"Daddy-longlegs" is one of the worst animal names there is. The name is widely used and generally recognised, but causes endless confusion because it is applied to no less than three very different animals. In a brief attempt at disambiguation, these are the animals involved:
First are harvestmen of the order Opiliones (picture from UMMZ). Harvestmen are often confused with spiders, but the body is not divided into a cephalothorax and abdomen, the opisthosoma (the posterior part of the body corresponding to the abdomen) is externally segmented, the chelicerae (mouthparts) are pincers rather than fangs, and harvestmen do not produce silk. The name "daddy-longlegs" as applied to harvestmen usually refers to the group known as "long-legged harvestmen" (Palpatores). There is some uncertainty about whether Palpatores are a monophyletic group, but that's a subject for another time.
Second are spiders of the family Pholcidae (picture is from Iziko Museums of Cape Town - the object the spider is holding is the egg-sac, which is carried by the female until the eggs hatch). Pholcids are true spiders, and so have a divided body, an unsegmented abdomen, fangs, and produce silk. Here in Australia and New Zealand, the 'daddy-longlegs' that are almost ubiquitously found in houses (particularly bathrooms) are pholcids, most often the introduced Pholcus phalangioides. Offhand, there is a common belief that daddy-longlegs (either pholcids or Opiliones) are "the most poisonous spiders in existence, but their fangs are too small to pierce human skin". I have come across this story many times, and have even been assured of it by people who really should know better. This story is absolute bunkum. The University of California, Riverside site has more info.
Finally, the third group accused of being 'daddy-longlegs' are crane flies of the family Tipulidae (picture from Wikipedia). Crane flies look a bit like giant mosquitoes, but they are not blood-suckers. They are a large family - the adults are nectarivores or do not feed, while the larvae, commonly called leatherjackets, feed on vegetation. Crane flies are easily distinguished from the other 'daddy-longlegs' - the wings are a bit of a give-away.
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