Parasites have some remarkable strategies with which they can manipulate the behaviour of their hosts in order to increase their chances of reproducing. For example, fungi of the genus Cordyceps produce chemicals which alter the host’s navigational senses. These chemicals induce the host to climb to the top of a plant, and secure itself to a stem with its mandibles. The fruiting body of the fungus then grows from the host’s head, and releases spores. The height at which this happens increases the area over which the spores are distributed. Similarly, the gordian worm produces chemicals which disrupt the geotactic sense of its terrestrial host, causing the insect to jump into water so that the mature worm, which is aquatic, can emerge.
But in this fascinating evolutionary struggle, insects have evolved their own strategies to avoid parasitic infection. Jane Brockman, a zoologist at the University of Florida, and her colleagues, have discovered one such host strategy. The findings are reported in two papers, one in the journal Ethology, the other in Animal Behaviour.
The insect host investigated by Brockman’s team was the Southeastern U.S. field cricket (Gryllus rubens), which is native to North Florida. Male crickets attract mates by producing a song. However, the song of the male cricket also attracts unwelcome visitors – a species of parasitic fly called Ormia ochracea.
Female flies deposit hundreds of tiny larvae on or near the host. The larvae enter the insect’s body by burrowing into an exposed membrane located between the pro- and mesothorax. Inside the host, the larvae moult and feed on non-essential organs for a week. The larvae then emerge from the host, pupate and metamorphose into adult flies. Normally, the cricket dies soon after the emergence of the larvae, maybe because the parasite empties the toxic contents of its gut before leaving the host.
The flies use highly specialized hearing organs to locate their hosts. The organ consists of two tympana (ear drums) fashioned out of thin sections of the chitinous exoskeleton. These miniscule ears are about 1 mm-squared in size and are highly sensitive to the frequencies at which crickets sing. The tympana are very close to each other and, because they are mechanically connected by a semi-rigid bridge, can resolve nanoscale time differences in an incoming sound and have hyperacute directional selectivity. With these tiny ears, the flies can accurately locate singing male crickets from distances of up to several metres, and drop larvae on them while flying overhead.
In field experiments, Brockman’s team captured male crickets in containers and recorded their songs. They have found that the crickets sing less during the autumn (fall) months, when there is an abundance of parasitic flies, than in spring. 75% of males captured in the spring sang, compared to just 43% of those captured in the autumn. In another set of experiments, female crickets were captured, and their behaviour observed when they heard recorded male songs being played back. It was found that females were more reluctant to approach males in the autumn, perhaps to avoid becoming hosts to the flies.
Although fewer males sang in autumn, those that did sing did so for longer periods of time. This seems paradoxical, as it increases the probability of attracting the flies. Brockman speculates that this may be because there are far more female crickets in autumn than in spring.
For the male crickets, this strategy poses somewhat of a dilemma: singing increases its likelihood of finding a mate, but also of attracting the parasitic flies. Some males appear to have another behavioural adaptation that may have arisen as a result – some do not sing at all, but instead found mates by walking around.
- Arthur, B. J. & Hoy, R. R. (2006). The ability of the parasitoid fly Ormia ochracea to distinguish sounds in the vertical plane. J. Acoust. Soc. Am. 120: 1546-1549
- Mason, A. C. et al (2005). The start of phonotactic walking in the fly Ormia ochracea: A kinematic study. J. Exp. Biol. 208: 4699-4705.
- Mason, et al (2001). Hyperacute directional hearing in a microscale auditory system. Nature 410: 686-690.