Touchy Harvestmen

Natural History, Oct, 2000 by Rogelio Macias-Ordonez

FOR A STRIPED DADDY LONGLEGS, WHEN IT'S TIME TO MATE, THE ONLY OPTION IS A BLIND DATE.

IMAGINE YOU ARE DEAF AND NEARLY BLIND and can hardly smell anything, but you have eight legs, each forty or fifty feet long. These legs are what enable you to assess your environment, detect rivals and predators, find mates, and procure food. That is the tactile perspective of most daddy longlegs, also known as harvestmen. As the British arachnologist Theodore H. Savory once stated, "The study of harvestmen is a study of legs."

Making up the order Opiliones (which together with spiders, scorpions, mites, and other groups form the arthropod class Arachnida), daddy longlegs are creatures that people commonly encounter, both indoors and out. About 7,000 species of harvestmen have been described, and many more remain to be identified. Relatively little is known, however, about how they lead their lives. During the past seven years, I have been trying to understand the mating strategies of one of the most abundant species in the eastern United States, the striped harvestman (Leiobunum vittatum). This species can easily be recognized by the characteristic blackish stripe on its back. The females range from yellow to light brown, while the males--which are smaller and have more elongated bodies--usually are tinged reddish orange (perhaps a warning to predators that they taste bad).

Each species of daddy longlegs has its preferred environmental conditions. Several species may share the same habitat but be active at different times of day, when the humidity and temperature fit their particular demands. Adult striped harvestmen, like adults of some other species, appear for only a few weeks during the year and then die. As is true for most species in temperate zones, they are active in late summer and early fall--hence the name "harvestman."

The population I study, in an old quarry on the campus of Lehigh University in eastern Pennsylvania, is most active between mid-September and mid-October. During this period, males station themselves on the moss-covered, football- to beachball-size rocks on the ground, frequently remaining on these territories for days at a time. As females wander amid the surrounding leaf litter, they climb over the rocks and find the waiting males. Not until they actually come into contact do a male and a female detect each other's presence. Exactly how an individual recognizes that it has encountered a member of the opposite sex is not yet known. Characteristic anatomy and behavior may be detected through touch. The animals may also have taste or short-range odor receptors on their legs that play a role.

If a male detects a female, he pounces on her, attempts to grasp her from the front with short appendages known as pedipalps, and tries to copulate. While other male arachnids use their pedipalps to insert sperm into the female, male daddy longlegs have true intromittent genitals. In both sexes, the genitals are located just below the mouth, so the position for copulation is halfway between face-to-face and belly-to-belly. I've observed that females usually resist the advance and wrestle with the insistent male. It is hard for a female to escape a male's grasp, but the male needs her cooperation; mating is impossible if she does not lift up her front end.

Larger males copulate more, perhaps because they are better able to subdue reluctant females. Another possibility is that females assess male size through such wrestling matches, choosing to copulate only with larger males. In other animal species in which males have mating territories, females can generally assess and compare males before entering their domains, but a striped "harvestwoman" cannot know whether a male is nearby--much less choose one--before she touches him.

After mating, the female usually remains on the rock and lays her eggs there. She repeatedly inserts her ovipositor under the moss cover, searching for crevices in the rock where her eggs can safely spend the winter. The male follows her while she does this, keeping contact with at least one leg. He fends off any male that approaches the female. (Similar behavior, common among other species in which fertilization occurs inside the female, helps the male assure paternity.) If the male loses leg contact with the female, he walks in circles until he locates her. He is apparently unable to distinguish his recent mate from other females laying eggs, however, and will guard the first female he finds in his search.

When the female leaves the rock, the resident male remains behind, waiting for more females to arrive. Since soon afterward the first female may copulate with another male on another rock, the obvious questions are, Which male's sperm will fertilize which of the female's eggs, and how is this regulated? Unfortunately, we don't yet know whether it is a matter of anatomy, physiology, behavior, or even direct competition between the sperm themselves.

Rocks are good mating territories not only because gravid females prefer them as places to lay eggs but also because these sites enable males to comfortably grasp a female and attempt to copulate. A male that encounters a female on leaf litter also will try to copulate, but if females want to avoid the advance, they typically succeed. Males can rarely achieve the balance and support there that they would get on the flat, hard surface of a rock.