Or rather, when various insects are attracted to mercury vapor lights at night in large numbers.
The Arizona desert is amazingly productive during the summer rainy season. During our trip, one night in particular we were overwhelmed with insects. Mercury vapor lamps (highly attractive to nocturnal insects) and sheets had been set up, and when we checked them a few hours after dark, some of the sheets could hardly be seen!
The photo on the left is me, being "attacked" mostly by sphingids (hawk moths). They flew around the lights at such high speeds, they ran into us with a considerable amount of force. It wasn't just the occasional bump, though, but a constant barrage of flailing and fluttering insect life. Moths of all sizes, beetles, true bugs - many of them finding their way under clothing or into eyes and ears. As much as I love insects and handle them frequently, there was something unnerving about becoming the substrate for dozens of tarsal claws to grip and crawl across.
Through the fracas we managed to accomplish some collecting, and emerged mostly unscathed (at least one blister beetle burn was reported). Multitudes of insects were shaken out of clothing and hair before returning to the vans, and still more were discovered when preparing for bed that night.
While other collecting trips produced impressive displays of invertebrate life, none were quite like that night. I hope to be lucky enough to have this experience again.