An adult caddis fly. Photo credit: Malcolm Burrows.

An adult caddis fly. Photo credit: Malcolm Burrows.

Malcolm Burrows is always on the look out for intriguing insects, especially if they jump. From fleas and locusts to praying mantises, Burrows has captured the elegant acrobatics of species from across the globe with his high-speed camera, detailing every element of their graceful launches. Some of the species that he has scrutinised have even come from his back garden. ‘I set up a light and an old white bed sheet on my washing line to attract moths,’ says Burrows, who was intrigued when the caddis flies that had blundered into his trap began jumping to escape as he attempted to collect them. ‘We could find no previous reports of this behaviour,’ Burrows recalls, so he and Marina Dorosenko transported the insects to his laboratory at the University of Cambridge, UK, to learn more about their jumping technique.

Unfortunately for Burrows, the caddis flies were less cooperative once in the lab. ‘[We had] to get them to jump in the appropriate direction so that we could get high-speed videos… that were in focus and that would enable us to understand the sequence of leg movements’, he explains – adding that the insects had a tendency to escape, ending up lodged in light fittings and other nooks. However, after months of patiently encouraging the insects to launch themselves into the air, Burrows and Dorosenko successfully captured 90 take-offs from three species ranging in size from 4.5 mg up to 70 mg.

Scrutinising the movies, the duo could see that the insects used two strategies to get themselves into the air. In the first, the caddis flies began pushing down with both rear pairs of legs 10 ms before take-off, lifting the front legs off the ground 5 ms into the launch preparation. However, Burrows and Dorosenko were surprised to see that even though the rear legs were longer than the middle legs, they lost contact with the ground first, leaving the middle legs to push down and provide the final thrust. The duo was also intrigued to see the caddis flies deploy their wings during a third of the leaps, spreading them wide before partially depressing them while the legs prepared to push off – although the wings were not fully depressed at lift off, probably to avoid crashing into the ground.

So, how do the insects power these leaps? Burrows explains that jumping is physiologically challenging because animals have to produce large amounts of power at high speed. However, muscles cannot satisfy both demands simultaneously. To resolve the paradox, some insects slowly store energy in catapult structures in the exoskeleton – which is released explosively at take-off – while others coordinate several muscle-powered limbs to push off more gently. Wondering which mechanism the caddis flies use, Burrows and Dorosenko calculated the power required for the take-off and found that it ranged from 226 to 343 W kg−1 – well within the range of muscle. So, the caddis flies power their launches by direct contraction of their leg muscles.

But why don't caddis flies resort to using an explosive catapult mechanism when it could give them a faster take-off? Burrows suspects that the answer could lie in the insect's environment. Explaining that catapult-powered take-offs exert high forces on surfaces during take-off, Burrows suggests that caddis flies may have opted for the more sedate muscle-powered take-off, to distribute the take-off forces over more limbs and longer times, and allow them to take to the air from more flexible structures, such as the leaves and petals of plants that skirt the watercourses where they lay their eggs.

References

Burrows
,
M.
and
Dorosenko
,
M.
(
2015
).
Jumping mechanisms in adult caddis flies (Insecta, Trichoptera)
.
J. Exp. Biol.
218
,
2764
-
2774
.