Location, Location, Location

Yesterday, UK photographer Paul Bunyard (@wildaboutimages) ran into a massive mating swarm of chironomid midges (family Chironomidae) while travelling through the Norfolk countryside, and shared some amazing images on Twitter (reproduced here with his permission).

After recording the spectacle, Paul had a great question:

https://twitter.com/wildaboutimages/status/202873537147584514

Much like we use landmarks for finding locations where we might attract a mate (i.e. neon signs disclaiming the availability of cheap beer and live music Thursday nights), many flies will also meet at specific landmarks. Some flies choose landmarks which rise the highest above the horizon, like a tall tree or the top of the highest hill, while other species congregate above recognizable patterns, like a grey slab of asphalt among a sea of green foliage. In many cases, a local population can become extremely “loyal” to their “hangout”, with generation after generation of flies returning to the same tree or section of road year after year to find mates of their own.

There are other factors which influence swarming locations and behaviours, including wind direction and strength. In order to remain in place above the landmark, many flies will orient themselves to fly into the wind, a process termed anemotaxis, and then match their flight speed with the wind speed by visually observing stationary items, like the ground or trees, around them¹. If the wind picks up too much, swarms will retreat into sheltered areas, like below the lip of the roadside ridges in Paul’s photos. Many swarms will occur where there are updrafts or potentially convection currents, which may further explain why this swarm was over the road; as the road heats up, the hot air rises, buoying the small flies up and reducing the amount energy they need to use to remain aloft, allowing them to remain in the swarm for longer. Other conditions which impact swarm formation and size include the time of day, size of the landmark, and whether there are even females around and willing to mate, all of which combine to make a swarm of this size something special to see.

Many species are so particular about their swarming landmark that closely related species can live together and still remain reproductively isolated. For example, two species of Cricotopus midge which live in the same area swarm over totally different objects: one species hanging out over local shrubbery with the other preferring light coloured objects in grassy areas². This can lead to potentially confusing situations as a taxonomist, especially if the species look similar or have nearly identical genetic sequences. Without knowing their natural history and mating behaviours, an entomologist studying dead, pinned specimens in a natural history collection may inadvertently mistake one species for another, or fail to realize the true diversity of their research subjects! This is one of many reasons why observational field work and an understanding of a species’ natural history are so important to taxonomy, even with the availability of “modern” molecular techniques.

But lets forget about identification headaches for now and enjoy Paul’s mesmerizing video of the swarm.

 

Amazing.

Thanks to Paul Bunyard for allowing me to use his images here. You can see more of his photography here.

——————————

1- Gibson, N.H.E. (1945). ON THE MATING SWARMS OF CERTAIN CHIRONOMIDAE (DIPTERA), Transactions of the Royal Entomological Society of London, 95 (6) 294. DOI: 10.1111/j.1365-2311.1945.tb00263.x
2- Sullivan, R.T. (1981). Insect Swarming and Mating, The Florida Entomologist, 64 (1) DOI: 10.2307/3494600