Definition: small modified wing structures which aid flight control in true fly species
Flies can seem both incredibly stupid and incredibly smart. They buzz inanely around the room, intermittently bashing into window panes. Yet they seem to have a preternatural ability to avoid being swatted. Before your hand gets anywhere near, the fly somehow veers off at an unlikely angle and carries on infuriatingly as if nothing happened.
How can flies be so agile? Well they have their halteres to thank for being able to fly another day. Halteres are small drumstick-shaped structures which stick out from the sides of the thorax. They are sense organs that detect body rotation and orientation in space, filling the same role as the liquid in our inner ears or an aircraft’s gyroscope. Indeed flies without working halteres are pretty unworthy of their name, being barely able to stay in the air—we know this because scientists have tried it.
Halteres are specialised features of the ‘true flies’ or Diptera family which includes house flies, hover flies and fruit flies. They evolved from the back wings of these insects, turning into tiny, rigid rods. In normal flight the halteres beat with the same frequency as the front wings, but in the exact opposite phase. So when the main wings sweep up, the halteres point down and vice versa.
Any changes in rotation will be detected by the halteres. If the fly gets buffeted by a sharp gust of wind, the halteres sense the direction and extent of its tumble. They then send this information directly to the wing steering muscles, which do what needs to be done to put the fly back on track.
This reflex is important because at 200 flaps-per-second, the fly’s tiny brain simply can’t actively control each wingbeat. Bypassing the brain in this way allows the fly to devote more attention to finding the next mate or food morsel, rather than something trivial like not plummeting out of the sky!
- There is a small group of insects called the strepsiptera which also have halteres, but the other way round. They evolved from the forewings, leaving the back wings as the main flight wings. They are hard to classify but are not directly related to the Diptera flies.
- The family name ‘Diptera’ means ‘two-wings’, unusual given most insects have four wings. Many insects that appear to have two wings actually have four. Bees and butterflies have their front and back wings hooked or glued together giving the appearance of two. In beetles, the front wings (called elytra) have evolved into a tough, coloured coating for the hidden flight wings beneath.