Cephalic anatomy of Zorotypus hubbardi (Hexapoda: Zoraptera): new evidence for a relationship with Acercaria

Abstract

External and internal head structures of Zorotypus hubbardi were examined. A detailed description is provided for the wingless morphs. The results are compared to the conditions found in alate specimens and in representatives of potentially related groups of insects. The pigmentation is strongly reduced in the wingless morphs and well-developed compound eyes, optic lobes and ocelli are absent. Kidney-shaped white spots between the paired ocelli and compound eyes were absent in all specimens examined. Most characters of the head (orthognathism, tentorium, musculature, mouthparts) are plesiomorphic. The wing and eye dimorphism, the strongly reduced head sutures, the reduced number of antennomeres, the presence of a prostheca on the left mandible and the presence of a transverse muscle of the head capsule and a retractor of the salivary sclerite are potential autapomorphies of Zoraptera. The unusual shape and large relative size of the brain and suboesophageal complex are also probably autapomorphic and a result of miniaturisation. Possible affinities with Dictyoptera were not supported by the results of this study. The only potential apomorphic feature of the head shared with Embioptera is the presence of a clearly delimited anteclypeus. However, this condition is also present in several groups of Acercaria. The very strongly developed M. clypeobuccalis, the medially divided prementum, and the slender, apically bifid laciniae without any mesally directed setae or spines are potential autapomorphies of Paraneoptera (incl. Zoraptera). If this group is indeed monophyletic, it is plausible to assume that the stem species was small like zorapterans and psocopterans and feeding on hyphae and spores of fungi, with mandibles with grinding molae and chisel-like laciniae suitable for loosening the food substrate. The feeding apparatus was then further modified within Acercaria, with a mortar-and-pistill apparatus in Psocoptera, and piercing–sucking mouthparts in Anoplura, Thysanoptera and Hemiptera.