Parasitic Mites as Sexual Display Ornaments

Non biting midges (chironomidae) are ubiquitous insects (Armitage, Cranston, and Pinder, 1995), and while we have a reasonably good understanding of the ecology of the larval stages, the adult part of the life cycle is poorly studied. As the part of the life cycle responsible for reproduction, the adaptive biology of the adults falls largely into the real of sexual selection but their mating behaviour is not easy to study. Yet there is an incentive to do so because they are the part of a life cycle of an insect whose aquatic larvae dominate  many freshwater habitats and are therefore major engines of energy turnover in freshwaters worldwide. Direct observation of the behaviour of midges in mating swarms has yielded much (McLachlan and Neems, 1995), but to get at the adaptive function of the swarms more is needed. A way must be found to manipulate a swarm experimentally, in principle like clipping the tails of mating peacocks by Marion Petri (Petrie, 1994). But a method of swarm manipulation with a similar aim is difficult to imagine. To this end I have been playing with the idea of spraying swarms with aerosol paint to test specific hypotheses. I believe it can be a useful method when combined with rigorous controls.

In an earlier experiment with aerosol paint I was attempting to test the hypothesis that the wing symmetry of swarming males influences mating success (McLachlan, 1997). The method depended on random hits with the droplets of paint upsetting wing symmetry (Fig. 1.).

Fig.1. Examples of males for an experimental mating swarm hit by aerosol paint droplets. Modified from (McLachlan, 1997).

My prediction was that male midges hit by paint (like those in Fig.1, would experience a reduction in mating success. The results of this experiment on load effects were unclear (McLachlan, 1997), but using the same method I wished to test a second hypothesis. Midges are frequently infested by a bright red mite Unionicola ypsilophora, and, counterintuitivly, male midges infested with mites achieve improved mating success (McLachlan, 1999). Michael Ryan’s work on sensory drive (Ryan, 2017), offers an explanation, that it is the red colour of the mites per se that determines mating success of the host midge. Droplets of red aerosol paint on male midges would act as a surrogate for mites. The prediction is that midges with red paint spots would gain in mating success. In other words, mites are here seen to promote mating success in males purely by their colour, i.e., by acting as a visual sexual display ornament like a peacock’s tail (Fig. 2). Let me explain how, given the chance, I would test such an hypothesis.

Fig.2. A male P. rufiventris baring 2 mites on his ventral surface. After  (McLachlan, 2010).

My test chironomid species is the common midge Paratrichocladius rufiventris.

Is it possible that we have here an example of choice by the female, which would be somewhat unusual (Andersson, 1994). Hence the above hypothesis to test the effect of colour as a sexual preference in the female midge, enhanced in the presence of mites.

The design of an experiment to test such an hypothesis requires an uninfected male swarm to be sprayed. This is a realistic requirement since not all male swarms are infected. In many locations dozens of separate swarms, each containing thousands of male midge can be found. As far as I can tell each swarm is independent with little cross-contamination. Swarms therefore act as statistically independent samples. Details of the procedures and precautions in the application of the paint are given in (McLachlan, 1997). I used commercially available water-based paint. It would be far better to make up ones own paint with known ingredients – i.e. non-toxic, inert water soluble pigment and water. The control is provided by an uninfected and unsprayed swarm.

At least two treatment controls are required:

1. Swarms sprayed with paint of another colour, e.g. white.

2. Swarms sprayed with water only.

The whole procedure should be repeated on as many occasion as feasible – control, treatment control and experiment all on each evening together.

Note that the mating behaviour of this midge, like that of most animals, is sensitive to environmental conditions and can readily switch between alternative mating behaviours (McLachlan, 2018). For this reason, it is important to ensure that experiment and controls are not infected with mites. Furthermore, the choice of P. rufiventris as the test subject is significant. This species has a red tinge to the ventral surface (see rufiventris), as the hypothesis of a supranormal sexual display is meaningful in that context. It would be interesting to carry out the same experiment on anther common chironomid, e.g. Chironomus plumosus which does not bare a red tinge. In the case of C. plumosus a positive result could be interpreted, again following Mike Ryan, as the mites revealing a hitherto hidden preference for red. In both cases, such an outcome would be interesting as flies (Diptera), are thought not to able to detect red (McLachlan, 2009). Note too that neither supranormal nor hidden preference carry fitness benefits (Coyne, 2009), p.181.

references

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

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

Coyne, J. A. (2009). Why Evolution is True. Oxford: Oxford University Press.

McLachlan, A. J. (1997). Size or Symmetry: An Experiment to Determine which of the two Accounts for Mating Success in Male Midges. Ecoscience, 4, 454-459.

McLachlan, A. J. (1999). Parisites promote mating success: the case of a midge and a mite. Animal Behaviour, 57, 1199-1205.

McLachlan, A. J. (2009). Do Flies See Red?, http://www.google.co.uk/atholmclachlan.blogspot.co.uk.

McLachlan, A. J. (2010). Fluctuating Asymmetry in Flies, What Does it Mean? Symmetry, ISSN 2073-8994. doi:10.3390/symf202f1099.

McLachlan, A. J. (2018). The Mating Behaviour of a Swarm Based Insect., http://www.google.co.uk/atholmclachlan.blogspot.com/.

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

Petrie, M. (1994). Improved Growth and Survival of Offspring of Peacocks with more Elaborate Trains. Nature, 371, 598-599.

Ryan, M. J. (2017). A Taste for the Beautiful. Princeton, New Jrsey.: Princeton University Press.