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 and 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 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 it is partly or largely retracted, and the sclerotized elements of the external head capsule are partly or fully reduced. The larval 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.

Patterns of morphological simplification and innovation in the megadiverse Holometabola (Insecta)

“…The adult head of true flies is mainly characterized by piercing sucking stylets, likely formed by the labrum, mandibles, maxillae and hypopharynx in the groundplan (Hennig, 1973;Schnee-berg and Beutel, 2014). However, the paired mouthparts are completely or largely reduced in the species-rich nematoceran Tipulidae, in Nymphomyiidae and Deuterophlebiidae, groups presumably very close to the root of the order, and in the vast majority of families of the megadiverse Brachycera (Schneeberg and Beutel, 2014). The thorax is mainly characterized by anatomical dipterism, with hind wings transformed into specialized gyroscopic sense organs (halteres) and a metathorax distinctly reduced in size and simplified.…”

“…The complete reduction of paired stylets in the majority of brachyceran groups is accompanied by the formation of complex premental labellae equipped with pseudotracheae, suitable for the uptake of liquids from surfaces (e.g. Schneeberg and Beutel, 2014). Similarly, the meso-(Strepsiptera) or metathoracic (Diptera) halteres can be seen as partially reduced wings not contributing propelling forces during flight, but also as specific and complex gyroscopic sense organs and structures stabilizing flight with their rotating movements.…”

“…A comprehensive work on beetle morphology and phylogeny was published by Lawrence et al (2011) and new Handbook of Zoology volumes on this megadiverse order and on Nannomecoptera and Neomecoptera are available (Beutel and Leschen, 2016;Leschen and Beutel, 2014;. Studies on specific groups and body regions include, for instance, Schneeberg et al (2011Schneeberg et al ( , 2012Schneeberg et al ( , 2013Schneeberg et al ( ), H€ unefeld et al (2012, Schneeberg and Beutel (2014), Liu et al (2017) and Richter et al (2019Richter et al ( , 2020.…”

Detailed morphological descriptions of the immature stages of the ant parasite Microdon mutabilis (Diptera: Syrphidae: Microdontinae) and a discussion of its functional morphology, behaviour and host specificity

“…Many fl ies, especially those in brachyceran groups with hook-like mandibles, move their mouthparts vertically and obliquely to hook on to the substrate and establish an anchor point on which they slide the rest of the body (Berrigan & Pepin, 1995, Schneeberg & Beutel, 2015, whereas, locomotion in M. mutabilis is performed without using the head skeleton. The highly developed ventral musculature in M. mutabilis, coupled with the enlarged body with a fl at "foot", is suffi cient to allow an autonomous movement without the need of mandibles, in way analogous to slugs with which they were initially confused (Reemer, 2012).…”