Harvestmen (opiliones) are arachnids, but are not spiders.
They do not have a two segmented body like a spider. The family
Phalangiidae are commonly called daddylonglegs. These mercenaries
of the arachnid order will eat mostly anything. They can be beneficial
in rock gardens which are common habitats for snails and pillbugs and the like.
However, in the absence of crittters to eat they will nibble on plants too.
More information visit: http://insects.tamu.edu/fieldguide/cimg369.html