The larval head of Agathiphaga (Lepidoptera, Agathiphagidae) and the lepidopteran ground plan

. The larval head of Agathiphaga vitiensis is described. There is a complete hypostomal bridge but no hypostomal ridges. Adfrontal ridges and distinct ecdysial lines are absent. There are two vestigial stemmata (without lenses) on each side. The antenna is one‐segmented. All ‘typical lepidopteran’ head setae have been identified. The corporotentorium is very slender; dorsal tentorial arms are present. Intrinsic labral muscles are lacking. The mandible has retained a tentorial muscle. The maxilla is without a discrete cardo and has but a single endite lobe; ‘intrinsic maxillary muscles’ and the ‘cranial flexor of the dististipes’ are lacking. The postlabium is undivided and without setae, the labial palp is one‐segmented and the lateral prelabio‐hypopharyngeal sclerotization is continued into an oral arm. Some of the ventral pharyngeal dilators arise on the tentorium; mouth‐angle retractors and dorsal post‐cerebral pharynx dilators are absent. The two brain lobes have almost parallel long axes and are united by a narrow (almost pure neuropile) bridge. The corpora cardiaca and callata are contiguous. The aorta is an open gutter in front of the retrocerebral complex. Available evidence on the ground plan structure of the lepidopteran larval head is reviewed. The ancestral head supposedly was prognathous and was autapomorphic in having the cranio‐cardinal articulation far behind the mandible; it had a complete hypostomal bridge but neither hypostomal nor adfrontal ridges, its tentorium was probably stout and with dorsal arms. Paulus & Schmid (1978, Z. zool. Syst. EvolForsch. 16) described a lepidopteran/trichopteran synapomorphy in stemma structure. A tentative table of homologies between cranial setae in Lepidoptera and Trichoptera is presented; it differs considerably from the scheme of Williams & Wiggins (1981, Proc. 3rd Symp. Trichopt.). The mouth parts and their musculature must have been overall very primitive for a panorpid larva, but the number of maxillary palp segments was reduced (three). The ‘dististipes’ sensu Hinton is considered to consist of complexly fused parts of the stipes and basal palp segments. The cephalic stomodaeum must have possessed all primitive groups of extrinsic muscles. The incomplete available information on Micropterigidae impedes reconstruction of some details of the lepidopteran ground plan. Larval head structures support the monophyly of an entity comprising the Agathiphagidae + Heterobathmiidae + Glossata. There is one suite of derived characters shared by Heterobathmiidae and Agathiphagidae only and another shared by Heterobathmiidae and the Glossata only; one of these must represent parallelisms.