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Some flies have specialised hindwings to help them take off faster, making them harder to swat.

Many flies can be notoriously hard to catch. They manage to dodge incoming hazards by taking off from a standing position in a fraction of a second.

They primarily use sight to escape danger, but Alexandra Yarger at Case Western Reserve University, Ohio, and her team have found a new mechanism that might be helping them get away.

All fly species have shortened hindwings called halteres. These don’t generate useful lift, but are used as sensory organs for balance to help stabilise the insect while in flight.

A group of flies known as Calyptratae, which includes houseflies and blowflies, rhythmically move these wings when standing.

“We know that they’re the only group that does this,” says Yarger. “It’s still a bit of a mystery why they do it.”

Yarger and her team tested to see if this behaviour affected their take-offs. Using high-speed cameras to film the flights of over 20 fly species, they found that, overall, Calpytrate flies were roughly five times faster at taking off than other flies. The team then removed the halteres and found that both speed and stability of take-offs reduced in Calyptratae species.

Yarger suggests this haltere movement increases the amount of sensory information these flies receive, but what they can sense and how it is processed remains unclear.

“We think there might be a pathway from halteres to the legs that’s causing them to take off faster,” says Yarger. “It doesn’t go through any central nervous system, it’s almost like a reflex,” she says.

Being able to have a speedy take-off allows this group of flies to better avoid harm.

“It’s part of the reason they’re so successful, they can escape very quickly,” says team member Jessica Fox. “Transitioning from taking off to flight is a challenging thing and using halteres to help both is clearly very advantageous,” she says.