Predator Morphology - Diptera: Asilidae compared with Odonata

Last updated: 20 Nov. 2002

Right: A robber fly in the genus Diogmites hangs by one leg and uses the other five to grasp a teneral dragonfly (a female Sympetrum vicinum, Yellow-legged Meadowhawk).  Photo used with permission of the photographer, Thomas Arendt.  The picture was taken at the Tri-County Fish and Wildlife Area in Kosciosko county in northern Indiana

I am interested in the functional morphology of insects. Two groups of insects that I study are the order Odonata (dragonflies and damselflies) and Order Diptera, Family Asilidae (robber flies). The adults of both of these groups are predatory insects that take their prey in the air. Both have some species that feed by perching on vegetation or the ground, watching for prey to fly by, and then swooping up and capturing the prey, taking it back to the perch to consume it (however, some Odonata eat their prey in flight). 

The Odonata consume their prey by chewing it (see the picture above right - click on it to enlarge the image - the damselfly has her mouth open very wide and is holding her prey in her jaws.  The coenagrionid damselfly is a female Enallagma clausum).  The Asilidae pierce the prey's body, inject saliva, and suck the dissolved soft tissue from their prey (see the picture at left - click to enlarge the image - the robber fly is hanging by its fore legs while manipulating the bee with its mid and hind legs so that it can insert its proboscis). While the feeding mechanisms are quite different, the methods of locating and capturing prey are similar. Both groups have very large compound eyes, very large thoracic muscles which power their wings, and legs with spines that help them to grasp and hold their prey. Both also tend to have long abdomens. The thoracic structures are quite different in the two groups. The first picture shows the thorax and head of a robber fly. Note the hump-backed appearance of the thorax, which is swollen and filled with muscle. The legs are huge, and are located directly beneath the wings.

The second figure shows the thorax and head of a damselfly. In this case, the thorax is skewed so that the wings are located behind and the legs forward; rather than a thick, massive thorax like that of the robber fly, the damselfly has evolved a long, stretched-out thorax, with long flight muscles.

As you look closely at these two pictures, you will see other similarities and differences. It is interesting to think about how and why these differences and similarities evolved. This page will be expanded in the future with more information and speculation about the shape and function of these insects. Email your thoughts and comments to me: Roy J. Beckemeyer.

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