Animals are commonly infested with ectoparasites which can be large relative to the host. Here a previously unrecognised consequence of infection for such hosts and for their parasites is examined. As a model host-parasite system the chironomid midge Paratrichocladius rufiventris and the hydracarinid mite Unionicola ypsilophora were adopted. Previous work has shown that mites have a profound effect on the mating success of infested midges and since midges mate on the wing this effect is likely to be due to interference with the mechanics of flight in the host. Mites depend on the host for dispersal but since host mating precedes dispersal there are presumably fitness advantages for the mite that attaches at a site which minimise detrimental effects on flight performance. One expected outcome is that mites should arrange themselves left-right symmetrically with respect to the host midline. In this regard three predictions were tested: first, that mites arrange themselves non-randomly on the host. Second, that the arrangement of mites affects mating success of the host and third, that the degree of symmetry in the arrangement of mites affects aerobatic performance. The first was achieved through measurement of the distribution of mites on wild caught midges, the second through samples from mating swarms in the wild and the third through an experiment in the laboratory. A highly significant departure from randomness in the arrangement of mites on wild midges was found. This finding suggests that mites are choosing the site of attachment on the host and that this behaviour may have evolved in the mite because it carries fitness benefits. Indeed, we show that mite-simulating artificial loads interfered with aerobatic ability of midges in the laboratory, the effect being most pronounced when the load was asymmetrical. However, we were able to detect no clear consequence of infection for host mating success
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