremely strongly developed pharyngeal dilators are apparently of Meruidae as sistergroup of all other aquatic lineages except for Gyrinidae.This appears unlikely as a sistergroup relationship with Noteridae was supported in an earlier morphological study (BEUTEL et al. 2006) and also in a recent molecular study using several mitochondrial and nuclear genes (BALKE et al. 2008).Modifi ed adult head structures of Meruidae may have resulted from adaptations to an unusual hygropetric habitat (see SPANGLER & STEINER 2005) and miniaturisation.Aside from this, the description of meruid larvae is not available yet (A.Short, pers.comm.).It is conceivable that our analysis was negatively affected by secondarily modifi ed head structures of Meruidae and a lack of larval data.
The relict family Trachypachidae with 6 extant species had its greatest distribution and diversity in the early Mesozoic, before the remarkable diversifi cation of Carabidae started (PONOMARENKO 1977).Structural features found in Trachypachus and its sister genus Systolosoma are arguably closest to the ground plan of the suborder, as specialisations of Carabidae on the one hand (e.g., antennal vestiture of fi ne hairs, elongation of mandibles) and the aquatic groups on the other (e.g., shortened streamlined head capsule) are missing.To corroborate this, however, potential non-adephagan outgroup taxa are not really helpful, as all of them show different modifi cations of head structures, mainly correlated with specifi c feeding habits (e.g., algophagy in Myxophaga, saprophagy in Hydrophiloidea).The head structures of adults of Archostemata, arguably the sistergroup of the remaining three extant suborders (e.g., BEUTEL et al. 2007;FRIEDRICH et al. 2009), are extremely derived as pointed out in BEUTEL et al. (2007).In any case, it appears plausible to assume that the last common ancestor of Adephaga was a predaceous beetle, lacking specialised head features that evolved in Gyrinidae, Haliplidae, and in the dytiscoid families, and possibly a preference for riparian habitats (e.g., BEUTEL 1997).Whether an aquatic or terrestrial origin of Adephaga is more parsimonious depends on the sistergroup of Adephaga and its ancestral life style.These issues, however, are presently still open to debate (e.g., BEUTEL et al. 2007;CATERINO et al. 2002;FRIEDRICH et al. 2009;HUNT et al. 2009; WILD & MADDISON 2008; see above).Considering the strikingly different adaptations in larvae and adults of Gyrinidae and the other hydradephagan groups, two independent invasions of the aquatic environment appear plausible at least.
Evolutionary tendencies in Carabidae are elongation of the head capsule and the mouthparts (especially mandibles), a proximal extension of the dense antennal pubescence, and an elongation and proximal shift of the protibial antenna cleaning organ.Adaptations for aquatic and semiaquatic life styles such as reductions of the surface vestiture, a shortened correlated with the exclusive diet of tubifi cid worms (BEUTEL 1986a).The evolution of adult noterid head structures is discussed in detail in BELKACEME (1991).A fully streamlined head has apparently evolved independently from Dytiscidae.This is clearly suggested by the sistergroup relationship between Noteridae and the remaining Dytiscoidea (e.g., BEUTEL et al. 2006).Apomorphic conditions occuring within Noteridae are mainly modifi cations of antennomeres as outlined in BELKACEME (1991).
As already pointed out further detailed morphological data of adults and larvae of Meru (and Phreatodytes) may lead to a reliable clarifi cation of the phylogenetic affi nities.Even though supported by morphological and molecular data in BALKE et al. (2005,2008) and in the present study, the monophyly of Aspidytidae + Amphizoidae is not yet suffi ciently supported.Furthermore, more representatives of the highly diverse ground beetles have to be examined and included in analyses in order to evaluate some features shared by Trachypachidae and subgroups of Carabidae.In contrast to morphology-based studies (e.g., BEUTEL et al.

Acknowledgements
We are grateful to Eric Anton (FSU Jena), Dr. Michael Balke (Zoologische Staatssammlung München), Dr. Ignacio Ribera (Museu de Zoologia, Barcelona), †Prof.Dr. Robert E. Roughley (University of Manitoba) and Dr. Andrew Short (University of Kansas) for providing valuable material.Our particular thanks go to PD Dr. Hans Pohl (FSU Jena) for kindly allowing us to use his extremely useful specimen holder for scanning electron microscopy.We are also indebted to Dr. Frank Friedrich (Biozentrum Grindel Hamburg) for the Bremer support calculation.Sincere thanks are due to the two anonymous reviewers for their helpful advices.

4. 1 .
10. Nervous system Cerebrum and suboesophageal ganglion (Figs.2A, 3A,C, 5D -F).The cerebrum is large in relation to the head size.Two anterior lobes comprise the anteriormost part of the protocerebrum and the deuto-and tritocerebrum.They are adjacent to the inner faces of the dorsal tentorial arms and reach the laminatentoria.Posterior protocerebral lobes nearly reach the occipital origin, (I) apicolaterally on the suspensorial cross-bar, with a slender sclerotised tendon; (F) elevator of the suspensorium, contraction of the anatomical mouth.-M.tentoriohypopharyngalis (M.42): absent.-M.clypeopalatalis (M.43): (O) paramedially on the clypeus; (I) dorsal wall of the preoral cavity, between the tormae and the median epipharyngeal bulge, widened at the insertion area; (F) dilator of the preoral cavity.-Mm.compressores epipharyngis (Mm.III): (Fig. 5B) Numerous transverse muscle bundles connect the upper edges of the posterior epipharynx.Between these bundles fi bres of M. clypeobuccalis (M.44) insert on the dorsal wall of the preoral cavity.The muscle functions as depressor of the posterior epipharyngeal wall, antagonistic to M. clypeopalatalis (M.43).-M.clypeobuccalis (M.44): (O) paramedially on the posterior clypeus, between M.43 and the epistomal ridge; (I) posteriormost epipharynx, immediately anterad of the mouth opening, between the muscle fi bres of Mm.III; (F) dilator of the posterior preoral cavity.General features.The anatomical mouth opening is defi ned by the insertion of M. frontopharyngalis anterior (M.45) and the frontal ganglion which separates this muscle from the epipharyngeal muscles.Further structures associated with the mouth opening are the dorsal transverse Mm. compressores epipharyngis (Mm.III) and the suspensorial cross-bar (see section 4.1.8.) (Fig.5B,C).

Musculature. M.
submentopraementalis (M.28): (O) medially on posterior gula, mesally of M.18b; (I) medially on ventral margin of the prementum; (F) retractor and depressor of the prementum.The bundles of the paired muscle are closely adjacent at the origin and not distinguishable at the insertion area.-M.tentoriopraementalis inferior (M.29a,b): (O) subcomponent a: ventral wall of the head capsule at the submento-mental border; subcomponent b: middle region of the gula, mesad of subcomponent a, both parts fan-shaped; (I) subcomponent a: apically on the process of the palpiger; subcomponent b: ventral base of the cylindrical part of the palpiger, subcomponent a is distinctly steeper than subcomponent b; (F) retractor and adductor (subcomponent a) of the ; (I) lateral branch of the internal cardinal process with a short slender tendon; (F) abductor of the maxilla.-M.craniocardinalis internus (M.16): absent.-M.tentoriocardinalis (M.17): (O) lateral face of the gular ridge, fan-shaped; (I) mesal branch of the internal cardinal process; (F) adductor of the maxilla, antagonistic to M.15.-M.tentoriostipitalis (M.18a,b): (O) subcomponent a: anterior face of the laminatentorium; subcomponent b: posterior gular region and postgular ridge, laterad of M.30, both parts strongly broadened at the origin; (I) membrane at the stipital base, tendons of both subcomponents fused, subcomponent a is distinctly steeper than subcomponent b; (F) adductor and retractor of the stipes, with vertical component (subcomponent a).-M.craniolacinialis (M.19): (O) posteroventral wall of the head capsule, posterad of M.15 and M.17; (I) base of the lacinia, with a long slender tendon, above the cardo process between the two branches; (F) adductor and retractor of the lacinia.-M.stipitolacinialis (M.20): (O) basal margin of the dorsal plate of the palpifer, mesad of M.22; (I) lateral margin of the mediostipes; (F) adductor of the lacinia and the galea (homology see section 4.1.6.).-M.stipitogalealis (M.21): (O) basal wall of the basistipes; (I) basal ventral margin of the galea; (F) movements of are closely adjacent.-M.praementopalpalis externus (M.34): (O) inner margin of the process of the palpiger, opposite to M.29a, fan-shaped; (I) ventral base of palpomere 1; (F) movements of the labial palp.palp (see section 4.1.7.).-M.tentoriopraementalis superior (M.30): (O) medially on posteriormost gula, posterad of M.28, mesad of M.18b; (I) medially on the dorsal margin of the prementum; (F) retractor of the prementum.The bundles of this paired muscle

4 . 4 .
. M. frontohypopharyngalis (M.41): (O) frons, between M.45, M.46 and M.47, strongly fanshaped; (I) apicolaterally on the suspensorial crossbar, with a slender sclerotised tendon, intercrossing with the bundles of M.45 and M.47; (F) elevation of the suspensorium, contraction of the mouth opening.-M.tentoriohypopharyngalis (M.42): absent.-M.clypeopalatalis (M.43): (O) paramedially on the clypeus; (I) anterior epipharynx, broad area between tormae and epipharyngeal lobes; (F) dilator of the preoral cavity.-Mm.compressores epipharyngis (Mm.III): Numerous dorsal transverse muscle bundles connect the upper edges of the posterior epipharynx.The fi bres of M. clypeobuccalis (M.44) insert between the transverse bundles.The muscle functions as a depressor of the posterior epipharyngeal wall, antagonistic to M. clypeopalatalis (M.43).-M.clypeobuccalis (M.44): (O) paramedially on the clypeus, posteriorly of M.43; (I) posteriormost epipharynx, immediately anteriorly of the frontal ganglion, between the muscle fi bres of Mm.III; (F) dilator of the posterior preoral cavity.Due to the rudimentary epistomal ridge the clypeal origin cannot be assessed unambiguously.The homologisation is based on the position closely anteriorly of the frontal ganglion (V.KÉLER 1963) and the characteristically insertion between the muscle bundles of the Mm.III.4.3.9.Pharynx Figs.14, 15 General features.The anatomical mouth opening is marked by the suspensorial cross-bar ventrally and by the frontal ganglion and the insertion of M. frontobuccalis anterior (M.45) dorsally.The pharynx is subdivided into an anterior precerebral and a posterior postcerebral part.It has a wide and approximately round lumen throughout its length and is located in the dorsal protruding.The colour of the head capsule is darkbrown.The surface (except for the gula) and also the exposed parts of the head appendages are densely covered with small pores bearing sensory styliform processes (Fig. 21B,G,K).Tiny setae originate among these grooves, especially close to sclerite margins, around the eyes, and near the postoccipital ridge.Longer setae are absent.The clypeus is almost twice as long as the labrum.The rounded protuberances articulating with the secondary mandibular joints are located at the ventroposterior angles.The clypeofrontal suture is a complete transverse furrow and medially arcuate.The anterior tentorial pits are located in the lateral parts of the suture, close to the antennal ridge.the central part of the cerebrum and reaches the cardinal processes anteriorly.It is posteriorly continuous with the connectives to the prothorax.Head morphology of Amphizoa lecontei 4.4.1.External head capsule Figs. 18, 21
head region.The postcerebral pharynx is continuous with the voluminous oesophagus.The ring musculature of the pharynx is well developed, whereas the intermediate section between the parts of the brain is only equipped with ventral and dorsal transverse muscle fi bres.A single layer of thin longitudinal muscle fi bres underlies the ring musculature.

3 Division of compound eyes: (0) absent
Thin lateral ridges close to the posterior margin of the compound eyes present in Haliplidae: single ridge in Peltodytes and Brychius and two ridges in Haliplus (BEUTEL & RUHNAU 1990).Postocular ridges surrounding the entire head capsule occur in Polyphaga (e.g., Helophorus, Catops).ascending to the dorsal half of the head capsule and has a distinctly wider lumen than the postcerebral pharynx which lies approximately in the mid level of the head capsule.Ring and longitudinal musculature of the pharynx are well developed.The length is measured from the straight basal margin to the anterior margin of the apical part of the mandible, the width measured at the base.]-Approximately twice as long as wide in many Carabidae and Priacma.Ratio in most taxa under consideration between 1,3 and 1,8.Compact and short in Gyrinidae, Amphizoa, Agabus and Aspidytes, and also in Catops.

Mouthparts 29 Vestiture of dorsal surface of labrum: (0) dense
Only separated by a narrow chitinous bar in Dytiscus.One larger fi eld in all other groups.