Biting Midges

I want to make two points about the mating system of biting midges (Culicoides), as subjects for comparative work with non-biting midges (Chironomidae). First, the biting midges are very small flies which introduce interesting biomechanical questions about flight in the mating arena. Second, females of biting midges are readily collected from swarms. Chironomid females do not aggregate like this and are therefore difficult to collect. These differences provides the opportunity to resolve some long standing questions about the mating system of these flies. Here I adopt the ubiquitous Chironomus plumosus as test species for the non-biters and the dreaded Scottish midge, Culicoides impunctatus, for the biters.   

Biomechanical aspect of flight

Chironomus plumosus has a wing length of c. 4.4mm in the female while for Culicoides impunctatus it is c.1.4mm, i.e. about three times smaller than Chironomus. Culicoides is a very small insect indeed, barely visible to the naked eye, hence the name ‘no-see-ums’ in parts of the States. Whereas the size of Chironomus appears to place individuals on the edge of the inertial and viscous universes when in flight (Crompton, Thomason et al. 2003), size would be predicted to place flying Culicoides individuals firmly in a viscous universe (Vogel 1994). Thus comparative work based on size introduces some interesting proximate questions. For example, when in flight Culicoides may encounter air somewhere near the viscosity of treacle.  In attempting a pairing in a mating swarm, how do the sexes of Culicoides manoeuvre under these conditions?

Behaviour of females in mating swarms

The comparative approach here is based on the fact that the mating system of midges and many other swarming Diptera are leks and therefore essentially similar (Downes 1969; McLachlan and Neems 1995; Hendry 2003). Most species in both taxa mate on the wing. These mating systems involve swarms of males that attract females which enter the swarm singly or in small numbers, soon to leave tightly paired with a male. Hence females are scarce in the swarm, i.e. the operational sex ratio is strongly male biased. Crucially, unlike Chironomus, females of Culicoides also form swarms, just like males. Female swarms here are feeding swarms where females search for blood meals. Consequently, Culicoides females are abundant and readily capture. This means that Culicoides females can be used to test hypotheses about the mating system that are very difficult with chironomids.

For example, hypotheses about the central role of female body size in mating success can be tested using Culicoides females. The determination of  body size in both mated and unmated females will help resolve interesting questions such as the role of sexual coercion by males (McLachlan 2011; McLachlan 2012), and of predation on mating swarms by flies such as empids (McLachlan, Ladle et al. 2003). These questions are interconnected with biomechanical effect because larger females are predicted to be slower in flight and hence to be the more readily captured by both males and predacious flies. Captured Culicoides females provide data on unmated females for comparison with those captured emerging from mating swarms paired with males.  However, few biologists have the motivation to voluntarily approach these hellish feeding swarms, so the curiosity driven research opportunities described here are at something of an impasse at present.

I have just read a paper by Tripet et al (2009), which suggests a resolution for one puzzle; i.e. what do the females of biting flies gain by swarming? It is not a part of the mating system so why do they do it? The explanation proposed is that communal feeding may carry fitness benefits for the flies in circumventing the host immune response. The reasoning here is that the host's immune system inhibits biting a by the flies and that the injection of saliva during feeding suppresses this. Thus swarming may be a necessary prerequisite for communal feeding. Not very nice for the host! Tripet's findings apply to sand flies, not biting midges, suggesting that communal feeding may be a widespread adaptation among biting insets for reasons connected with the immune response of the host.

References

Crompton, B., J. Thomason, et al. (2003). "Mating in a viscous universe: the race is to the agile, not to the swift." Proceedings of the Royal Society, London (B). 270: 1991-1995.

Downes, J. A. (1969). "The swarming and mating flight of Diptera." Annual Review of Entomology 14: 171-297.

Hendry, G. (2003). Midges in Scotland. Edinburgh, Mercat Press.

McLachlan, A. J. (2011). "Homosexual Pairing within a Swarm-Based Mating System: The Case of the Chironomid Midge." Psyche ID 854820: 5 pages.

McLachlan, A. J. (2012). "Phenotypic plasticity and adaptation in a holometabolous insect, the chironomid midge." ISRN Zoology, 8 pages.

McLachlan, A. J., R. Ladle, et al. (2003). "Predator-prey interactions on the wing: aerobatics and body size among dance flies and midges." Animal Behaviour 66: 911-915.

McLachlan, A. J. and R. M. Neems (1995). Swarm based mating systems. Insect Reproduction. S. R. Leather and J. Hardie. New York, CRC Press.
Tripet, F., Cleg, S., Elnaiem, D. E. and Ward, R. D. (2009). Cooperative blood feeding and the function and implications of feeding aggregations in the sand fly, Lutzomyia longipalpis (Diptera: Psychodidae). PLoS Negl. Trop. Dis. 3, e503. 

Vogel, S. (1994). Life in moving fluids. Princeton, Princeton University Press.