The hairless stems of this vicia americana (American or Purple Vetch) are under full-scale attack by pestiferous aphids, who may have been aided by ants who transport, protect and milk the aphids for the honey they excrete when properly tickled. But an insect bigger than the local ants, some sort of spider, is on the scene, and looks to be gobbling up the juicy little parasites. Judging by the size of the spider’s belly, it’s had a few already. But it’ll need help to save this plant, with almost every square millimeter covered by the sapsuckers. Lady bugs also eat aphids, but none were in view when I visited.
I couldn’t identify the spider. Luckily, a week later I ran by chance into emeritus Prof. Robert Lane at the park, an entomologist, who cautioned that he was not a spider person, but thought that this was a harvestman or “daddy longlegs.” He checked with a colleague who is a spider specialist, Dr. Rosie Gillespie, who confirmed the ID. Consulting Wikipedia, I find that harvestmen are actually not spiders at all. They are opiliones. They’re arachnids, as are spiders, but otherwise they’re not closely related. The most obvious difference between harvestmen and spiders is body structure. The harvestmen, like the one in this photo, have a single fused body, while spiders have a narrow waist that joins separate abdominal and head regions. Less obvious is that harvestmen have only one pair of eyes in the middle of the head, while spiders have three or four sets of eyes on the edges of the head.