HOLLOWS, PEEPERS, AND HIGHLANDERS: AN APPALACHIAN MOUNTAIN ECOLOGY

10. Nuptial Gift of the Hangingfly

67 10 Nuptial Gift of the Hangingfly H erbs tickle my arms as I sit on the ground in a cove forest watching the sex life of the black-tipped hangingfly. I’m close enough to see the medium-sized (0.8 inch), slim, brownish body, long thin legs, and four narrow, black-tipped wings of each hangingfly. Named for their habit of dangling from foliage by their front legs, they hang with wings outstretched rather than folded over their abdomens. Hangingflies superficially resemble long, slender, delicate flies but are not true flies, which are members of the order Diptera; they just look like them. In central and southern Appalachia, you will find the hangingfly’s story easy to follow — they fly slowly over short distances, catching prey and mating throughout the day. Research on the black-tipped hangingfly’s behavior has revealed a complex blend of courtship, nuptial giving, thievery, and sexual prevarication. Although the male and female hangingflies look similar, their behaviors are markedly different — and therein lies the tale. The male hangingfly uses his strong rear legs to capture house- flies, daddy longlegs, and other arthropods. While holding prey with the prehensile segment of his last leg, he catches onto a leaf or twig. He prepares the meal by piercing the prey with his needlelike snout and injecting digestive juices that paralyze the insect and liquefy its innards. After a brief taste, the male hangingfly may discard the prey; but if he retains it, he begins making short (less than twelve feet) flights through the ground cover. After each flight, he hangs from a leaf or twig and everts a pair of abdominal glands that release a 68 pheromone, a specific molecule that carries a message to a receiver. (Some biologists have suggested that a man’s sweat contains a sexual pheromone that excites women. Not so in my experience!) A female hangingfly responds to the male’s pheromonal invitation by settling beside him, then hanging by her forelegs facing him. She signals sexual readiness by lowering her wings, which stimulates the male to present his gift of food. Both are initially hesitant and noncommittal: While the female evaluates the prey by feeding on it, she keeps her abdominal tip pulled away from the male. The male retains a firm grip on the prey with both hind legs and one or two middle legs. With these behaviors, each member of the consort pair enters a conditional contract: If the prey is unpalatable or too small, the female flies away unmated or after having allowed copulation for a short time, an average of only five minutes. If on the other hand, she finds the nuptial gift to be satisfactory, she allows a lengthy copulation . In this latter situation, mating lasts an average of twentythree minutes, during which the female feeds on the offering. Female black-tipped hangingflies select or reject mates on the basis of their food offering — much like humans. Dr. Randy Thornhill of the University of New Mexico has discovered a tense battle between selfish individuals underlying this apparent romance, each fly pursuing its own reproductive priorities. Another look at the hangingfly’s mating behavior reveals the brutal details. Huntingisadangerousactivityforhangingflies.Itnotonlycosts time but exposes them to the webs of predatory spiders. Hangingflies have evolved several methods to reduce these costs. Female hanging flies do not hunt when their own males are abundant. When they do hunt, they retain the prey items. In contrast, males discard about 32 percent of the prey items as unpalatable or too small. This intersexual difference suggests that females reduce their odds of falling prey by allowing copulation in exchange for food. Trading a meal for sex makes the male more vulnerable than the female to predation by spiders. Males have evolved a strategy for reducing their risks during hunting. They may capture free-rangHollows , Peepers, and Highlanders 69 ing arthropods, as reported above, or they may steal prey from other males. Although a male with prey is a potential theft victim while feeding, flying, and copulating, he is most vulnerable when releasing pheromone because at that particular moment he is physiologically programmed to receive a female. Fully 50 percent of a male’s time between copulations is spent searching for prey. Males that steal rather than hunt for themselves reduce that unproductive...