Snakes and Lace

The holometabolous insects—that is, the clade containing most insects with a complex life cycle including differentiated larval and pupal stages—is one of the most extensive radiations of animals on this planet. Much of this diversity is assigned to four major orders: wasps, moths, beetles and flies. But there are also a number of smaller lineages making up the holometabolous insects. Among these are the lacewings and their relatives in the clade Neuropterida.

Female snakefly Puncha ratzeburgi, copyright Hectonichus.

Modern members of the Neuropterida are generally recognised as belonging to three orders—the lacewings and ant-lions in the Neuroptera, the snakeflies in the Raphidioptera, and the alderflies and dobsonflies in the Megaloptera—though go back a few decades and you may find texts referring to a single order Neuroptera. A number of authors have advocated for use of the name 'Planipennia' for the lacewing order to avoid confusion with the broader sense of Neuroptera but, while a case could certainly be made for this usage, it's just never really caught on. Most neuropteridans are fairly similar in overall appearance: long-bodied insects with well developed wings with numerous crossveins. Of the living holometabolous insects, they probably bear the greatest overall resemblance to the clade's ancestors and hence they are commonly thought of as 'relicts'. However, they do possess their own specialisations and are not primitive in every regard (for instance, the most primitive egg-laying apparatus among holometabolous insects belong to wasps). Species of Neuropterida are mostly predators as larvae. The larvae of the lacewing family Ithonidae may possibly feed on decaying plant matter though we don't know for certain (Grimaldi & Engel 2005). Adults are predators and/or pollen-feeders, or may not feed at all in some short-lived forms.

Male (above) and female dobsonflies Corydalus cornutus, copyright Didier Descouens.

The exact relationships between the neuropteridan orders have been debated over the years. Though most of their obvious similaities to each other represent shared ancestral features, there is a broad consensus that they do indeed form a clade. There has also been little, if any, question of the monophyly of the Raphidioptera and Neuroptera; the monophyly of Megaloptera has been more debated but seems more likely than not. Most recent studies have suggested that the Raphidioptera are the sister group to a clade of the other two orders (Engel et al. 2018). Raphidioptera are the least diverse of the generally recognised living orders of insects with about 250 known species. They are found in cooler regions of the Northern Hemisphere—in the temperate zone or at higher elevations in lower latitudes—and are completely absent from the Southern Hemisphere (Aspöck & Aspöck, 1991, refer to a failed attempt to introduce them to Australia and New Zealand but provide no details why such a thing was tried in the first place). They are characterised by a notably elongate prothorax (the first segment of the thorax) which explains the vernacular name of 'snakefly'. Larvae live under bark or in litter and moult into pupae with the onset of cold weather. The pupae of Raphidioptera and Megaloptera are primitive in aspect, with legs separate from the body wall, and are highly mobile. Engel et al. (2018) even refer to the pupae of Raphidioptera as 'active predators' but I've not been able to find corroborating details for that remarkable description.

The Megaloptera are often particularly large neuropteridans, reaching up to twenty centimetres in wingspan, and comprise a bit less than 400 species worldwide, mostly found in temperate regions. Larvae are aquatic, living under rocks and debris, and characterised by the presence of filamentous lateral gills on the abdomen. Adults are short-lived and feed little if at all. Male dobsonflies (of the subfamily Corydalinae) possess spectacularly large, curved mandibles of largely unknown purpose; certainly they do not seem to use them for biting.

Mantisfly Mantispa styriaca, a raptorial lacewing, copyright Gilles San Martin.

The largest of the three orders, by a considerable margin, is the Neuroptera with over 5700 known species. Needless to say, this level of species diversity is associated with a high diversity of appearances and lifestyles, too many to cover adequately here. The larvae of two families of Neuroptera, the Nevrorthidae and Sisyridae, are aquatic and there has been a long-running debate whether this aquatic habit is an ancestral feature of the order shared with the Megaloptera (Nevrorthidae larvae are generalist predators, Sisyridae are specialised feeders on freshwater sponges and bryozoans). However, recent phylogenetic studies (e.g. Vasilikopoulos et al. 2020) do not agree with earlier hypotheses that the Nevrorthidae represent the sister taxon of the remaining Neuroptera. Instead, Nevrorthidae and Sisyridae may form a clade with the Osmylidae, a family whose larvae are not aquatic but often inhabit damp stream banks. The aquatic Neuroptera probably entered the water independently of the alderflies. The current favourites for the sister clade of other neuropterans are the dustywings of the Coniopterygidae, a group of small neuropterans with reduced wing venation that have historically been difficult to place owing to their derived features.

An unidentified dustywing, Coniopterygidae, copyright Katja Schulz.

A fourth order has often been associated with the Neuropterida, the extinct Glosselytrodea. Glosselytrodeans are small insects known from the Late Permian to the Jurassic, characterised by wings bearing dense cross-veins of which the fore pair would have had a leathery appearance in life (not dissimilar in texture to the fore wings of grasshoppers and other Orthoptera). Other than the wings, the features of glosselytrodeans are poorly known: they seem to have been hypognathous (i.e. had the head directed downwards) with slender legs (Grimaldi & Engel 2005). Connections to Neuropterida are based on features of the wing venation but cannot be considered strongly supported. Other authors have regarded them as of uncertain position within the broader holometabolous clade, or even as more closely related to the Orthoptera than any Holometabola. Unless more complete remains should come to light, it seems likely that the question will remain open.

REFERENCES

Aspöck, H., & U. Aspöck. 1991. Raphidioptera (snake-flies, camelneck-flies). In: CSIRO. The Insects of Australia: A textbook for students and research workers 2^nd ed. vol. 1 pp. 521–524. Melbourne University Press.

Engel, M. S., S. L. Winterton & L. C. V. Breitkreuz. 2018. Phylogeny and evolution of Neuropterida: where have wings of lace taken us? Annual Review of Entomology 63: 531–551.

Grimaldi, D., & M. S. Engel. 2005. Evolution of the Insects. Cambridge University Press: New York.

Vasilikopoulos, A., B. Misof, K. Meusemann, D. Lieberz, T. Flouri, R. G. Beutel, O. Niehuis, T. Wappler, J. Rust, R. S. Peters, A. Donath, L. Podsiadlowski, C. Mayer, D. Bartel, A. Böhm, S. Liu, P. Kapli, C. Greve, J. E. Jepson, X. Liu, X. Zhou, H. Aspöck & U. Aspöck. 2020. An integrative phylogenomic approach to elucidate the evolutionary history and divergence times of Neuropterida (Insecta: Holometabola). BMC Evolutionary Biology 20: 64.