The Phalangids (legacy name) or Opiliones (better known as "harvestmen") are eight-legged invertebrate animals belonging to the order Opiliones in the class Arachnida, in the subphylum Chelicerata of the phylum Arthropoda. As of 2005, over 6,300 species of Phalangids have been described worldwide. The order Opiliones can be divided in four suborders: Cyphophthalmi (Simon, 1879), Eupnoi (Hansen & Sørensen, 1904), Dyspnoi (Hansen & Sørensen, 1904) and Laniatores (Thorell, 1876). Well-preserved fossils have been found in the 400-million year old Rhynie cherts of Scotland, which looks surpringly modern, indicating that the basic structure of the harvestmen hasn't changed much since then. Their closest relatives are probably the mites (Acari).

Harvestmen are also known by the name "daddy longlegs" or "granddaddy longlegs", but this name can also refer to two other unrelated arthropods: the crane fly (Tipulidae) and the cellar spider (Pholcidae).

Physical description

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These harmless arachnids are known for their exceptionally long walking legs, compared to body size. 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 nearly joined, so that they appear to be one oval structure. In more advanced species, the first five abdominal segments are often fused into a dorsal shield called the scutum, which is normally fused with the carapace. Sometimes this shield is only present in males. The two most posterior abdominal segments can be reduced or separated in the middle on the surface to form two plates laying next to each other. The second pair of legs are longer than the others and works as antennae. This can be hard to see in short-legged species. They have a single pair of eyes in the middle of their heads, oriented sideways. They have a pair of prosomatic scent glands that secrete a peculiar smelling fluid when disturbed. Harvestmen do not have silk glands and do not possess poison glands, posing absolutely no danger to humans (see below). They do not have book lungs, and breath through trachea only. Between the base of the fourth pair of legs and the abdomen a pair of spiracles are located, one opening on each side. In more active species, spiracles are also found upon the tibia of the legs. They have a gonopore on the ventral cephalothorax, and the copulation is direct as the male has a penis (while the female has an ovipositor). All species lay eggs.

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