Stilt Walker
by William H. Amos, USA
As a boy I was filled with childish questions. Despite a lifetime spent in biology, many of them remain unanswered. One puzzle was how a daddy-long-legs, or harvestman, could possibly control legs no thicker than a human hair. Even then I knew muscles had to be involved, but how could they be present in such extremely long, hairlike appendages? No doubt about it—a harvestman is one weird creature.
If you were ant-size, you would never know a harvestman was nearby. Its eight legs would be so widely spaced, straddling such a wide area, if you happened to see one leg, the others would be far away and seem unrelated—if they were noticed at all. You would have no idea that hovering high above, a compact egg-shaped body (an ovoid) was supported on such spindly shafts.
A harvestman, like spiders and insects, is a creature with an external skeleton. If you examine one of its legs closely, it proves to be tubular, consisting of seven major sections with finely-articulated joints, and a highly flexible foot terminating in a single curved claw. Nerves, free-flowing blood (but not in vessels), and threadlike muscles are enclosed within this slender living pipe. The muscles, attached to the inner walls of the leg, control the leg’s movement with precision, ordered to do so by their attendant nerves. Other nerves carry messages back to the distant body and small brain, for even the lightest touch causes the animal either to withdraw a leg or run away. If the touch is more vigorous, the leg will simply detach and fall to the ground, wriggle a bit, possibly attracting a would-be predator, while the erstwhile owner runs safely away. This is true for only some of the legs, not all. The legs are much more than moving, articulated stilts, causing one specialist who worked with these odd animals to say, "A study of harvestmen is a study of legs."
Although all legs of a harvestman are sensitive, the second pair of legs is so vital that the loss of one is a serious matter, and the loss of both is quickly fatal. Why? These legs—and to a lesser degree all the others—are the animal’s main sources of information, serving as ears, nose, tongue, and perhaps even as supplementary "eyes." In other words, the legs are loaded with nerves and literally thousands of tiny sense organs that lie inside microscopic slits which open to the outside world. When there is motion nearby, a harvestman energetically waves its second pair of legs in the air, picking up scent and vibration, or taps nearby surfaces, sampling the "taste." And instead of elaborate courtship rituals found in spiders, harvestmen touch each other gently with the same two legs, exchanging chemical messages unknown to us. Even the male sex organ is uniquely associated with the legs, having a location found in no other arachnid.
If you take a strong magnifying glass and examine the body of a harvestman, be prepared to see a mimic of Hollywood’s elevated mechanical creatures in "The War of the Worlds." Its rotund, plated body is decorated with small spikes and high on its back, toward the front, is a knob or pedestal with two back-to-back eyes pointing out to either side, "like the two faces of a clock-tower," said Theodore Savory, an English biologist who studied arachnids and their kin. The way in which they are elevated reminds me of a conning tower, the "eye" of a submarine. Situated as they are, they watch for danger from above and off to the side.
There are a lot of different kinds of harvestmen in the world—3500 species in all—some not in the least resembling the few species we find in New England. Here you will find the familiar dark brown or gray daddy-long-legs on a tree trunk or on the side of your house. On the summit of Burke Mountain, another species has much shorter legs and a light-colored body. Elsewhere in the world, especially in southeast Asia and Central and South America, some groups of harvestmen are hardly recognizable as such. They have short, heavy, knock-kneed, spiny hind legs, a pair of big sharp hooks emerging on each side of a fat, pear-shaped body—and so on. In every part of the world you will find still other shapes and sizes, some of them flat as a pancake and very slow and deliberate in their movements. Certain odd harvestmen live only in caves. Even our common lowland species have distinct preferences for habitat, living in particular zones according to age. For example, the older they become, the higher they ascend in trees, while youngsters stay below. Harvestmen need and seek moisture, and for this reason have been described as having an undying thirst. It is said they acquired their name long ago when an early biologist first studied them during harvest time.
Daddy-long-legs are often thought to be spiders, while in fact they are only distant cousins. Like spiders they have eight legs, but where a spider has a pair of "fangs," a harvestman has a pair of minute pincer-like claws and no venom. A pair of short, spiny leglike appendages extending out in front help with the feeding process. So even with feeble jaws, harvestment are able to pick up food and pluck it apart into smaller pieces. Local species feed mostly on dead insects, but will kill very small insects and mites whenever possible. There are always a few harvestmen around our porch light after dark, avoiding big moths, awaiting tiny flying insects to drop down upon after straddling such unwary prey. They also suck juice from soft berries and fruits; throw a rotting peach outside and watch the nightly jamboree.
Harvestmen are vulnerable and have only a few effective means of protecting themselves. Speed and detachable legs are obvious advantages, but another defense—one we can barely detect—is the secretion of an odorous fluid. This substance, described as smelling like walnuts or horse radish, emerges from a pair of pores on each side of the front of the body, just above the first pair of legs. To predators, the fluid is obnoxious enough to assist in a daddy-long-legs’ survival. I’ve watched both birds and toads studiously ignore harvestmen, but that doesn’t mean they don’t get eaten.
Probably only a specialist can truly care about several thousand species of harvestmen with their peculiar assortment of characteristics, yet would not the world be a poorer place without them? From a human point of view, they are totally harmless—and fun to have around. But please leave their legs alone!
© 1998 William H. Amos
Comments to Bill Amos welcomed.
First published in July 1998 Micscape Magazine.
Please report any Web problems
or offer general comments to the Micscape Editor,
via the contact on current Micscape Index.
Micscape is the on-line monthly
magazine of the Microscopy UK web
site at Microscopy-UK