Adult head structures of representatives of all major trichopteran lineages were examined by using a combination of well-established morphological techniques as histology, light microscopy, scanning electron microscopy, and micro-computed tomography in combination with computer-based 3D-reconstruction. Internal and external cephalic features of the annulipalpian representative, Philopotamus ludificatus McLachlan, 1878, are described and illustrated in detail. The sclerites of maxilla and labium are often fused and inseparable leading to different homology hypotheses in the literature, concerning for instance the identity of the maxillary endite lobes (galea or lacinia), the composition of the basal maxillary sclerites and the origin of the haustellum. The detailed description of the skeleto-muscular system of Philopotamus allows for a thorough evaluation of these structures and a critical discussion of their identity. Furthermore, character complexes showing the most relevant variability among the major trichopteran lineages are presented. The ground plan conditions of the adult head in Trichoptera and Amphiesmenoptera are reconstructed for several characters. The trichopteran ground plan contains a Π-shaped tentorium with short dorsal arms, moderately sized mandibles equipped with three well-developed muscles, a small but distinct lacinia, five-segmented maxillary palps and a small haustellum. Additionally, the unusual configuration of the extrinsic dilator muscles of the salivarium is regarded as a potential autapomorphy of Trichoptera. Adult head structures observed in the annulipalpian family Philopotamidae show a remarkable number of presumably plesiomorphic features, as moderately developed and sclerotized but functionless mandibles, a small protrusible haustellum without channels on its surface, extrinsic antennal muscles originating exclusively from the tentorium, a small lacinia closely associated with the mainly membranous galea (i.e., galeolacinia), extrinsic dorsal muscles of the salivarium originating fromthe hypopharynx and from the premental sclerite, respectively, and small labial endite lobes.

Head morphology, skeleton, musculature, nervous system, Amphiesmenoptera, Integripalpia, Spicipalpia, Rhyacophilidae, Phryganeidae.