Hanging flies are unusual insects that often get overlooked. They aren't really flies at all. In fact they don't even look like flies. They are about an inch long and thin, and look a bit like large crane flies but they have four very obvious wings. Flies, of course (including crane flies), only have two wings.
Hanging flies are members of the Mecoptera, an order of insects commonly known as scorpion flies. This name is derived from certain species (in the genus Panorpa) where the males bear what looks like a stinger at the end of the abdomen. Hanging flies, however, do not have this threatening terminus and very often go undetected by even well-informed naturalists. This is unfortunate. They are fascinating insects to watch.
One easy way to tell them apart from crane flies or even damsel flies is to look at the head. Hanging flies (like most other mecopterans) have a pointed head. This picture is of Bittacus chlorostigma that I took earlier this year (in April) along the Merced River in California. It is a fairly common insect (if you know what to look for) in the spring throughout the open oak woodlands of the Sierra Nevada foothills. I have found them most commonly when the California poppies are in full bloom. This particular individual was out when a redbud was flowering a few feet away. The yellow spots (stigmata) at the end of the wings are very diagnostic, even if you can't get a good look at the head.
Hanging flies are best known for their unusual mating ritual where the males capture a prey item (usually a fly or small moth), hang from a small branch (or long blade of grass) with their forelegs, and then release a pheromone that attracts females. This is unusual among insects that use pheromones in that it is the male and not the female that sends the signal. Once a female finds the gift and accepts it, the insects couple. If the gift is too small the female looks elsewhere.
Most of the research that has been done on this interesting behavior comes from the Eastern and Midwestern species of Bittacus. The common California species (B. chlorostigma) is largely un-represented in these kinds of studies. A paper published in 1977 suggested that it was a nocturnal species because so little was known about it. This is certainly not true. I saw a pair exchanging gifts in April of last year above Pine Flat Reservoir. It was mid-morning and the pair was very approachable (unfortunately, I didn't have a good camera with me at the time). Some enterprising student might take this species on as a research project. They shouldn't be hard to find and they are quite attractive.