Common Name: Brown Daddy-long-legs, Harvestman, Harvest spider, Opilione - The rather puerile common name is an obvious reference to legs that exceed the body size by an order of magnitude, a seemingly unintelligent design. Harvestman and Harvest spider are thought to have been applied to the species as it was first noted and described in the autumnal season, the harvest time.
Scientific Name: Phalangium opilio - The generic name is from phalanges, which is a variation of the plural of phalanx, a Greek military formation in which the soldiers were massed in deep ranks with overlapping shields and extended spears. The plural term phalanges was used in lieu of phalanxes when they were five in number. The Greek and Latin term phalanges also meant the bones of the five fingers or toes at each extremity, a meaning that it retains in present day biology. It is probable that the phalanxes on a field of battle looked something like fingers, and, when they extended in five ranks, were like the five appendages of the hand. The reference in the case of the harvestman genus is to the many segments, or phalanges, of the legs. Opilio means shepherd in Latin; there is no coherent explanation for its use for the harvestman species name.
The entrenched cultural notion that harvestmen have the most deadly venom of all spiders but that their fangs (chelicera) are too short to penetrate human flesh is erroneous. Daddy-long-legs are not spiders, though they are in the same phylum (Arthropoda) and the same class (Arachnida). They are members of the order Opiliones, characterized by having a body that appears to be unsegmented, though it actually consists of a cephalothorax broadly joined to the abdomen. Harvestmen do not have venom and do not make webs, since they do not capture and demobilize their food like spiders. They are therefore not poisonous. Some of the confusion is due to the fact that the ubiquitous cellar spider is also known as the daddy-long-legs spider. It is a true spider (of the order Pholcidae); however, there is no evidence that the daddy-long-legs spider is poisonous to humans either, though this has never been confirmed by an actual test.
The long and filamentous legs of the harvestman are its namesake feature. As it ambles forward with its characteristic awkward, unsteady motion, it uses the second set of legs, which are the longest, as sensors. Unlike the spiders, which have eight eyes that provide essentially circumferential vision, the daddy-long-legs has only two eyes mounted on a raised protuberance on the top of the body. It must rely on the long legs to find food which can then be investigated by tilting the body forward to see it with the eyes, and, if appropriate, to grasp it with the specialized appendages just in front of the legs called pedipalps.
The legs of the harvestman are so long that they need a special neural node at the end of the first long segment to assist in the execution of muscle extensions and retractions. The embedded "pacemaker" in each leg constitutes one of its primary defense mechanisms. When it is attacked by a predator, it sacrifices a removable leg, which will twitch due to the continuing nerve signal pulses. This twitching continues for up to an hour in some species, and is responsible for distracting the predator so that the daddy-long-legs can escape. A new leg will grow back to replace the sacrificed limb.
The harvestman also differs from spiders in matters of sexuality. Most spiders have fairly elaborate rituals associated with mating. The wolf spider is a case in point. This is thought to be due to the need for the male spider to establish a recognizable intent with the generally larger female who might otherwise react with a potentially fatal, venomous response. The mating process of spiders consists of the deposition of a spermatophore, fertilization occurring without direct sexual contact. Harvestmen males have a penis and the sexes engage in copulation that lasts for several minutes, frequently repeated several times. There is no mating ritual, as there is no need to signal against a poisonous reprisal. The female deposits the eggs in soil or wood crevices with a long tube called an ovipositor.
Harvestmen are omnivores, and are generally considered to be beneficent in their impact from the human perspective. Feeding mostly at night, they subsist mostly on decomposing plants and animals. However, in spite of their lack of venom, they are opportunistic predators and will kill and eat small insects and even other harvestmen. There have been instances of harvestmen taking the food away from other predators including other spiders, a phenomenon known as kleptoparasitism.