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

Abstract

We analyzed patterns of complexity and simplicity in holometabolan insects using parsimony and maximum-likelihood. By contrast with other groups of arthropods (and most other groups of animals), insects have undergone a stepwise process of structural simplification in their evolution. The megadiverse Holometabola are characterized mainly by structurally simplified larvae, which differ strongly from the adults in their morphology and usually also in their life habits. Although smaller groups such as Neuropterida have largely maintained their structural complexity in adults and immature life stages, a series of reductions occurred with the appearance and diversification of Coleopterida, Mecopterida and especially Antliophora. Parasitic Strepsiptera or fleas display conspicuous patterns of reduction in different life stages and body regions, and high degrees of simplification also occur in groups with short-lived adults. Larvae living in moist substrates display far-reaching structural simplifications and also morphological uniformity, especially in the species-rich Diptera, but also in other groups. Liquid feeding leads to correlated simplifications and innovation of adult head structures, especially of the mouthparts. Functional or anatomical dipterism leads to an optimization of the flight apparatus in most holometabolous groups, which is correlated with reductions in one of the pterothoracic segments, and coupled (e.g. by hamuli), partly reduced or transformed wings (e.g. halteres). In flightless groups, the pterothoracic skeleto-muscular apparatus is strongly simplified. In the abdomen of adult females a stepwise reduction of the lepismatoid ovipositor occurs. By contrast, the male genital apparatus often undergoes an extreme diversification. Our evaluations revealed a highly correlated complexity between larval and adult stages.

Cephalic anatomy highlights morphological adaptation to underground habitats in a minute lacewing larva of Dilar (Dilaridae) and conflicting phylogenetic signal in Neuroptera

“…Evolution in the larval stages was apparently accelerated in Neuroptera, whereas the adults are mostly characterized by plesiomorphic features compared with other groups of Holometabola (Beutel et al, 2010a(Beutel et al, , b, 2011(Beutel et al, , 2022. This underlines the importance of immatures stages for understanding adaptations and major evolutionary shifts in Neuroptera.…”

The morphology of the free‐living females of Strepsiptera (Insecta)

“…The results of this study show that the whole body of female Mengenillidae is highly modified and simplified as already shown for the female abdomen in Tröger et al (2019). The degree of simplification is higher compared to adults of other insect orders and even compared to other groups within Coleopterida, in which structural simplification is a general pattern (Beutel et al, 2021). This correlates with their cryptic and more or less stationary lifestyle.…”

Xenos vesparum(Strepsiptera: Xenidae)—A New Insect Model and Its Endoparasitic Secondary Larva

“…The strepsipteran secondary larval stages belong to the most strongly simplified larvae of Holometabola (Beutel et al 2022). Conspicuous features distinguishing secondary larvae of Strepsiptera from prepupal stages of other holometabolous orders are the presence of compound eyes and wings in males, and the non-functional (males) or missing legs (Tröger et al 2020, present study).…”