The Asteiids: Overlooked Flies

Flies are incredibly diverse, but they may be one of the least appreciated of the major insect groups. There are many significant fly lineages whose presence goes all but unnoticed by a small number of afficionados.

Asteia amoena, copyright Mick E. Talbot.

One of the largest clades of flies is the Schizophora, including many such familiar animals as house flies, blowflies, and fruit flies (of both varieties). The most distinctive feature marking this lineage is the ptilinal fold, a groove that runs around the face of the flies along the inner margin of the eyes and across above the antennal insertions. This groove marks the position of a large fold of soft cuticle, the ptilinum, that is used by the fly when it emerges from the hardened case, the puparium, in which it metamorphoses from a larva. The ptilinum expands like a bellows as the fly pumps its head full of liquid until the pressure causes the cap of the puparium to pop open. After this, the excess cuticle is folded away inside the head, never to emerge again, but the mark of its presence remains.

House fly Musca domestica emerging from its puparium, showing the inflated ptilinum. Copyright Alex Wild.

Schizophorans have commonly been divided between two main groups, referred to as the calyptrates and acalyptrates. The basis of this division is the presence (calyptrates) or absence (acalyptrates) of the calypters, lobes at the base of the wings that help in controlling flight. This is not an entirely phylogenetic system: the calyptrates are a single clade but the acalyptrates are not. The most familiar flies belong to the calyptrates (which include house flies and blowflies) despite the fact that acalyptrate flies are considerably more diverse. This is in part because many acalyptrates are very small flies, easily overlooked by the casual observer.

The Asteiidae are one such group of overlooked flies. They are found pretty much world-wide and can be very abundant in some habitats. Nevertheless, they are apparently not common in collections: their soft-bodiedness makes them tricky to preserve, and Grimaldi (2009) noted that tropical species living on rolled leaves of herbs such as bananas and gingers were reluctant fliers and so unlikely to be collected by passive intercept traps. Noteworthy features of asteiids compared to similar flies include a reduced wing venation, and an antennal arista bearing alternating rays (Friedberg 2009). In two genera, Polyarista and Anarista, the arista is reduced or lacking, replaced by a collection of long setae arising from the first flagellomere (Papp 2013).

Diagnostic features of Asteia amoena, from Walker's Insecta Britannica Diptera.

Because of their low collection rates, the natural history of asteiids is poorly known. As already noted, a number of species are found in association with vegetation; others have been raised from fungi. Grimaldi (2009) described Asteia species running "over the surface of a leaf in all directions with uniform effort, including backwards and sideways, which gives them an appearance of floating over the surface". Some species have mating rituals involving trophallaxis, in which a male attempts to entice a female by offering her a regurgitated droplet. If his offering meets her standards, they will collaborate to produce a new generation that will carry on in the same obscurity as the last.

REFERENCES

Freidberg, A. 2009. Asteiidae (asteiid flies). In: Brown, B. V., A. Borkent, J. M. Cumming, D. M. Wood, N. E. Woodley & M. A. Zumbado. Manual of Central American Diptera pp. 1093–1096. NRC Research Press: Ottawa.

Grimaldi, D. A. 2009. The Asteioinea of Fiji (Insecta: Diptera: Periscelididae, Asteiidae, Xenasteiidae). American Museum Novitates 3671: 59 pp.

Papp, L. 2013. A new genus of Asteiidae with a key to the Old World genera (Diptera). Annales Historico-Naturales Musei Nationalis Hungarici 105: 199–205.