The symphony of cricket trills, gracing grassy fields in many parts of the world, arises from an all-male orchestra. Each male advertises his presence and prowess by scratching together the bases of his forewings, which are ridged like tiny washboards. Each stroke creates a single chirp, and the cricket’s song consists of a series of chirps called a trill. Like the communications of many males, the trill carries two meanings, a come-hither message for females and the opposite for competing males.
To human ears, the field may sound full of crickets all calling at once, but the males take turns. If two males call at the same time, one will fall silent and move a respectful distance away. A female, on the other hand, homes in on the most robust trill, with mating on her mind. Female crickets will not respond to just any male cricket. The male must sing a song specific to her species. This discrimination is so specific, one scientist found, that female hybrids prefer the song of identically crossbred males. The scientist crossed two specieslet’s call them A and B. Females of species A respond only to A males. And B females respond only to B males. But what if these species can mate and produce hybrids? The researcher found that hybrid females respond to hybrid calls. But their powers of discrimination surpassed even this. By recording the songs and making graphical representations, the researcher found subtle differences between the songs of males with an A father and a B mother (AxB) and males of the opposite cross (BxA). And hybrid females detected the difference as well. Females of AxB crosses preferred AxB males over all three other possibilities. (A, B and BxA). Cricket scientists explain this amazing pickiness by suggesting that the nerve cells of cricket brains contain a pattern generator. In males, the genetically determined generator makes them sing their species-specific song and no other. In females, the same pattern generator gives them an internal ideal with which to compare any song they hear. Although genetically programmed to play a particular song, male crickets can determine when, where and how loud to play, unlike female mosquitoes, where the signal results directly from beating wings.
A successful male attracts a female of his species, mates and passes his genes on to the next generation. But in an insect version of fatal attraction, he may attract a completely different kind of female. This female is looking not for sex, but a place to lay her eggs. She’s a parasitic fly of the genus Ormia. Once she’s found a male cricket, Ormia lays an egg. The developing maggot burrows into the male and grows, eventually eating him from the inside out. But how can Ormia home in on the song of a species so different from her own? Ormia is a true fly, the size of a house fly and related to mosquitoes. And like mosquitoes, Ormia flies detect sound as their antennae sway, buffeted by the movement of air molecules. In a sense, a female Ormia has the same goal as the female cricket, to hear and home in on a male cricket. To succeed, she must hear sounds from a much greater distance than the maximum distance at which her antennae are effective.
Over the millennia, flies in the genus Ormia have evolved the same solution to this problem that crickets have: an eardrum. Crickets hear with a small eardrum on their legs. Ormia females have the same sort of structure, unlike other flies. And Ormia’s evolution has produced a more sensitive solution than even the cricket’s. Scientists measuring the output of the nerve cells connected to the eardrum of one Ormia species found the flies 100 times as sensitive to male cricket songs as female crickets.
In places where the parasitic flies and crickets coincide, such as Hawaii, cricket evolution must balance the desirability of mating with the fatal possibility of becoming a meal for a growing fly maggot. The presence of the parasite has favored Hawaiian crickets with a shorter chirp. The males also sing only when the flies are least active, during the hours of darkness. This way they attract female crickets but mostly avoid attracting female Ormia flies. Crickets of the same species on an island without parasitic flies have a fuller call and begin calling at dusk, persisting until dawn.