The monsoon, which has been hit-or-miss so far this year, was very active the last several days, culminating in choking humidity this morning before dawn. I thought I saw rain falling, and briefly opened the sliding door, but smelled no moisture. When it began to get light, I saw that something indeed was falling: thousands of small insects!
I have seen these creatures before. They are flying ants--the sexually mature members of a tribe of harvester or leafcutter ants--which form huge, rotating mating vortices up to twenty feet high. They are very like dust devils, made up entirely of mating insects. They are often thick in the neighborhood at the height of the monsoon. This is the first time I have ever seen a mating cloud form directly above my little patio, giving me a front-row seat.
At one point the fallen ants were so thick you could scarcely see the brick floor of the patio.
My cats stared in disbelief, occasionally batting at an ant that had somehow made it inside the house. These photos give only an approximation of how intense this event was. When it got a little lighter I went out front and took a video of the mating vortex, showing hundreds of insects falling, while others continue to join the swarm and rise into the air.
By later in the morning most of the ants had disappeared. I didn't see anything eating them, but in the past I've seen birds congregate at the bottom of a swarm to feast.
I found an excellent description of the ant mating swarms, with great closeup photos, on the “Tortoise Trails” blog, which is written by a fellow Tucsonan named Pam: Flying Ants. In it, Pam explains how these mating vortices form, and what happens to the ants afterward. (Spoiler: they couple and fall to the ground still mating; the males soon die and the females go off to form new colonies. I got an excellent look at this in person this morning, though did not see any of the spent couples sharing a tiny cigarette.)