Culicoidea (Insecta: Diptera)

The Culicoidea is a superfamily of nematoceran flies (see crudely-modified phylogeny above, from Grimaldi & Engel, 2005). It includes one the most well-known group of organisms, the mosquitoes (Culicidae), as well as three other families: the Dixidae, the Corethrellidae and Chaoboridae. Their sister group is the Chironomidea (containing the Chironomidae, Ceratopogonidae, Simuliidae and Thaumaleidae), all together forming the Culicomorpha (Sæther, 2000); this grouping is supported by many characters, mostly from the specialised-for-filter-feeding larval mouthparts, but also from sperm morphology (they share 15 protofilaments in the accessory tubules, as opposed to 13 in other flies) and male genitals (they have no sclerotised aedeagus [penis] as in other flies).

It’s worth noting that in some older molecular analyses, this two superfamily system isn’t recovered (e.g. Miller et al., 1997). These are flawed due to insufficient taxon sampling.

The culicoids are best recognisable as larvae: in contrast to the larvae of other nematocerans with their very short antennae, culicoid larvae have long antennae.

The earliest fossil record of the Culicoidea stems from finds of dixids and chaoborids in the basalmost Jurassic of Germany and Krygyzstan (Lukashevich, 1996), with numerous fossils scattered around the Jurassic of Siberia. Since the earliest chironomids are found in the Triassic (Krzemiński & Jarzembowski, 1999), it’s expected that the stem of the culicoids remains to be found in Triassic rocks, as suggested even by Hennig (1981).

The earliest representatives of the other 2 families are known from the Cretaceous: Corethrellidae from Lebanese amber (Szadziewski, 1995) and Culicidae from Canadian amber (Poinar et al., 2000) – their lack of earlier representation is likely a bias in the fossil record, as the earlier record of insects comes mostly from lake sediments, where predation by fish would have destroyed any possibility of larval preservation.

The Dixidae (drawing above from McAlpine et al., 1981) is the basalmost family, with 175 species in 7 genera. Their larvae are unique among the culicoids, being the only non-aquatic ones – they feed only on matter that is found on the surface of the water. Their adults don’t feed at all.

This stands in contrast with all the other families, whose larvae are either at the top layers of the water (corethrellids, many culicids) or planktonic and bottom feeders (many culicids, chaoborids). However, even those last ones need to swim to the surface to breathe. The exception is are mosquito larvae of the genera Coquilletidia and Mansonia, who have evolved telescopic siphons that allow them to suck oxygen from vegetation.

Among the bottom and planktonic feeders, there are critical differences in larval lifestyle. Chaoborid larvae are active predators of water fleas and other aquatic larvae; this is derived and is characteristic for the chaoborids. Culicid larvae are herbivorous or filter-feeding, with protein only needed by the adult (hence why they drink blood).

It has been proposed that the shift to planktonism and bottom-feeding is a critical one, leading to the chaoborids and culicids being set apart from the other two families as a monophyletic group, with the culicids arising from a chaoborid stem (Borkent & Grimaldi, 2004). This clade is well-supported, including by wing venation (Colless, 1979).

The Chaoboridae (drawing above from McAlpine et al., 1981) contains ~50 species in 3 subfamilies and 6 genera (Borkent, 1993), and they are hard to tell apart from mosquitoes at all life history stages. They can be up to 10 mm large as adults, with somewhat shorter wings than mosquitoes (their wings don’t reach the end of the abdomen).

The Culicidae (drawing above from McAlpine et al., 1981) is the most diverse of the culicoid families, containing over 3400 species in over 30 genera (Harbach & Kitching, 1998); refer to Harbach (2007) for taxonomical information. They range between 3 and 9 mm, and most adults have adapted to feeding on vertebrate blood, hence their characteristic proboscis-forming mouthparts. Only females sting (and thus are a typical health risk as vectors, most famously of malaria and yellow fever); males feed by sucking up nectar from flowers (and hence also act as small-time pollinators).

In the female mosquito, the piercing part (called the fascicle) is formed by the labium, hypopharynx, a pair of mandibles and of maxillae. The latter two are what pierce the skin for blood sucking, the labium forms a sheath and the hypopharynx pierces the skin as well, but is immobile and just serves for saliva transfer from the mosquito to the host (there are no muscle attachments, and it’s linked to the salivary canal).

In the males, the hypopharynx and the labium are fused together (forming the creatively-named labio-hypopharynx). This means that it cannot pierce at all, but that the sheath formed by the labium is linked to the salivary canal. The saliva thus drips on the food (nectar) or covers it while it’s getting sucked up, starting the digestion a bit earlier, and also providing some protection against any bacteria swimming in the nectar.

The sister group to the Culicidae+Chaoboridae clade is the Corethrellidae (Wood & Borkent, 1989; drawing above from Borkent, 2008), a monogeneric family with 62 species (Borkent, 1993). This relationship is confirmed by the male genitals, which are secondarily simplified. The plesiomorphic condition is for a two-chambered spermatophore to be produced by specialised glands to transport sperm (Sinclair et al., 2007); Corethrellidae, Culicidae and Chaoboridae do not produce these, instead ejaculating their sperm straight into the female.


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Research Blogging necessities :)

Saether, O. (2000). Phylogeny of Culicomorpha (Diptera) Systematic Entomology, 25 (2), 223-234 DOI: 10.1046/j.1365-3113.2000.00101.x

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