Harvestmen display an unusual ability to cluster in large numbers. Some experts believe this activity may occur as a defense mechanism against predators and accomplishes several different things. For one thing, daddy long legs excrete chemicals that put off a “distinctly, unpalatable smell,” from special glands in their bodies. In a large group, the effect is much more pronounced and pungent, working to repel larger predators, such as birds.
Another more obvious reason harvestmen gather in large groups revolves around the illusion of looking bigger as a mass in warding off large predators. When some harvestmen species become agitated, they are capable of rocking up and down in an undulating, rhythmic motion. When large groups of harvestmen perform this choreography, the effect can be quite disconcerting to predators. Additionally, the entire mass can move as a synchronistic unit, traveling in a wobbling motion, scaring predators and humans as well.
Nocturnal animals, daddy long legs are active at night and may form groups during the daytime while they are resting because they lack the silk-making ability required to make webs and they do not burrow. Disturbing a mass like this in the daylight can cause harvestmen group reactions that can be quite spectacular as well as offensive to the olfactory systems of humans and animals.