Winter Gnats

Winter Gnats

Fig 1 with the help of Andrew Mortley

These flies (Trichocera), look rather like chironomid midges (Fig. 1), and, also like chironomids (McLachlan & Neems, 1995), form mating swarms to acquire a mate. Chironomids are well known because of dominance of the larval stages in populations of many, perhaps most, lakes and streams (Armitage, Cranston, & Pinder, 1995). But does anyone know anything about winter gnats? They are not mentioned in Gullan and Cranston (Gullan & Cranston, 1994) but do appear in Imms (Imms, 1957), with a brief description, on p. 610. To anyone used to chironomids the proportions of wing to body are all wrong. Evidently the morphology is like that of the Tipulidae, to which they are related. What interests me about these flies is their ability to swarm on very cold December days. Indeed this is the only time mating swarms are seen. There are clear adaptive advantages to flying under these conditions because of the absence of insect predators such as bibionids and empids. Does their morphology, in contrast to the chironomids, provide an explanation? I want to suggest that the relatively large wings act as solar panels, picking up radiant heat even at very low air temperatures. Indeed the possibility of insect wings functioning as solar panels has been proposed by Reynolds (2020,) p159, as an intermediate step in the evolution  of insect flight. 

Turning to the adaptive aspects following from the possibility of wings as solar panels, some conjecture is possible. For example, the Trichoceridae share their distinctive morphology with another major family, the Tipulidae or crane flies. Both families have exceptionally long legs and exceptionally large wings (Imms, 1957), pp. 608-610). As far as I know, of these two families, it is only the trihocerids that have a swarm based mating system (McLachlan & Neems, 1995). It is within this type of mating system that the large wings may have a fitness advantage permitting conspicuous mating swam to function in the virtual absence of invertebrate predators. We need to know something of the phylogeny of these two families but it is conceivable that they share a common ancestor. In which case large wings came first and can be regarded as a pre-adaptation (Dawkins, 1996), p.95, to mating in predator free conditions.

Fig. 1. Composite drawing to illustrate the relative wing to body size of a male chironomid midge (a), and a winter gnat (b). Legs not shown.

Fig. 2. A sketch of the tether with a fly in position – a, stand; b, wire; c, drop of ‘typex’; d, midge or gnat. The attachment ‘c’ must leave the wings free of interference.   Details are from (McLachlan, 1983), p550.

An hypothesis that wings act as solar panels is, as far as I can ascertain, a novel one. Furthermore it is testable. Here is the design of a simple experiment.

Treatment: Fly winter gnats in tethered flight in a controlled temperature cabinet at various low temperatures.  A lamp provides solar radiation.

Control:  Fly chironomids under the same conditions. Choose species of approximately the same body size as the gnats in the treatment.

Treatment control: A treatment control is required with the light as a source of heat

 radiation removed.

Unknowns: Will gnats fly on a tether? I have not attempted this. I know chironomids perform well under these conditions (McLachlan, 1983).

Field work: Record air temperatures and solar radiation in mating swarms in the wild for both gnats and midges. Results should inform the treatment temperatures.

If this works it will be a neat little experiment but it may not be as straight forward as it seems. There could be other, perhaps better experiments to test the same hypothesis. 

references

 Armitage, P., Cranston, P. S., & Pinder, L. C. V. (1995). The Chironomidae. The biology and ecology of non-biting midges. . London: Chapman & Hall.

Gullan, P. J., & Cranston, P. S. (1994). The Insects: An Outline of Entomology. London: Chapman & Hall.

Imms, A. D. (1957). A General Textbook of Entomology. (9th ed.). London: Methuen.

McLachlan, A. J. (1983). Life-history tactics of rain-pool dwellers. Journal of Animal Ecology, 52, 545-561.

McLachlan, A. J., & Neems, R. M. (1995). Swarm based mating systems. In S. R. Leather & J. Hardie (Eds.), Insect Reproduction. New York: CRC Press.

Dawkins, R. (1996). Climbing Mount Improbable. London: W. W. Norton.

Imms, A. D. (1957). A General Textbook of Entomology. (9th ed.). London: Methuen.

McLachlan, A. J., & Neems, R. M. (1995). Swarm based mating systems. In S. R. Leather & J. Hardie (Eds.), Insect Reproduction. New York: CRC Press.

Renolds, S. (2020). Success! When, Why and How insects got their wings. Bull. Ent. Soc. Antenna, 44, 155-160.