Harvestmen are not spiders. Daddy-longlegs can be confused with spiders in the Family Pholcidae - the long-legged or cellar spiders. Pholcid spiders build webs but harvestmen do not.
With harvestmen, the head and thorax is broadly joined to the abdomen so the body appears rounded or oval like it was one piece (spiders have two distinct body regions - the head-thorax and the abdomen). Harvestmen only have one pair of eyes (most spiders have 4 pairs of eyes).
I did see a large aggregation of daddy-longlegs at the Caverns of Sonora on a visit in the middle of the summer, but the exact purpose of this was unclear. I have also seen daddy-longlegs "bounce" their bodies when disturbed.
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