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External and internal head structures of Osmylus fulvicephalus were examined and described in detail. Exoand endoskeleton, musculature, elements of the central nervous system and tracheae are compared to conditions found in other groups of Neuropterida and other endopterygote lineages. Thirty-six adult cephalic characters were compiled, combined in a datamatrix with 64 characters of the larval head, and analysed cladistically. Mainly because many data on adults remain missing, most branches in the cladogram are mostly or exclusively supported by larval features. The shortening of the mesal mandibular wall and the resulting anterior shift of the adductor tendon possibly constitute an adult groundplan apomorphy of Neuropterida. Raphidioptera and Megaloptera share distinct prognathism and the presence of a sclerotised gula. However, the orthognathous head and the absence of a gula resulted as autapomorphies of Neuroptera in our analyses. Further potential autapomorphies are the asymmetry of the mandibles as well as the respective presence of dorsolateral furrows on the head capsule, of a shovel-like extension on the ventral mandibular cutting edge, and of a row of stiff hairs on the mandible's ventral surface. The systematic affinities of Osmylidae remain ambiguous. Osmylus is mainly characterised by plesiomorphic features of the adult head such as a complete endoskeleton, long filiform antennae, largely unmodified orthopteroid mouthparts, and particularly the nearly complete set of muscles. The placement with a clade also comprising Hemerobiidae and Chrysopidae is poorly supported. The presence of a dense vestiture of long microtrichia on the distal galeomere resulted as a synapomorphy of the three families. An apparent plesiomorphy preserved in Osmylus but absent in all other groups of Neuroptera is the presence of well developed ocelli. The present study underlines the severe shortage of detailed morphological data on the adults. Intensive study of adult structures is required for a solid reconstruction of the phylogeny of Neuropterida, especially of the hemerobiform lineage of Neuroptera.

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