Many insects are predators on other insects. Dragonflies and
damselflies (order Odonata) come to mind immediately, as all of them eat
smaller insects and spiders. But put them up against robber flies (order
Diptera, family Asilidae), and they have not only met their match but been
bested with ease.
Robber flies eat dragonflies and damselflies regularly, but
there are almost no records of odonates turning the tables. One type of
predator is clearly superior to the other. I have seen robber flies take
insects from most orders, including their own. Size is no limit, as an
inch-long robber fly can latch onto a flying dragonfly three times its size and
bring it down to the ground instantly with a paralyzing bite. Presumably if the
fly was captured, it could do the same thing to its captor.
The two wings of a robber fly are narrow but strong, and
they propel their owner through the air with an audible—sometimes impressively
loud—buzz. Their flights are usually short, and when you hear that buzz you can
often find its source resting on a branch, rock or the ground. They usually
perch right out in the open, again like a dragonfly, where they can see
potential prey. They have relatively large, forward-pointing eyes as befits a
predator.
Many insects are poisonous and distasteful and brightly colored to advertise their unpalatibility. This adaptation must be against birds, because robber flies freely feed on such insects, as do dragonflies.
Robber flies are very bristly. The legs have long, sharp
spines to hold onto the prey, much as in dragonflies. The face has a dense coat
of bristles, called the mystax,
presumably to protect it from the legs and mandibles of struggling prey (but
it’s a sure thing that they don’t struggle for long).
Many robber flies are sleek and pointed at the rear, the jet
fighters of the insect world. Others are fat and fuzzy, very effective mimics
of bumble bees but just as effective as predators. They have been called aggressive mimics, mimicking their
favored prey species to get close enough to make a kill.
Fly larvae are legless and look like maggots, and robber
flies are no exception. Slim and pointed at both ends, at least some of them feed
on the larvae of other insects, usually in rotting organic material such as
logs and dead trees or in the soil. Surprisingly little is known about the
larval life of this group, however.
With over 7000 species in the world, robber flies are
diverse on all the continents. They are relatively uncommon in the wet western
lowlands of the Pacific Northwest, just as many groups of insects are less
common in our cool, cloudy summer climate. Head across the Cascades to see a
lot more of them in the dry, open areas that they prefer.
Robber flies are easily observable, as they are fairly tame,
but capturing one in an insect net and looking at it closely allows you to
appreciate its adaptations even more. Be cautious, however, as their bite can
be painful. I know enough about their adaptations that I have never allowed one
to bite me!
Dennis Paulson
