The Harvestman, or Grand-daddy Longlegs Is Not a Spider

By Renn Tumlison, tumlison@hsu.edu

Dolomedes

The term “arachnid” refers to a group of animals that possess four pairs of legs, chelicerae (fang-like mouthparts) and appendages near the mouth called “pedipalps.” The term “arachnophobia” refers to a fear of such animals. Usually, the fear is most focused on one group of arachnids known as spiders, but the other members, scorpions, mites, ticks, and harvestmen seldom are viewed favorably by humans.

Many people see the similarity of form and think that the harvestman, perhaps better known as the grand-daddy longlegs, is a kind of spider. Spiders have two body segments (see image above). The cephalothorax is the fused head and thorax regions, which is distinct from the abdomen. Harvestmen essentially have an oval body without the separation (see right photo). They also do not produce silk or a web. They are not true spiders. Venom produced by spiders helps them in their role as predators, but harvestmen do not need venom because they are scavengers. They feed by searching over the ground or over plants for small insects, eggs, or dead material (either plant or animal). For protection, harvestmen can produce defensive chemicals that taste and smell bad.

harvestman
Mites Detail

Small red mites often are found hanging onto the legs or bodies of harvestmen. It is not known what those mites are doing. If they feed on the harvestman, they would be considered parasites, but they would be “phoretic” mites if they merely hang on to be transported to locations that might provide a good source of food. The long legs of harvestmen are easily detached and will twitch for some time after removal. “Pacemakers” located in the first segment of the legs (called the femur) send signals that make the muscles in the broken leg tighten, but the leg relaxes between signals. Repetitive pulses of the signal create the twitches. The twitching probably attracts attention of possible predators and allows the harvestman to escape.

Harvestmen often will aggregate into groups in which the legs seem to be twined together and the bodies close to each other. When disturbed, a single harvestman typically pushes its body up and down in a slow, vibrating motion. The large groups will perform the same behavior if disturbed, so the pulsating mass of harvestmen may be an even greater deterrent to potential predators. Furthermore, the defensive chemicals produced by a mass may be collectively more effective, so grouping might promote greater security while the aggregation rests or hibernates.

Longlegs Group
 
 
 
 
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