singing male crickets but also to recognize those dhe cricket and require no od~er cues, such as Overhearing Cricket Love that belong to her own species and, possibly, to smell. If dhe flies' ears are damaged, they cannot Songs assess the adequacy of the male by the quality of locate dhe sound. On the other hand, preliminary his song. For the fly, the task is also to find a observation suggests dhat they may be reluctant to cricket of the right species. The possibility that a deposit their larvae unless they actually find a Some flies bear an eary resemlance to their victims female fly also assesses the quality or health of her cricket at the source of the sound. In contrast, host before entrusting her brood to him seems slim, entomologist Tom Walker and his colleagues at the by Daniel Robert and Ronald R. Hoy but cannot be ruled out. University of Florida, Gainesville, have observed that a related species from Brazil, O. depleta, will For 200 million years, on any warm evening, male Our probing of its anatomy reveals that the hearing readily deposit from one to eight maggots right on crickets have been eagerly rubbing their forewings organ of O. ochracea is composed of a pair of very a piece of paper covering a loudspeaker. together, "singing" to attract mates. Early on, they thin membranes situated on the front of the thorax, were pioneers, inventing new ways to advertise near the neck and just behind the head. These So far, six members of the genus Ormia are known their presence to their fellow crickets. But about membranes act much as human eardrums do, to have ears for detecting their insect hosts, an forty million years ago, their serenades began to converting sound energy into mechanical ability they must have inherited from their attract some eavesdropping newcomers—tachinid movements. Each membrane is backed by an air common ancestor. In addition to field crickets, flies. For crickets, this was bad news. chamber and attached internally to a vibration their specific hosts include some katydids and Tachinid flies are parasitoids, parasites that use an sensor. The ear appears to have evolved from a mole crickets. Two genera of flesh flies have also animal host as a food source for their young, chordotonal sensory organ, a type of evolved, independently, a remarkably similar althongh the adults are freeliving. The female "mechanoreceptor." In nonhearing flies this organ hearing organ to listen for the singing of cicadas. tachinid fly, a tiny creature, deposits her eggs or serves as a sort of strain gauge that senses stresses larvae on or near a host insect, typically a species around the neck region and probably helps monitor To be a successful bacterium, fungus, animal, or much larger than herself, such as a beetle or a the movement and posture of the head and front plant depends on detecting crucial features of the caterpillar. The larvae then burrow inside and legs. environment. Survival often requires diverse gorge themselves on the host's generous muscular sensory capacities. From an evolutionary mass or other tissues. After a week or so, they Although an exceptional anatomical development perspective, there is always a potential advantage emerge to pupate. This strategy is very successful, among flies, the tiny ears resemble those found in in doing something a little differently. When some judging by the abundance of tachinid fly species various other insects, including crickets. In all paräsitoid flies gained ears, a whole new sensory (8,000 have been identified worldwide, 1,000 in cases that have been studied, insect ears seem to world became accessible to them. They reaped the North America alone). The family Tachinidae is have evolwd from such chordotonal organs. In advantage of locating a dispersed, concealed host. the second largest in the order of true flies, crickets, for example, ears evolved from sensors Other flies could find crickets by sight or smell, Diptera, after the very diverse Tipulidae, or crane situated on the tibia of the front legs, which but might miss some that are easily located by flies. Another successful, large family of originally functioned merely to detect low- sound. The hearing flies filled a new niche, where parasitoids is the Sarcophagidae, or flesh flies, frequency ground vibrations. Various lines of competition was reduced and resources lay which counts some 2,000 species worldwide. evidence suggest that the original sensory structure untapped. was duplicated, and that this duplicate gained a separate function, the sensing of air vibrations. As Despite the advantage hearing has conferred on in other insects with ears, these structures have certain species of parasitoid flies, the phenomenon nothing in common with the feathery antennae thät is not widespread. Shelley Adamo, of Cornell enable some insects (mosquitoes, for example) to University, who studies the effects of parasitism on detect low-pitched sounds, such as the buzzing of cricket behavior, physiology, and reproductive other insects, at close range. success, has concluded that at least in North America, relatively few singing insect species have The fly's ear resembles the cricket's not only in bodies large enough to support a tachinid structure and sensory origin but also in sensitivity. infestation. Probably more remain to be discovered One way to understand a fly's sensitivity to sound in the tropics, where singing insects are is to measure the electrical activity of the sensory numerous— and often large. nerves leading from its ears to its central nervous As far as we know, the vast majority of tachinid system. To determine which pitches O. ochracea is Many questions remain to be explored in the flies (lilce nearly all flies) are deaf to high-pitched most sensitive to, we inserted tiny recording relationship between ear-equipped tachinid flies sounds, such as the chirping and trilling of electrodes into dhe thorax, at dhe base of dhe and their hosts. What effect do the parasitoids have crickets, and find their hosts by sight and smell. auditory nerves, and tested dhe reaction to on the cricket population as a whole? How But a few species of tachinid flies have evolved the computer-generated simulations of the cricket's detrimental is infestation to a male cricket's ability ability to home in on a cricket's chirp, getting the song. to leave offspring? And will the cricket's tendency drop on their viclim, no matter how well it may be to chirp eventually be eliminated by natural concealed by vegetation or the darkness of night. Our experiments have demonstrated that this tiny selection? fly is most sensitive to sounds at dhe frequency of Among them is the fly Ormia ochracea, which five kilohertz (a litde above dhe highest pitch on Some cricket species have lost their ability to sing, lives along the gulf coast from Florida to Texas, dhe piano), a pitch close to d~e frequency dbat and we and others suspect that parasitism played a preying on the southwestern or southeastern field dominates the cricket's song. This is a striking key role in this loss. Males of the species Gryllus cricket. Diving out of the night sky, the fly example of a phenomenon known as convergent ovisopis, whose common name is the taciturn field deposits one or more tiny maggots on or near a evolution, where superficially similar structures cricket, lack a long-range call, although they chirping male cricket and takes off. The active evolve in distandy related organisms as adaptations conserve enough of the sonnd-producing wing maggots latch on to the cricket and penetrate it. to similar requirements or circumstances. anatomy to scrape out a short-range courtship song (They may even end up parasitizing a female if a female wanders into range. According to Tom cricket attracted by the same song.) By the time the The fly's sensitivity—and especially dhat of dhe Walker, who has studied them, they do not seem to maggots have matured and are ready to emerge, female fly—even surpasses dhe cricket's. We have evolved any other long-range signals, such as the cricket is at death's door. estimate d~at a female cricket can detect a male chemicals. An entirely mute species (which has cricket from at least twenty yards away, while dhe also lost its ears) is Larandeicas bicolor of As biologists interested in the evolution fly can hear it from twice dhat distance. (Humans southern Africa. Unlike its singing relatives, it of sensory systems, we wanted to know how—and we tested are even more sensitive, discerning dhe attracts a female's attention with its brightly how well—this species of tachinid fly could hear cricket song at sixty yards—but they are not as colored wings. Crickets may never regain the the field crickets. In the course of their lives, both quick and precise at locating it in the grassy freedom of action they lost when tachinid flies the female cricket and female fly faee the same meadows, perhaps for lack of practice.) arose forty million years ago. But if the going gets reproductive problem: finding a male cricket too tough, they may evolve some new tricks of singing in the dark. The female cricket uses her In field experiments using loudspeakers, we have their own. sense of hearing not only to detect and locate shown tbat dhe flies are attracted by dhe sound of