Understanding Swarm-Based mating systems.

In September last year the Royal Entomological Society held a conference at St Andrews University entitle, Thirty Years of Thornhill and Alcock, The Evolution of Insect Mating Systems. The  conference has prompted to reread their book. The authors reveal, as never before, the adaptive complexity of the mating behaviour of the insects. In this light I examine again the taxon I know best, the chironomid midges, and attempt resolve the central question - is the mating system monogamous (males with only one partner)? 

We are slowly gaining ground in understanding the proximate aspects of the functioning of swarm-based mating systems, (see  ‘Swarm Based Mating Systems – the latest. 12/03/2012), but we are further behind  in understanding the ultimate, that is evolutionary, aspect of the functioning of these swarms. I have consistently assumed that the mating system of the chironomids is essentially monogamous. My reason for this is that, like mayflies, chironomids are short lived as adults and hence probably have only a single opportunity to mate. Under this assumption life time mating success and mating in a single evening are equivalent. By contrast with the vertebrates, this feature of the life cycle greatly simplifies studies of chironomids. An assumption of monogamy finds support in the fact that the heavily male biased operational sex ration, which is a feature of mating swarms, also favours an explanation of monogamy (Thornhill and Alcock 1983), p231. If not monogamy we are confronted by a mating system comparable in complexity to that of  the (Andersson 1994). And there is reason to be cautious about an assumption of monogamy. Confounding observations are of at least two kinds. First, the male external genitalia of chironomid midges are extraordinarily elaborate (Fig. 1.).  From (Armitage, Cranston et al. 1995)

Arnqvist (Arnqvist 1998), has shown that such complexity is the result of sexual selection on males in competition with rivals in polygyny (males have multiple partners) and/or polyandry (females have multiple partners). Second, unlike the mayflies, female chironomids possess spermathecae. This feature gives the female choice over what sperm to use in fertilising her eggs. It is difficult to imagine how the presence of spermathecae could have evolved if only one male fertilises her (Thornhill and Alcock 1983), p.325 et seq. Tests of these hypotheses are rare and when present often contradictory. For example, there is a contradiction between the findings of Downe (Downe 1973) and Arqvist (Arnqvist 1998), for the chironomid Chironomus riparius. But I  cautiously stand by an assumption of monogamy until further evidence is forthcoming.

For anyone with the inclination and expertise to do so, there are several tried and tested methods to explore the question of the mating system of the chironomid midge. For example, Downe (Downe 1973), uses radioisotopes to distinguish between the ejaculate of different males. Arnqnist (Arnqvist 1998) reviews the use of spermatophore counts in the female spermathecae. Some simpler observations could help. For example, the presence of various forms of plug left in the female genitalia after copulation could suggest monogamy (Thornhill and Alcock 1983), p240.  

References

Andersson, M. (1994). Sexual Selection. Princeton, Princeton University Press.

Armitage, P., P. S. Cranston, et al. (1995). The Chironomidae. The biology and ecology of non -biting midges. . London, Chapman & Hall.

                       

Arnqvist, G. (1998). "Comparative Evidence for the Evolution of Genitalia by Sexual Selection." Nature 393: 784-786.

Downe, A. E. R. (1973). "Some Factors Influencing Insemination in Laboratory Swarms of Chironomus riparius (Diptera: Chironomidae)." Canadian Entomologist 105: 291-298.

           

Thornhill, R. and J. Alcock (1983). The Evolution of Insect Mating Systems. London, Harvard University Press.