Dragonflies and other ‘flies’

| Updated on September 12, 2014

Photo: Ambika Kamath

What makes insects capable of catching prey nearly as large as themselves?

Most of the creatures I pick to be featured in this column are ones that I’ve seen somewhere, creatures that give me a reason to find them fascinating — they may thrive in an unexpected place, exhibit unusual behaviour, or interact interestingly with another organism. Once I zero in on the subject, I sift through scientific literature to see if others have found the same animal compelling for reasons similar to mine. I usually find what I’m looking for; this time, however, my search came up more or less empty.

I wanted to write about an interaction between a robber fly (an actual fly, cousin of the common housefly) and a dragonfly (not a fly at all, since it has four wings instead of two like the flies). It was a decidedly one-sided duel — clasped between the legs of the robber fly was the corpse of a dragonfly nearly double its own size. The robber fly seemed to struggle with its oversized prey, pausing on a tree trunk to reposition its load, like a harrowed traveller hauling too large a suitcase across the airport. As I stopped to photograph the duo, I wondered at the robber fly’s hunting prowess. What makes it capable of catching prey larger than itself?

The robber fly’s feat is made all the more impressive by the fact that dragonflies are no sluggards. Just a few weeks earlier, I had seen a dragonfly catch and feed upon another equal-sized dragonfly. What makes this insect capable of catching prey almost as big as itself?

Unfortunately, we don’t really know how robber flies or dragonflies vanquish large prey. But we know quite a bit about how dragonflies hunt smaller prey, specifically fruit flies (also ‘real’ flies). Dragonflies outperform fruit flies completely — they fly faster, speed up and slow down quicker, and usually the fruit fly doesn’t even realise it’s being hunted. But despite their sniper-like precision, even dragonflies can fail. We learn a lot about dragonflies from these failures, from fruit flies that evade capture.

How on earth does a fruit fly manage to outwit a dragonfly? It doesn’t, and it doesn’t even try. Most fruit flies that escape from dragonflies aren’t doing anything out of the ordinary. When flying around to explore their environment, fruit flies routinely make erratic turns. And while the fruit fly may have a perfectly good reason for turning, the dragonfly doesn’t know it and can’t predict it, thereby leading to a failed predatory excursion.

While it’s possible that dragonflies choose only to chase fruit flies whose turns they can predict, it seems to me that whether a fruit fly lives or dies in the jaws of a dragonfly depends a lot on chance — the chance that a fruit fly will take a sudden turn just as a dragonfly swoops down towards it. And I guess chance also drives those more noticeable hunting encounters between dragonfly and dragonfly or dragonfly and robber fly. Though ‘chance’ is not a satisfying answer to this or any question really, it’s an answer, one must accept, that is often true.

Ambika Kamath studies organismic and evolutionary biology at Harvard University

Published on August 08, 2014

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