The evolution of head structures in lower Diptera

Abstract

The head of adult dipterans is mainly characterized by modifications and more or less far reaching reductions of the mouthparts (e.g., mandibles, maxillae), linked with the specialization on liquid food and the reduced necessity to process substrates mechanically. In contrast, the compound eyes and the antennae, sense organs used for orientation and for finding a suitable mating partner and oviposition site, are well developed. Some evolutionary novelties are specific adaptations to feeding on less liquefied substrates, such as labellae with furrows or pseudotracheae on their surface, and the strongly developed pre- and postcerebral pumping apparatuses. In some dipteran groups specialized on blood the mandibles are still present as piercing stylets. They are completely reduced in the vast majority of families. Within the group far-reaching modifications of the antennae take place, with a strongly reduced number of segments and a specific configuration in Brachycera. The feeding habits and mouthparts of dipteran larvae are much more diverse than in the adults. The larval head is prognathous and fully exposed in the dipteran groundplan and most groups of lower Diptera. In Tipuloidea and Brachycera the head is partly or largely retracted and the sclerotized elements of the external head capsule are partly or fully reduced. The head of Cyclorrhapha is largely reduced. A complex and unique feature of this group is the cephaloskeleton. The movability of the larvae is limited due to the lack of thoracic legs. This can be partly compensated by the mouthparts, which are involved in locomotion in different groups. The mouth hooks associated with the cyclorrhaphan cephaloskeleton provide anchorage in the substrate.