The cephalic morphology of the American cockroach Periplaneta americana (Blattodea)

The skeleto-muscular system of the head of the American cockroach Periplaneta americana is described in detail. The results are compared with previous partial descriptions of the cephalic morphology of this species and other dictyopterans. The head of Periplaneta is, as in other cockroaches, mostly characterized by plesiomorphies such as the typical orthopteroid mouthparts, the lateral position of the compound eyes, 5-segmented maxillary palps and 3-segmented labial palps, as well as long antennae. Periplaneta shows sexual dimorphism with the compound eyes of the males reaching further ventrally. The epistomal ridge is medially interrupted so that the frons and the clypeus are confluent. The cephalic musculature is typical for a polyneopteran insect and includes 59 muscles. Potential apomorphies for Blat-todea in the cephalic area include the absence of the median ocellus, the bipartite condition of M. verticopharyngealis (0ph1) and of M. hypopharyngosalivaris (0hy12), and the presence of oesotendons. The lacinula, a subapical lobelet on the lacinia, is present in almost all studied blattodeans but its potential homology to the dentisetae of Palaeoptera or the lamellae of apterygotes cannot be addressed at the moment. A “perforate” tentorium, a membranous postmola, and a lacinia that fits into the concave mesal wall of the galea are confirmed as autapomorphies of Dictyoptera.


Introduction
Blattodea includes about 4,000 described species of cockroaches (= paraphyletic "Blattaria"; Klausnitzer 2007) as well as about 3,000 species of termites (Isoptera) and is most diverse in the tropics.Most cockroach species are solitary to gregarious and omnivorous but few developed brood care and a subsocial family-based lifestyle, which is often related to wood feeding (e.g.Cryptocercidae or certain Blaberidae such as Salganea) and culminated in the states seen in termites.Cockroaches have colonized almost every habitat including forest, grassland, heath, steppe, salt marshes and deserts (Bell et al. 2007).Some even prefer aquatic habitats.Despite the large species richness and different lifestyles of cockroaches, the few synanthropic species, which are also pests and vectors for diseases (Bell et al. 2007), such as the American cockroach Periplaneta americana or the German cockroach Blattella germanica, dominate the general and quite negative perception of the group.
Phylogenetic studies of recent years have consistently identified the following principal lineages within Blatto-dea: Blattidae, Tryonicidae, Lamproblattidae, Noctico li dae, Corydiidae (or Polyphagidae), Cryptocercidae + Iso ptera, and Blaberoidea (including Ectobiidae, or Blattellidae, and Blaberidae, the latter being phylogenetically subordinate in the former) (Klass & Meier 2006;lo et al. 2007;inward et al. 2007;Pellens et al. 2007;ware et al. 2008;Murienne 2009;roth et al. 2009;djernaes et al. 2012, 2015;legendre et al. 2015; for details on the history of Blattodea systematics see djernaes et al. 2015 and legendre et al. 2015).djernaes et al. (2015) have tentatively identified Anaplectidae as another major lineage -a group that has been strongly neglected in previous studies and had been considered as representing the basal-most offshoot(s) of the blaberoid lineage (Klass & Meier 2006).lo et al. 's (2007) finding of Nocticolidae being sister to Mantodea is the only recent phylogenetic result that challenged the monophyly of Blattodea, but the more extensive studies of djernaes et al. (2015) and legendre et al. (2015) found Nocticolidae subordinate in Corydiidae.While there is agreement regarding these principal lineages of Blattodea, recent phylogenetic studies contradict each other regarding the relationships among these lineages.During the last decade nearly all possible combinations between these groups have been proposed (see djernaes et al. 2012, 2015).
Despite the facts that synanthropic cockroaches are common in zoological institutes around the world and that Periplaneta americana is a common species for students to dissect (seifert 1995; storch & welsch 1999), no description of the entire head capsule including the musculature is available for this species or any other cockroach.Nevertheless, several studies focused on specific parts of the head of the American cockroach: craMPton (1917,1921) and snodgrass (1960) provided a treatment of the head capsule.craMPton (1923) described the maxilla, craMPton (1925) the labium, walK er (1931) the clypeus, labrum and labium, PoPhaM (1961) and zhuzhiKov (2007) the mouthparts, willey (1961) the nervous system, Butler (1973) the compound eyes, we Ber & renner (1976) the ocellus, Pass (1985) the antennal heart, Klass & eulitz (2007) the anterior head sulci and the tentorium, and Buder & Klass (2013) the hypopharynx.wiPfler et al. ( 2011) provided a list with head muscles present in Periplaneta but no further information such as the exact origin or insertion.Additionally, Periplaneta is used to study the function and biomechanics of the mouthparts, especially the mandible and maxilla (schMitt et al. 2014;weihMann et al. 2015a,b).The lack of a complete documentation and the role of Periplaneta americana as a model organism in bio logy (e.g. rotte et al. 2009;Matsui et al. 2013) induced us to present a coherent and well-documented description of the entire cephalic skeleto-muscular system for this species.willey (1961) provided an excellent study of the stomodeal nervous system of Periplaneta americana, so we will only describe the parts necessary to understand the skeleto-muscular system.

Material and methods
Specimens.This study is based on male and female specimens of Periplaneta americana (Linnaeus, 1758).
For morphological examination they were fixed in 70% ethanol.Animals were acquired from the breeding of Jörg Bernhardt (www.schabenspinnen.de)and taken in culture at the Institut für Spezielle Zoologie und Evolutionsbiologie mit Phyletischem Museum of the Friedrich Schiller-Universität Jena.Dissection.Specimens were dissected in 70% ethanol under a stereo microscope (Leica MZ 125).To study the interior (epidermis-facing) surface of the head capsule, specimens were macerated with a 10% KOH solution and checked regularly until maceration was completed.
Scanning electron microscopy (SEM).Four animals were transferred from 70% to 100% ethanol, dried with HMDS (Hexamethyldisilazan; Brown 1993) and subsequently sputter coated with gold (Emitech K500).SEM was performed using a Philips XL30 ESEM with a special sample holder (Pohl 2010).µ-CT and 3-D reconstruction.One female was critical point dried and mounted on a special sample holder for µ-computed tomography (µ-CT).The scan was performed at the Department of Functional Morphology and Biomechanics of the Christian-Albrechts-Universität Kiel on a Skyscan 1172 with a beam strength of 40 kV and 250 µA.The exposure time was 720 ms and a 360° scan with steps of 0.130° was performed.The resulting spatial resolution was 3.07 µm.The µ-CT data was segmented with Visage Imaging Amira 5.2.2 and the individual materials were separated with the algorithm function of Amira.Subsequently they were exported as stacks of tifffiles that were imported in VG Studiomax 2.2.Rendering was done with a combination of volume rendering (Scatter HQ; muscles and internal structures) and isosurface render (used in some images for the skeleton).The CT scan is deposited in the collection of the Phyletisches Museum in Jena, Germany (Polyneoptera/ Blattodea/ Periplaneta americana 12).Additionally a binned version can be downloaded from morphDbase (groBe & vogt 2009) under the link www.morphdbase.de/?B_Wipfler_20160818M3.1.The Electronic Supple ment file 1 provides an interactive 3dimensional PDF of the musculature of Periplaneta americana.Digital microscopy and image processing.For digital microscopy specimens or dissected parts such as mouthparts were critical point dried and subsequently photographed with a Keyence VHX 2000 using the same sample holder as for SEM (Pohl 2010).For figure 1 they were imaged without critical point drying in natural coloration.Final images of SEM, digital photography and µCT based reconstructions were assembled, edited and labeled with Adobe Photoshop CS6 and Adobe Illustrator CS6.Terminology.Terminology follows Beutel et al. (2014a) for general exoskeletal morphology, wiPfler et al. (2011) for the cephalic musculature, and Buder & Klass (2013) for exoskeletal elements of the hypopharynx (which are only partly covered in Beutel et al. 2014a).We use the term 'Blattodea' for the monophyletic group comprising cockroaches and termites and 'Blattaria' for the paraphyletic cockroaches (as in e.g.Klass & eulitz 2007).
There has been a long discussion concerning the terminology of strengthening and weakening linear structures on the insect head (e.g. duPorte 1946(e.g. duPorte , 1957;;snodgrass 1947;Klass & eulitz 2007;wiPfler et al. 2011).We cannot solve this definition problem but to provide maximum clarity in description, we distinguish between three categories for the structures relevant in this manuscript: (1) Ridges: strengthening ridges on sclerites, which are based on inward folding or thickening of the cuticle, project into the lumen of the body, and are externally either visible as grooves or leveled (termed sulci by Klass & eulitz 2007, snodgrass 1960: p. 15, weidner 1982: p. 38, and griMaldi & engel 2005: p. 121; and costae according to their internal appearance in griMaldi & engel 2005: p. 121).(2) Cleavage lines: linear weakenings of the sclerotization or of the entire cuticle in pre-imaginal stages which are predefined breaking lines during ecdysis; they are often still recognizable in the adults (where we did not analyze their detailed structure).(3) Syndeses: narrow stripes of membrane between sclerites which in a more or less hinge-like fashion allow their reciprocal movement (e.g. between the clypeus and the labrum).This terminology is not meant as a set of strict definitions (which would require much research on the cuticular fine structure) but as a categorization convenient for descriptive purposes.
We use the standard terminology of Beutel et al. (2014a) to name the various internal ridges of the head.However, we follow griMaldi & engel (2005) by subsuming the epistomal and subgenal ridges under the term 'costa'.In the text we only provide a few references to the illustrations.A complete list of used abbreviations in the morphological description and reference to the respective figures is provided in Electronic Supplement file 2.
Different terminologies for the mandibular incisivi have been proposed (e.g.ahMad 1950).However, the homology of these teeth is difficult to infer even within one traditional order.We therefore avoid any terminology or homology hypothesis and simply number them.
In the descriptive part, we distinguish between (1) structures or formative elements (abbreviated in small letters) such as the labrum, galea, or tentorial arms and (2) sclerites (abbreviated in capital letters) such as the labral or the stipital sclerite (see e.g.Klass & Matush Kina 2012).

Head capsule
The orthognathous head capsule of the female is overall brownish (Fig. 1 shows it in its original coloration) and   flattened in posteroanterior direction.In frontal view it is reversely drop-shaped with the maximum width on the level of the antennal bases.It is sparsely covered with short setae on its frontal surface.Along the occipital ridge there is a row of long setae.
The foramen occipitale (foc, Figs. 2, 3) is nearly square but with its dorsal and ventral boundaries convexly arched.Ventrally it is closed by the posterior base of the labium.The margin of the foramen is surrounded and reinforced by the postoccipital ridge (por, Figs. 2, 3, 4; in Fig. 3F partly broken on left side of head), so no postoccipital area is present.Dorso-laterally the postoccipital ridge bears a condyle, which articulates with the lateral cervical sclerite (*, Fig. 2) on each side.
The large kidney-shaped compound eyes (ce, Figs. 2, 3) are positioned laterally on the dorsal part of the head capsule.In the female they end ventrally on the level of the antennal articulations.Each eye is surrounded by a circumocular ridge (cor, Figs. 2, 3, 5).The two lateral ocelli (oc, Figs. 2, 3) are located mesally of the compound eyes and meso-dorsally of the antennal bases.The median ocellus is missing.The antennal bases are positioned in fronto-mesal recesses of the compound eyes.They are facing frontally.Each antennal socket is surrounded by a circumantennal ridge (car,Figs. 2,3,4,5), whose lateral to dorsolateral part is confluent with the circumocular ridge.From its ventral part a well-developed antennifer (an,Figs. 3,4,8) extends to the base of the scapus.The distance between the antennal bases is approximately twice their diameter.
From the fronto-ventral part of the circumocular ridge the laterally arched subantennal ridge (sar, Figs. 2 -5) continues ventrally, where it unites with the dorsally arched pleurostomal part of the costa (psr, Figs. 3 -5) at its dorsalmost point.From here the costa descends posteriorly towards the posterior mandibular articulation and anteriorly towards the anterior articulation.The lat-ter branch curves dorso-mesally upon the articulation to continue only for a short distance as an epistomal part of the costa.The entire stretch of the costa anteriorly of the junction with the subantennal ridge represents the anterior tentorial pit (atp,Figs. 2,4,5), i.e. these parts are deepened to form the anterior tentorial arms.The pleurostomal and hypostomal parts of the costa delimit the dorsal gena (ge, Fig. 2) from the ventral subgena.Between the anterior and posterior mandibular articulations the subgena is wide due to the arching of the pleurostomal part of the ridge.Posteriorly of the posterior mandibular articulation the hypostomal ridge (hsr, Figs. 3, 4, 5) of the costa runs along the margin of the head capsule.Dorsally, near the beginning of posterior tentorial arms, the hypostomal ridge forms a sclerotized process (hyp, Fig. 3) which articulates with the cardo of the maxilla.The occipital ridge (ocr, Figs. 1 -5) starts from the posterior mandibular socket and runs dorso-mesally on the posterior side of the head capsule.This supportive ridge bifurcates near the latero-dorsal edge of the occipital foramen.One branch continues for a short distance dorsomesally while the other one fuses with the postoccipital ridge.
clypeus is trapezoid and bears a few long setae.It consists of a proximal strongly sclerotized part (postclyp eus pcl; Figs. 1, 4) and a distal weakly sclerotized part (ante clypeus acl; Fig. 1, 3); in the latter the median part is especially weakly sclerotized.Distally the clypeus is separated from the labrum by a fairly narrow membranous stripe, which is transversely folded inward (a syndesis in a wider sense, but too wide for a distinctly hinge-like contact).The coronal cleavage line (cc, Fig. 3) runs medially from the foramen over the vertex.In between the compound eyes it branches into the two frontal cleavage lines (fc, Fig. 3), which continue laterally towards the ocelli, where they end.
The head capsule of the male is in almost every respect similar to the female one.Figure 3 compares the head capsules of the two sexes.The only difference refers to the bigger compound eyes.In contrast to the female they reach further ventrally than the antennal articulations.Thus the circumocular ridge is in contact with the hypostomal part of the costa, and there is no discrete subantennal ridge.

Tentorium
The tentorium is composed of paired anterior (ata,dorsal (dta,Figs. 4,5), and posterior tentorial arms (pta, Fig. 4) as well as two unpaired connective bridges, the corpotentorium (ct, Figs. 3 -5) and the anterior tentorial bridge (atb,.The massive and flat anterior tentorial arm (ata, Figs. 3 -5) originates from the anterior tentorial pit, which is located on the anterior costa.Each arm is twisted about 120° (left arm clockwise as seen from the pit) and has reinforced margins on both sides.Left and right anterior tentorial arms are medially interconnected by the anterior tentorial bridge (atb, Figs. 3 -5).The epidermis-facing surface of this bridge is smooth on its anterior side but shows a median ridge (atr, Fig. 4) on its posterior side.In their last third the anterior tentorial arms laterally border a round "perforation", i.e. a medial gap between the anterior tentorial bridge and the corpotentorium, through which the pharynx passes.On the anterior side of this part of the anterior tentorial arm the thin and wing-like dorsal tentorial arm (dta,Figs. 4,5) has its broad origin.It narrows distally into a slender thread-like part and does not reach the cuticle of the head capsule.On the anterior margin of the corpotentorium paired elongate laminar tendons, the oesotendons (os, Fig. 4), originate.They run ventrally and serve as attachment site for M. tentoriosuspensorialis (0hy5).Dorso-posteriorly the anterior tentorial arm continues into the corpotentorium, which is transversely plate-like and laterally receives the short posterior tentorial arms.The posterior tentorial arms (pta, Fig. 4) originate from the posterior tentorial pits (ptp, Fig. 3) at the transition between the hypostomal ridge and the postoccipital ridge.Laterally on the ventral side of the posterior tentorial arms small trabeculae tentorii are located, which serve as attachment site for musculature.

Antenna
The base of the scapus articulates with the head capsule via the antennifer.The scapus (SC, Figs. 2, 3, 8) is a slightly bent sclerite cylinder, half as wide as long and covered with various setae.The pedicellus (PE, Figs. 2, 3, 8) is about 1/3 the length of the scapus.Its setation is less dense than that of the scapus.Along most of the circumference of the antenna the scapus and the pedicellus are separated by a fairly wide membrane, but laterally and mesally there are small areas of close contact.The pedicellus has conspicuous longitudinal grooves on its distal half (Fig. 8A).The flagellum (fl, Figs. 2, 8) has at its base approximately the same diameter as the pedicellus and consists of about 120 flagellomeres.The first is about as long as the pedicellus while the ones in the following third of the antenna are only half as long as wide.In the middle of the flagellum they are as long as wide, while in the distal third they become longer than wide.The distal 15 flagellomeres narrow strongly.The musculature of the antenna is partially illustrated in Fig. 7 and described in Table 1.

Labrum and epipharynx
The trapezoid labrum (lbr, Figs. 1, 2, 4) and the clypeus together form the clypeolabrum.The labral sclerite (LBS, Fig. 3) is separated from the clypeal sclerotization by a syndesis and takes the entire anterior wall of the labrum.Its distal half and lateral sides bear large setae.The distal edge of the labrum is slightly notched (ln, Fig. 6).This notch is densely setose.
The epipharynx (epi,Figs. 3,6) is the posterior wall of the clypeolabrum.It forms the roof of the praeoral cavity and ends dorsally in the anatomical mouth opening (opening of foregut).The epipharynx is membranous except for the torma (TO, Fig. 6) and the epipharyngeal suspensorium (ESU, Fig. 6), which are interconnected.
Together they form a central loop-like sclerotization with a long distal extension (main part of suspensorium) and shorter lateral and proximal extensions (parts of the tormae).The lateral tormal extension is firmly connected to the labral sclerite very close to its basal margin.The ribbon-like, irregular, and partly fragmented distal extension serves as attachment for M. frontoepipharyngealis (0lb2) and bears several spike-like setae (eps, Fig. 6).Alongside the torma and the proximo-lateral part of the epipharyngeal suspensorium the cuticle is thickened  and sclerotized internally, while the surface is soft (thus showing the reverse condition of what is usually found in sclerites, including the remaining suspensorium).The median epipharyngeal area is covered by a brush of microtrichia (epb, Fig. 6).The brush is very strongly developed in the median-most area around the clypeo-labral border.Distally it decreases but the distal margin including the labral notch (ln, Fig. 6) is again densely covered by microtrichia.Proximally of the tormae an unpaired sclerite is embedded in the epipharyngeal membrane.It is laterally more strongly sclerotized and serves as attachment site for M. clypeopalatalis (0ci1, Fig. 18).
The musculature of the labrum and epipharynx is partially illustrated in Figs.7 and 18 and described in Table 1.

Mandible
Each mandible (md, Figs. 9 -11) bears a single and massive sclerite, which articulates with the head capsule via two ballandsocket joints: the anterior (ama, Figs. 2 -5) and posterior (pma, Figs. 3 -5) mandibular articulations.The mandible provides the socket for the anterior articulation (aa, Fig. 9) and the ball for the posterior one (pa, Fig. 9).Both mandibles are slightly bent posteriorly in their distal third.
The mandibles show some left-right asymmetry.The left mandible has four distal incisivi (including the tip; Fig. 9).The distal two (I + II, Fig. 9) are subequal in size, followed by the largest one (III, Fig. 9).On the posterior side a small dent-like tooth is positioned at the base of incisivus III but not counted as an incisivus on its own.The proximalmost incisivus (IV, Fig. 9) is the smallest.The right mandible has three distal incisivi.The proximal one (III) is also the smallest while the second one (II) is slightly longer than the first (I).
Proximally of the incisivi the mola (mo, Fig. 9) follows.In the left mandible, it is a concave area which is approximately twice as high as wide.The mola of the right mandible is convex with a ridge in its middle, thus perfectly fitting into the concave left mola.
The proximal end of the mesal surface of the mandible is formed by the membranous postmola (pm, Fig. 9).Seen from anterior, it is a rhomb-shaped structure with a slightly convex mesal edge.It is densely covered with microtrichia (Fig. 10C), which are longer in the lateral areas.The proximal edge of the postmola bears a dense cluster of thin hair-like setae (Fig. 10D).The anterior and posterior surfaces of the gnathal armature each bear a row of short, more robust setae (Fig. 10A).Both rows The strong mandibular adductor tendon originates mesally from the basal margin of the mandibular sclerite and reaches deeply into the lumen of the head capsule.Internally the tendon gradually widens and then divides into three winglike branches: Quite close to the mandible a sail-like basal wing of the tendon (bwt, Fig. 11) protrudes laterally; on the distalmost point of the basal wing the tendon forks into a narrow lateral wing (lwt, Fig. 11) and a fairly wide mesal wing (mwt, Fig. 11).The mesal wing reaches further mesally than the attachment of the tendon on the mandible.All wings serve as attachment for M. craniomandibularis internus (0md1).The mandibular abductor (0md3) inserts on a slim and unbranched tendon arising laterally from the basal mandibular margin.
The musculature of the mandible is illustrated in Fig. 11 and described in Table 1.

Maxilla
The maxilla articulates with the hypostomal ridge.On its postero-lateral side the maxilla is well sclerotized while the antero-mesal side, which faces the head capsule, is membranous.The stipital sclerite (ST, Figs. 12 -14) articulates with the cardinal sclerite mesally on its posterior side; the articulation is supported by the posterior end of the cardinal ridge.Apart from this articulation the sclerites are separated by a wide membrane anteriorly and by a narrow membrane posteriorly, the latter forming a cardinostipital syndesis (css, Figs.13).Proximally, near the syndesis, the stipital sclerite is sparsely setose.On the posterior side of the maxilla, the stipital sclerite has a distal semi-circular extension, the stipital disk (sd, Figs. 12, 13).It is synsclerotic with the lacinial and basigaleal sclerites.On the posterior surface of the stipes, the stipital ridge (sr, Fig. 13) runs from the cardino-stipital articulation distally and ends shortly proximally of the lacinial The galea (ga, Figs. 12 -14) bears 3 sclerites, the basi (BGA, Fig. 13), disti (DGA, Figs. 12, 13) and mesogaleal sclerites (MGA, Figs. 12, 13).On the posterior maxillary surface the basigaleal sclerite is synsclerotic with the lateral and distal margins of the stipital disk (sd).Distally of the basigalea the distigaleal sclerite extends over the disto-lateral half of the galea except for the apex.Basally it is well sclerotized while distally the sclerotization gradually decreases and obliterates.Disto-mesally of the distigaleal sclerite, well separated by a membranous area, the galeal brush (gb, Figs. 12, 13) is located on the mesoapical part of the galea.The mesogalea is an L shaped sclerite on the antero-mesal side of the galea.Its transverse basal part is widely separated from the stipes;  laterally it is interjacent between the bases of the disti and basigaleal sclerites (and somewhat synsclerotic with both), while mesally it shows a 90° bend to continue in its elongate distal part.This part reaches the meso-proximal end of the galeal brush; the distal two thirds of the mesogalea bear long mesally directed setae.
The lacinia (la, Figs. 12 -14) has its base postero-mesally of the galea.It bears a single sclerite, whose posterior surface is heavier than the anterior one.It is narrowly synsclerotic with the stipes along the mesal margin of the stipital disk, while further mesally the stipes and lacinia are separated by a membranous syndesis, which allows for meso-lateral movements of the lacinia.The entire mesal surface of the lacinial sclerite bears rows of setae on both the anterior and posterior sides; the setae increase in length distally.Distally the lacinia narrows strongly and ends in two strongly sclerotized lacinial teeth (or incisivi) (li,Figs. 12,13).At the distal end of the mesal setation and at the base of the lacinial teeth, the lacinula (lac, Figs. 12, 13) is located.It is a membranous lobe slightly longer than the setae.Only its tip is strongly sclerotized and bears three small teeth.In resting position the distal part of the lacinia fits into the concave mesal wall of the galea, being buried in the galeal brush.
The musculature of the maxilla is illustrated in Fig. 14 and described in Table 1.
The submental sclerite is a strongly sclerotized plate.Its lateral margins are densely setose, contrasting the sparse setation of its remaining parts.The basal margin is concave and connected with the cervical membrane.Basolaterally it articulates with the head capsule at the ventral end of the postoccipital ridge (Fig. 3F).In the distal half, the lateral margins are strongly convex.Distally it is separated from the mental sclerite via a narrow proximally arched membranous stripe, which is shaped as a pair of syndeses in the larger lateral parts but is distinctly wider in the median part.
The mental sclerite is also a strongly sclerotized plate and nearly oval.It is approximately half as long as the submental sclerite at midline and bears two pairs of setae, a mesal and a lateral one.The mental and the praemental sclerites are separated by a broad membranous area.
The praemental sclerite is an almost square plate with a slightly convex basal margin.Laterally it has concave emarginations embracing the base of the labial palps.Its surface is sparsely setose.In the distal half, a median praemental cleft divides the sclerite.From the base of the cleft a midline ridge runs proximally; it soon forks, the branches enclosing a near-triangular area upon the base of the praemental sclerite.The main body of the palpiger sclerite (PG, Fig. 15) is located in the lateral labial wall proximally of the palpal base.From there it extends distally in the anterior labial wall towards the base of the labial palp, where it forms an articulation (not shown).The second articulation of palpomere I is located at the lateral margin of the praementum (art 0-I, Fig. 15).The palpal vestiture with setae strongly increases along the three palpomeres, with palpomere III being densely covered.Palpomere I is clubshaped and approximately as long as wide.II is slightly longer than I but has the same width.Palpomere III is three times as long as wide.Its apical surface is flat and extremely densely covered with setae (Fig. 16F).Palpomeres I and II articulate upon each other on the anterior (not shown) and posterior surfaces (art III, Fig. 15).Palpomeres II and III articulate via a lateral joint (art IIIII, Fig. 15).Distally on the praementum, two lobes originate on each side.The mesal glossa bears a single sclerite and is narrowed distally.It bears a field of tapering microtrichia distally.The lateral paraglossa is longer than the glossa.Its lateral margin is concave while its mesal margin follows the shape of the glossa.Its basal two thirds bear a sclerite and several long setae.Distally the paraglossa is membranous with a distomesal field of tapering microtrichia.Disto-mesally the right and left paraglossae are in contact with each other.
The musculature of the labium is illustrated in Fig. 15 and described in Table 1.

Hypopharynx
The hypopharynx (hph, Figs.16 -18) is a tongue-like, largely membranous structure with several sclerites in its wall.It forms a slope towards the anatomical mouth opening and is divided in a distal lingual and a proximal suspensorial part.On the distal tip there is a small notch or low recess.The tip and the anterior and antero-lateral surface of the lingual part are densely covered with long tapering microtrichia (fch, Fig. 17).The suspensorial part is bare of such vestiture, except for a small field of microtrichia located disto-medially shortly behind the border to the lingual part.
The sclerites, many of which are interconnected, are classified into lingual and suspensorial ones.In the lateral area of the distal hypopharynx the lateral lingual sclerite (LLS, Fig. 17) is embedded.Its weakly sclerotized anterior part is triangular and in touch with the field of microtrichia.Proximally it is synsclerotic with the platelike distal part of the suspensorial sclerotization (SDP,Figs. 16,17).Posterodistally it continues into the second, more strongly sclerotized part of the lateral lingual sclerite, which is also triangular.This part does not reach the field of microtrichia anteriorly.Proximally it reaches underneath the suspensorial plate but is well separated from it by membrane.At its proximal-most point it continues, via a near180° bend, into the ventral lingual sclerite (VLS,Figs. 16,17).The latter continues disto-mesally, where the sclerites from both sides nearly unite in the posterior wall of the hypopharynx.The base of each ventral lingual sclerite additionally forms a postero-mesally directed arm    From the junction between the lateral lingual sclerite and the platelike distal part an armlike sclerotization (SMP, Fig. 17) extends antero-mesally.It is shaped as a deep, thickwalled groove.Left and right armlike sclerotizations abut upon each other in the dorsal midline of the hypopharynx (but are not connected).These arms are largely hidden due to their groove-like condition and a slight overfolding by the microtrichia-bearing area (fch) that immediately joins the armlike sclerotization distally.The oral arm (SOA) is a long and slender bar which runs far dorsally in the lateral wall of the pharynx (ph, Figs. 3 -5) where it ends in a pharyngeal pouch (php, Fig. 17).Several hypopharyngeal muscles are attached to the oral arm, which span the opening of the pharynx.The loral arm (SLA) is a sickleshaped sclerite located in the postero-lateral hypopharyngeal wall.The anterior border of the loral arm is very sharp while on its postero-distal side it continuously fades out into membrane.The linguacutal sclerite (LAC, Figs.17, 18) is positioned proximally of the loral arm, narrowly separated from it by membrane.The proximal part of the sclerite forms a strongly curved apodeme (lact, Figs.17, 18), which extends ventrally into the lumen of the mandible (in Fig. 17 it is twisted backwards due to the preparation of the hypopharynx, while Fig. 18 shows its natural orientation).
In the membranous proximal lateral wall of the hypopharynx a weak, isolated lateral sclerite ("basitendinal" sclerite, SBT) is embedded on each side.Mesally the sclerites of both sides approach each other (by invagination of the hypopharyngeal wall) and are linked by M. loroloralis.
The musculature of the hypopharynx is partially illustrated in Fig. 18 and described in Table 1.

Pharynx
The pharynx (ph, Figs. 3 -5) is a tube with a transversely oval cross section slightly wider than high.Its opening (the functional mouth) is located slightly ventrally of the frontal ganglion and is held in position by the oral arms of the hypopharyngeal suspensorium and M. oralis transversalis (0hy9).The posterior wall of the pharynx continues into the hypopharynx while the anterior wall continues into the epipharynx.At rest the pharyngeal wall is folded in a longitudinal direction, which likely allows for considerable expansion when food is taken up.The pharynx is equipped with an inner layer of ring muscle fibers (M.anularis stomodaei; closer to the pharyngeal wall) followed by a layer of longitudinal muscle fibers (M.longitudinalis stomodaei).
The musculature of the pharynx is illustrated in Fig. 18 and described in Table 1.

Central nervous system
The supraoesophageal ganglion (sog, Figs. 4, 5) is well de veloped; an external distinction between proto, deutero-and tritocerebrum was not possible based on our data.Frontally a pair of ocellar nerves (oc, Figs. 4, 5)

Muscle name
Abb.

Muscle name
Abb. arises from it; no vestige of a median ocellar nerve was found.The antennal nerves (ann, Fig. 4) and the optical lobes are strongly developed.The frontal connectives (frc, Figs. 4, 5) originate from the anterior side of the circumoesophageal connectives and turn ventro-anteriorly around the pharynx to join the frontal ganglion placed on the anterior side of the pharynx (fg, Figs. 4, 5).The unpaired nervus recurrens (ner, Figs. 4, 5) runs from the frontal ganglion dorsally along the anterior side of the pharynx.It passes beneath the supraoesophageal ganglion and joins the corpora allata (coa, Fig. 5).The supraoesophageal ganglion is connected with the suboesophageal ganglion (sug, Figs. 4, 5) via massive circumoesophageal connectives.The tritocerebral commissure (not illustrated) runs ventrally around the pharynx.The suboesophageal ganglion is much smaller than the supraoesophageal ganglion.

Circulatory organs
The aorta (ao, Fig. 5) is a thin tube located on the dorsal side of the pharynx.It ends below the supraoesophageal ganglion.At its end, M. ampulloaortica (0ah2, not illustrated) inserts.The paired antennal ampullae (am, Figs. 5, 7) are attached to the frontal head wall mesally of the antennal base, approximately on the same level as the antennifer.The antennal vessels (av, Fig. 7) extend from the ampulla into the antenna.Along their course in the head capsule they are strongly curled.

Comparison with previous studies
In general, our description provided herein is in most points congruent with previous studies.The main differences will be discussed in the following.
Head posture.In several articles the head position of Periplaneta and other cockroaches is described as being hypognathous (PoPhaM 1961;wiPfler et al. 2011).
Cockroaches have a highly moveable head which can be bent under the prothorax.Living individuals of Peripla neta hold their heads most of the time in a position where the mouthparts point directly downward (orthognathy) (snodgrass 1944; pers.obs.) and the antennae forward.We thus consider this as the typical head orientation.The overall proportions of the head also agree with the orthognathous condition rather than with the hypognathous one.However, after their death, the head often takes a backward position, which might have caused most previous authors to consider the head posture in this species as being hypognathous.

Cephalic ridges.
Periplaneta americana shows sexual dimorphism in its head morphology.In females the com-pound eyes end on the level of the ventral end of the antennal base; the circumocular and subgenal ridges are thus far apart from each other, and the subantennal ridge connecting them is long and distinct.In males, the compound eyes are bigger and reach far ventrally, whereby the circumocular ridge is in close contact with the subgenal ridge and a subantennal ridge is absent (likely fused with the circumocular ridge, see Klass & eulitz 2007).Klass & eulitz (2007) based their study on dictyopteran head ridges ("sulci" therein) mostly on male specimens and also illustrated their state in male Periplaneta americana.
Regarding other Blattidae studied therein, Shelfordella lateralis (only a female examined) shows a similar condition as females of Periplaneta americana, though with the eyes somewhat smaller, whereas Eurycotis floridana males show much smaller eyes than female Periplaneta americana and Shelfordella lateralis.The differences in the Blattidae are thus related to both interspecific variation and sexual dimorphism.The complex morphological correlations between compound eye size and the outline of the circumantennal ridge and the circumocular, subantennal, and subgenal ridges are discussed in Klass & eulitz (2007).snodgrass (1960) apparently studied a female of Periplaneta americana.
Several authors including Klass & eulitz (2007) described short lateral vestiges of the epistomal ridge in Peri planeta.However, statements on whether lateral parts of the epistomal ridge are absent or present depend on the definition of the border between the epistomal and subgenal ridges.Klass & eulitz (2007) define this border as being directly above the anterior mandibular articulation.On this basis, Periplaneta has short lateral parts of the epistomal ridge.seifert (1995) and wiPfler et al. ( 2012) define the mesal end of the anterior tentorial pit as the border.According to this definition, Periplaneta has no epistomal ridge.Both definitions are obviously problematic in some respect and cannot be reasonably applied to all insects.In the present contribution, we thus follow gri Maldi & engel (2005) by subsuming the epistomal ridge and subgenal ridge as costa.On this basis, the part of the costa in between the anterior tentorial pits is absent -with consensus across the literature.Periplaneta has lower mandibular biting strength and a comparatively smaller M. craniomandibularis internus (0md1) than other studied insects (weihMann et al. 2015a).It is likely that it thus requires less structural integrity between the anterior mandibular articulations, which led to the loss of this part of the ridge.Another reason could be the far anteriorly located transverse stabilization of the left and right halves of the head by the additional anterior tentorial bridge; however, this is also present in Mantodea, which have a well-developed epistomal part of the costa (Klass & eulitz 2007).

Mandibles.
A slight asymmetry in the structure of the left and right mandibles is confirmed for Periplaneta ameri cana.However, for the left mandible zhuzhiKov (2007) describes a second small tooth, which was not found in the present study.Possibly some specimens have two small teeth while others have only one.
Hypopharynx.The hypopharynx of Periplaneta ameri cana was described in detail by Buder & Klass (2013).Both descriptions are mostly congruent.However, herein we describe short sclerite ribbons extending ventromesally along the posterior base of the hypopharynx (SBH, Fig. 17), which are synsclerotic with the lateral and ventral lingual sclerite.Buder & Klass (2013) found only a pair of tiny isolated sclerites near the opening of the salivary duct, representing vestiges of median parts of the SBH clips.However, we cannot exclude that there is intraspecific variability in this character.For several other Blattodea they reported similar conditions as observed here in Periplaneta, with a connection to the lingual sclerites and a gap at midline.Head muscles.Some differences to previous descriptions were also found in the musculature (see Table 2).dorsey (1943) only addresses selected muscle systems but information for the antenna, the mandible and the maxilla is missing.Additionally some muscles of the foregut such as M. verticopharyngealis (0ph1) were not described.wiPfler et al. ( 2011) provided a list with muscles present in Periplaneta but no details on their origin and insertion.The present findings contradict that study in several points: M. tentoriomandibularis medialis inferior (0md8) and M. tentoriofrontalis anterior (0te2) were reported as present by wiPfler et al. ( 2011) but are found absent in the present study, while for M. loroloralis (0hy10), M. tentoriobuccalis lateralis anterior (0bu4) and M. stipitopalpalis medianus (0mx9) the situation is contrariwise.Our reevaluation of the CTscan studied by wiPfler et al. ( 2011) revealed that M. tentoriomandibularis medialis inferior (0md8) and M. tentoriofrontalis anterior (0te2) are indeed absent, whereas M. tentoriobuccalis lateralis anterior (0bu4) is present.M. loroloralis (0hy10) is an extremely small muscle which is hard to find.It is therefore possible that previous authors overlooked it.M. stipitopalpalis medianus (0mx9) is absent in both the left and the right maxilla of the specimen studied by wiPfler et al. (2011) but present in the specimen studied here.Apparently there are individuals which have three extrinsic muscles of the maxillary palp while others have only two.

Phylogenetic implications
In the context of our current insights for Periplaneta americana, the following studies on other blattodean species permit consideration of some phylogenetic implications: yuasa (1920), snodgrass (1935), strenger (1942) and Pradl (1971) studied the head capsule of Blatta ori entalis (Linnaeus, 1758);snodgrass (1943, 1944) and Pradl (1971) the one of Blattella germanica (Linnaeus, 1758); and Pradl (1971) the one of Blaberus giganteus (Linnaeus, 1758(Linnaeus, ). vishnoi (1956(Linnaeus, , 1962) )  In general the blattodean head is characterized by several plesiomorphies such as the lateral position of the compound eyes, biting mouthparts, 5-segmented maxillary palps, 3-segmented labial palps, and long antennae.However, the blattodean head also shows several apomorphies.The most striking is the loss of the median ocellus.This loss under retention of only the lateral ocelli has, for example, also occurred in some species of the dipteran group Mycetophilidae (söli 1996) but not in any other lower neopteran group (e.g.yuasa 1920).We found the loss of this ocellus to be accompanied by the loss of the associated ocellar nerve, showing that the plesiomorphic pathway of light perception has been lost (which would not necessarily be the case with the absence of the cuticular differentiation of an ocellus).Other potential apomorphies of Blattodea corroborated by the present study are the bipartite state of M. verticopharyngealis (0ph1) and of M. hypopharyngosalivaris (0hy12) (wiPfler 2012).Another potentially derived character is the presence of oesotendons.They are cuticular tendons of the corpotentorium which serve as attachment structures for M. tentoriosuspensorialis (0hy5) and M. tentoriobuccalis (0bu5+6).Klass & eulitz (2007) report oesotendons for all studied cockroaches with the exception of Cryptocer cus and Mastotermes.In these two taxa the "perforation" of the tentorium is comparatively small, which might correlate with a reduction of the tendons.lo et al. (2007) provided the only study which places these two taxa jointly as sister group to the remaining dictyo pterans, thus challenging the monophyly of Blattodea.All other studies on blattodean phylogeny consider Cryptocercus and Mastotermes as nested deeply within monophyletic Blattodea.Structures similar to oesotendons also seem to occur in the silverfish Thermobia (chaudonneret 1950) and the caeliferan Acridella (hudson 1945), which caused Klass & eulitz (2007) to assume that their presence in Blattodea might be plesiomorphic and their absence in Mantodea apomorphic.However, oesotendons are neither described in most polyneopteran groups (e.g.

Muscle name Abbreviation
Periplaneta americana (present study) Periplaneta americana (Wipfler et al. 2011) Periplaneta americana (Dorsey 1943) Blatella germanica (Snodgrass 1943(Snodgrass , 1944) ) Odontotermes obesus (Vishnoi 1956(Vishnoi , 1962) ) Hymenopus coronatus (Wipfler et al. 2012) Stagmomantis carolina (Levereault 1938 (Marquardt 1940;friedeMann et al. 2012), andEphemeroptera (staniczeK 2001).However, in Phasmatodea, Embia and Thermobia this muscle interconnects the loral arms of the hypopharynx rather than the lateral hypopharyngeal sclerites and lacks transverse striation and innervation.In Thermobia it is described as having a large portion of tendinous fibers (chaudonneret 1950).Moulins (1971) reports a similar structure in the  (Wipfler et al. 2011) Periplaneta americana (Dorsey 1943) Blatella germanica (Snodgrass 1943(Snodgrass , 1944) ) Odontotermes obesus (Vishnoi 1956(Vishnoi , 1962) ) Hymenopus coronatus (Wipfler et  cockroach Blaberus, which connects a sclerite very close to but separate from the loral arm.This sclerite is most likely homologous to the lateral hypopharyngeal sclerite (SBT) described here in Periplaneta.Whether this distinct sclerite is only present in cockroaches but absent in other groups cannot be answered since detailed studies of the hypopharynx are missing for most lower neopteran groups.If absent in other groups, the lateral hypopharyngeal sclerite (SBT) found in Blattodea may well be a detached part of the loral arm, as indeed suggested by the attachment of M. loroloralis (see also discussion of "M14" = M. loroloralis in Buder & Klass 2013).Another intriguing question is whether or not this muscle has in all groups a high proportion of tendinous fibers or is even a collagenous structure devoid of fibers in some species.Our CTscans do not allow a more detailed description of its composition in Periplaneta americana.
One of the most interesting structures observed in the head of Periplaneta is the lacinula, the subapical lobelet(s) on the lacinia.Similar structures have been reported for various cockroaches (zhuzhiKov 2007).The only studied cockroach without a lacinula is Polyphaga aegyptica (Polyphagidae) (zhuzhiKov 2007).zhuzhiKov (2007) described various forms of this structure ranging from a "rigid pigmented process resembling the apical claw" to highly complex ones as observed in Periplaneta americana.He divided them into 4 categories which correlate to the clades Cryptocercidae (type I: a rigid process resembling the lacinial teeth), Ectobiidae (type II: 2 -3 soft processes in a row), Blattidae (type III: a soft process with distal sclerotized processes) and Blaberidae (type IV: conical bases with sclerotized apical setae).eidMann (1923) and craMPton (1923) reported structures similar to those of Cryptocercus in Orthoptera and Plecoptera.Additionally craMPton homologized them with a distinct lamella ("midappendix, or modified lacinula") on the lacinia of apterygote hexapods such as Collembola, Archaeognatha and Zygentoma.BlanKe et al. (2012BlanKe et al. ( , 2013BlanKe et al. ( , 2014) ) distinguished between sclerotized dentisetae, which are present in Odonata and Ephemeroptera, and hyaline lamellae in similar position, which are found in Zygentoma (including Tricholepidion) and most other apterygote hexapods as well as in Ephemeroptera (in the latter next to sclerotized dentisetae).They thus concluded that the presence of dentisetae is a potential autapomorphy of Palaeoptera.The presence of structurally similar elements in polyneopteran insects requires a reconsideration of this discussion.However, little and preliminary information is currently available and a homology between the dentisetae in Palaeoptera, the hyaline lamellae in apterygotes, and the lacinula in Blattodea cannot be clearly addressed at the moment.Nevertheless it is highly interesting that the lacinula in Blattodea shows a high degree of variability within the group including sclerotized and non-sclerotized forms.
Termites (Isoptera) are the morphologically most derived group within Blattodea.One major difference to the remaining Blattodea is the prognathous head in this group (vishnoi 1956, 1962;walKer 1933).Other parts of the head also differ from cockroaches, such as the labium, which is only bipartite (post-and praementum) compared to the tripartite state in cockroaches (submentum, mentum, praementum).However, as for most cockroaches, little information is available on the termite head and important species for the understanding of the cockroachtermite transition such as Mastotermes or Cryptocercus have not been studied so far.Thus no precise conclusions can be made at the moment.
In general the head is far too sporadically studied across Blattodea to derive detailed phylogenetic information from this character system.The present study shows clearly that even for specimens where information is already present, a detailed re-evaluation might be necessary.In the past, characters of the polyneopteran head provided ambivalent phylogenetic signal: the hypopharynx (Buder & Klass 2013) and the anterior head ridges (Klass & eulitz 2007) contributed little or conflicting phylogenetic signal for Dictyoptera, whereas the antennal heart (wiPfler & Pass 2014) and analysis of the entire head morphology (e.g.BlanKe et al. 2012BlanKe et al. , 2013;;wiPfler 2012;MatsuMura et al. 2015) provided valuable evolutionary and phylogenetic information for our understanding of polyneopteran interrelationships.The present contribution forms a sound basis for further investigations on the blattodean head.

Fig. 10 .
Fig. 10.Scanning electron micrographs of details of the right mandible of Periplaneta americana.A: setation at mola and proximal incisivus III on anterior side of mandible, scale bar: 20 µm; B: overview of anterior side of mandible; C: surface of postmola in anterior view, scale bar: 20 µm; D: setation on proximal part of edge of postmola in anterior view, scale bar: 100 µm; E: mola in mesal view, scale bar: 200 µm; F: overview of mesal side of mandible, scale bar: 500 µm; G: postmola in mesal view, scale bar: 200 µm.

(
SBH, Figs.17) located at the very base of the hypopharynx; the left and right posteromesally directed arms are only narrowly separated at midline, each ends at the flanks of the opening of the unpaired salivary duct (sad, Fig. 18) into the salivarium.The postero-mesally directed arms span the largely membranous posterior wall of the hypopharynx, which immediately distally of the arms forms on each side a dome-shaped invagination (see ilg of Buder & Klass 2013).The suspensorial sclerites are composed of the platelike distal part (SDP), a wide loral arm (SLA), and a slender oral arm (SOA)(Figs.17, 18).All three parts are synsclerotic in the lateral wall of the hypopharynx.The sclerotization of the plate-like distal part consists of a more strongly sclerotized distal portion and a more weakly sclerotized, somewhat domed proximal portion bearing a row of long setae.The plate-like distal part is distally synsclerotic with the lateral lingual sclerite.

Fig. 16 .
Fig. 16.Scanning electron micrographs of details of labium and hypopharynx of Periplaneta americana.A: labium in posterior view; B: labium and hypopharynx in lateral view; C: labium and hypopharynx in anterior view; scale bar for A -C: 500 µm; D: detail of ventral view of labium and hypopharynx, scale bar: 200 µm.E: ventral view of labium and hypopharynx, scale bar: 500 µm; F: tip of labial palp in ventral view, scale bar: 200 µm; G: hypopharyngeal field of microtrichia in border region between lingual and suspensorial parts in anterior view (distal is to the left), scale bar: 500 µm; H: hypopharynx in anterior view, scale bar: 1 mm; I: Setation of hypopharyngeal sclerite SDP and microtichia anteromesally of it in lateral view (distal is towards the bottom, anterior to the right), scale bar: 100 µm.-Abbreviations: fch: field of microtrichia on dorsal and distal surface of hypopharynx, gl: glossa, hph: hypopharynx, lp: labial palp, MNT: mental sclerite, pgl: paraglossa, PR: praemental sclerite, SDP: platelike distal part of hypopharyngeal suspensorium, SM: submental sclerite, SMP: arm like disto-mesal part of suspensorium.
posterior 0mx5 anterior area of anterior tentorial bridge, in between 0mx3 and 0mx4 dorsal inner stipital wall, close to stipito-cardinal ridge, in between distal parts of 0mx6 adductor of stipes and protractor of maxilla M. stipitolacinialis 0mx6 with two bundles on lateral inner wall of stipes and one bundle on postero-mesal inner wall anterior lacinial margin, together with 0mx2 adductor of lacinia, supports 0mx2 M. stipitogalealis 0mx7 dorsal inner stipital wall, directly ventrally of insertion of 0mx5 baso-lateral distogalea abductor of galea M. stipitopalpalis externus 0mx8 dorsally on lateral wall of inner stipital ridge dorsally on maxillary palpomere I abductor of maxillary palp M. stipitopalpalis medialis 0mx9 lateral wall of inner stipital ridge, ventrally of 0mx8 anteriorly on maxillary palpomere I adductor or flexor of maxillary palp M. stipitopalpalis internus 0mx10 ventral lateral wall of inner stipital ridge, ventrally of 0mx9 ventrally on maxillary palpomere I adductor of maxillary palp M. stipitalis transversalis 0mx11 outer stipital wall, close to stipitocardinal border stipital ridge a very delicate structure M. palpopalpalis maxillae primus 0mx12 basally on anterior wall of maxillary palpomere I meso-basally on maxillary palpomere II adductor of maxillary palpomere II M. palpopalpalis maxillae secundus 0mx13 basally on anterior wall of maxillary palpomere I, ventrally of 0mx12 latero-basally on maxillary palpomere III abductor of maxillary palpomere III M. palpopalpalis maxillae tertius 0mx14 basally on anterior wall of maxillary palpomere III meso-basally on maxillary palpomere IV adductor of maxillary palpomere IV M. palpopalpalis maxillae quartus 0mx15 basally on anterior wall of maxillary palpomere IV internal apodeme of maxillary palpomere 0la11 on inner praemental wall, dorso-laterally of end of praemental cleft, directly dorsally of 0la12 on baso-mesal edge of paraglossa adductor of paraglossa M. praementoglossalis 0la12 on inner praemental wall, slightly laterally of end of praemental cleft, directly ventrally of 0la11 on baso-mesal edge of glossa adductor of glossa M. praementopalpalis internus 0la13 inner wall of praemental cleft, near end of cleft ventral basal margin of labial palpomere I adductor of labial palpomere I M. praementopalpalis externus 0la14 antero-lateral part of base of praementum dorsal basal margin of labial palpomere I adductor of labial palpomere I M. praementomembranus 0la15 absent M. palpopalpalis labii primus 0la16 antero-lateral part of base of praementum, directly ventrally of 0la14 lateral basal margin of labial palpomere II adductor of labial palpomere II M. palpopalpalis labii secundus 0la17 meso-basal wall of labial palpomere II in 2 bundles, the one mesally and the other laterally on basal edge of labial palpomere middle between antennal base and anterior mandibular articulation laterally on oral arm of hypopharynx, slightly proximally of insertion prementum, mesally of articulation of labial palpus proximally on ventral side of basal hypopharyngeal apodeme protractor of opening of salivary duct, antagonist of 0hy3 M. prementosalivarialis posterior 0hy8 antero-lateral wall of base of prementum, slightly mesally of 0la14 distally on ventral side of basal hypopharyngeal apodeme dilator of opening of salivary duct M. oralis transversalis 0hy9 oral arm oral arm of opposite side, fibres running on both sides of pharynx contractor of anatomical mouth opening, antagonist of 0hy2 and 0bu1 M. loroloralis 0hy10 mesal wall of lateral ("basitendinal") hypopharyngeal sclerite mesal wall of lateral hypopharyngeal sclerite of opposite middle between antennal base and anterior mandibular articulation, directly dorsally of 0hy2 lateral pharyngeal wall, directly dorsally of insertion of 0hy2 dilator of pharynx, antagonist of 0st1 M. verticopharyngealis 0ph1 in two bundles on vertex, in between bundles of 0md1 two bundles on anterior surface of pharynx, first bundle on same level as insertion of 0bu4, directly dorsally of brain, second bundle further dorsally dilator of pharynx, antagonist of 0st1 M. tentoriopharyngealis 0ph2 ventro-anterior margin of posterior tentorial arms and corpotentorium posterior wall of pharynx, same level of insertion of 0ph1 dilator of pharynx, antagonist of 0st1 M. postoccipitopharyngealis 0ph3 on postoccipital ridge on lateral side of occipital foramen broadly along lateral pharyngeal wall, from the insertion of the dorsal bundle of 0ph1 to the point where the pharynx leaves the head capsule dilator of pharynx, antagonist of 0st1 M. annularis stomodaei 0st1 ring muscle layer around pharynx, ends where the pharynx leaves the head capsule constrictor of pharynx, antagonist to pharyngeal dilators M. longitudinalis stomodaei 0st2 longitudinal muscles along pharynx, ends where the pharynx leaves the head capsule contractor of pharynx gave a detailed description of the head of the termite Odontotermes obe sus.walKer (1933) studied the exterior head capsule of Termopsis angusticollis.Additionally hudson (1945), Klass & eulitz (2007), Buder & Klass (2013), and zhu zhiKov (2007) provided information about the anterior head ridges, tentorium, hypopharynx, and mouthparts of various blattodean species.

Table 2 .
Presumed homologies of the cephalic muscles of Periplaneta americana with muscles reported in other studies.-Abbreviations: +: muscle present, -: muscle absent, /: muscle not studied, ?: unclear homology, *: apparently present in some individuals while absent in others (see Discussion).