While walking my dog on a warm autumn day, noticed some insects buzzing around in a cloud and wondered what they were up to.
I didn't catch any, but contacted Karen Needham, the curator of the Spencer Entomology collection at UBC for hints. She told me they were probably mating swarms of chironomids, which resemble mosquitoes but don't bite. Over 760 species of chironomid take wing in North America and these comments are for various species, so your mileage may differ.
Chironomid larvae live in water. Some have hemoglobin and are called bloodworms, much prized as fish bait. During warmer seasons, adults can emerge at various times but seem to synchronize this, possibly through changes in weather patterns. At lower temperatures, they swarm when it is brighter in the day, and when it is warmer, darker.
Chironomid adults live only a few days and so they need to get busy in a hurry. The clusters I have seen comprised maybe a few dozen individuals, but in some places, they can number in the thousands or even millions, and cause traffic accidents when their tiny bodies get smeared on windshields.
Swarms tend to form near some prominent point or something conspicuous and often light-coloured, including cars, furniture or people. The shape of the swarm tends to be egg-shaped, a little taller than wide and a little thicker at the bottom than the top.
Some scientists used high-speed video cameras and some specialized software to study the movements of individuals in a swarm. These midges are not just random particles, though unlike flocking birds or a school of fish, they do not align with neighbours. Still they demonstrate collective behaviour, as subgroups of individuals can form within the swarm. The speed and spacing of individuals tends to even out after about ten individuals have gotten together. They tend to fly faster horizontally than vertically and on average tend toward the centre of the swarm. Studying how properties of swarms emerges from the motion of individuals could have applications to drones and other robots.
The swarm is primarily all males. A female can find a swarm more easily than she could an individual, and the swarm seems to be some kind of stimulus in itself. When the amount of swarming is reduced experimentally, less mating and less oviposition takes place. When a female approaches, males detect her wing beats, possibly with their big fluffy antennae. But sound isn't totally reliable and males often pair up with other males by accident. And if you clap your hands or speak near a swarm they might try to mate with you. A smaller swarm gives better odds for a male to find a mate and the smallest males occur in the smallest swarms. But predators are also attracted to swarms and being in a smaller swarm also increases the chance of getting eaten. And you thought human dating was tough.
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