Every year about the middle of April, depending on the temperature in southern Arizona, eggs begin to hatch on mesquite trees and small, strange, blue bugs appear in large numbers. People don't notice these tiny insects until sometime in May, when they appear as inch-long, red- and white-banded insects with antennae that have an enlarged segment toward the tip.
These odd-looking beasts are giant mesquite bug nymphs, Thasus neocalifornicus.
The term "nymph" is used to describe the juvenile growing stage of insects that develop wing pads externally during such a period. The brilliantly colored bugs stick together because their advertising is more effective in large groups. This seemingly dangerous exposure is nature's way of warning predators that these insects have some kind of defense. Each nymph has glands that produce a smelly substance normally repugnant to predators.
The giant mesquite bug lives and feeds in the top of the mesquite tree but moves toward the base of the tree as temperatures rise. The bugs are harmless to plants and people. They don't bite people because their mouthparts are designed for feeding on the mesquites. (We wouldn't taste good to them, anyway.) Their smell may not be your perfume, but it is not dangerous.