Opiliones are an order of arachnids commonly known as harvestmen. Well-preserved fossils have been found in the 400-million year old Rhynie cherts (sedimentary deposit) of Scotland, which look surprisingly modern, indicating that the basic structure of the harvestmen has not changed much since then. Although they belong to the class of arachnids, harvestmen are not spiders, which are of the order Araneae rather than the order Opiliones.
These arachnids are known for their exceptionally long walking legs, compared to body size, although there are also short-legged species. The difference between harvestmen and spiders is that in harvestmen the two main body sections (the abdomen with ten segments and cephalothorax, or prosoma and opisthosoma) are broadly joined, so that they appear to be one oval structure; they also have no venom or silk glands. The feeding apparatus (stomotheca) differs from other arachnids in that ingestion is not restricted to liquid, but chunks of food can be taken in.
Many species are omnivorous, eating primarily small insects and all kinds of plant material and fungi. Some are scavengers, feeding upon dead organisms, bird dung and other fecal material. Most hunting harvestmen ambush their prey, although active hunting is also found. Because their eyes cannot form images, they use their second pair of legs as antennae to explore their environment. Unlike most other arachnids, harvestmen do not have a sucking stomach or a filtering mechanism. Rather, they ingest small particles of their food, thus making them vulnerable to internal parasites.
Harvestmen have a pair of prosomatic defensive scent glands that secrete a peculiar smelling fluid when disturbed, confirmed in some species to contain noxious quinones. Harvestmen do not have silk glands and do not possess venom glands, posing absolutely no danger to humans.
Typical body length does not exceed 7 millimeters (0.28 in), with some species smaller than one mm, although the largest species Trogulus torosus (Trogulidae) can reach a length of 22 millimeters (0.87 in). However, leg span is much larger and can exceed 160 millimeters (6.3 in). Most species live for a year.
ENTRY AND DAMAGE
Under most conditions outdoors, spiders are considered beneficial because they feed on insects. However, they are undesirable to most homeowners when indoors, and the unsightly webbing spiders use to catch insect prey usually outweighs this beneficial behavior.
Some spiders are associated with moisture and, therefore, are typically found in basements, crawlspaces and other damp parts of buildings. Others live in warm, dry places such as subfloor air vents, in upper corners of rooms, or in attics. Most species hide in cracks and darkened areas.
Thorough inspections are necessary to find all spider harborage, such as in landscaping, indoors or under the home, for later sanitation, removal or insecticide treatment. Space treatments of insecticides are often useful for cleanouts and for eliminating outdoor species that may be found indoors.
Long-term residual control of spiders is difficult to achieve. All areas where the spiders have been found should be treated, paying particular attention to dark corners. Dusts are especially useful for treating inaccessible void areas, crawlspaces and attics.