Opiliones are arachnids, but a separate order from spiders. Their cephalothorax and abdomen are broadly joined into a single oval body with no narrow waist, they have a single pair of eyes rather than the spider's usual eight, and they possess neither silk glands nor venom glands. Their chelicerae are small pincers used to tear food apart.
Because they cannot subdue prey with venom, harvestmen eat differently from spiders. Many are omnivores and scavengers, taking small soft invertebrates, fungi, decaying plant matter and droppings, and swallowing solid particles rather than liquefying a meal externally. Defence is chemical instead: paired scent glands on the body release quinones and phenols that repel ants and spiders.
One source of confusion is the name. Daddy longlegs is also used for cellar spiders in the family Pholcidae and for crane flies, and cellar spiders genuinely are venomous spiders, though not dangerously so. The myth appears to be a claim about pholcids, transplanted onto harvestmen and then exaggerated.