The name "daddy longlegs" applies to two different kinds of invertebrates (animals without a spine). The first is a harmless, nonbiting long-legged arachnid. (Arachnids are a class of invertebrates having jointed legs, including spiders, scorpions, mites, and ticks.) The second is a cranefly—a thin-bodied insect with long, thin legs that has a snoutlike proboscis (protruding mouth-part) with which it sucks water and nectar.
The arachnid daddy longlegs, also called a harvestman, is often mistaken for a spider, but it lacks the segmented body shape that a spider has. Although it has the same number of legs (eight) as a spider, the harvestman's legs are far longer and thinner. These very long legs enable it to raise its body high enough to avoid ants and other small predators. Another difference between harvestmen and spiders is that harvestmen never spin webs.
Harvestmen are largely carnivorous, feeding on a variety of small invertebrates, such as insects, spiders, and mites. They also eat some juicy plants. In captivity they will eat just about anything, from bread and milk to meat. Harvestmen also need to drink frequently.
The crane fly is only nicknamed "daddy longlegs" in English-speaking countries besides the United States. These slow-flying, mosquito-like insects, which are often found near water, grow no more than 1.2 inches (3 centimeters) long. Crane flies lay their eggs in damp places. They hatch into larvae (worm-like juvenile organisms) which feed on plant material all winter. The larvae go through a dormant (resting) period in the spring before transforming into the adult form.
Sources: Burton, Maurice, and Robert Burton. Encyclopedia of Insects and Arachnids, pp. 63, 226-27; "Crane fly." Encyclopedia Britannica CD 97; Palmer, Ephram Laurence. Field Book of Natural History, 2nd ed., p. 433; World Book Encyclopedia, vol. 5, p. 3.