JANUARY
2012

 

By
Tom Turpin
 
Professor of
Entomology
Purdue University

 

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These Insects Rob
for Food, Not Money


Human history has had its share of infamous robbers. In the United States, Jesse James, Bonnie and Clyde, and John Dillinger come to mind. England was home to Robin Hood and Dick Turpin; individuals who it is said sometimes helped themselves to the money of others.

In the interest of full disclosure, to my knowledge, the English outlaw Dick Turpin is not one of my ancestors. While being related to a legendary robber might not seem to be a good thing, in this case, it does have its perks. People with the surname Turpin are sometimes given a free drink at Dick Turpin British pubs!

The insect world also harbors a band of robbers. Called robber flies, these insects are just as ruthless as robbers of the human kind. But robbers of the insect kind are after food, not money.

Robber flies are members of the fly family Asilidae. This is a large group of flies with some 7,000 species identified worldwide. All are predators on other insects. In general, robber flies look mean. The top of their head is sunken, which makes their large eyes - with four thousand lenses - even more prominent. They also have a long beak that protrudes from a mass of bristling hairs and creates a look similar to that of a bearded man.

Robber flies are aerial predators. Like birds of prey, they hunt from the wing. The flies capture prey insects in their long legs. Once a capture is made, the robber fly will alight and use its sharp beak to puncture the body of the prey. The helpless insect is then injected with digestive enzymes that liquefy the innards of the prey so that the robber fly can suck up a meal.

A robber fly captures all types of flying insects, even those that are larger than the predator itself. All kinds of insects become food, including other flies, mosquitoes, tiger beetles, bumble bees and even Japanese beetles.

Humans consider some species of insects hunted by robber flies to be pests. Therefore, robber flies are generally regarded to be beneficial. But not always. As it turns out, robber flies willingly make meals out of one of our favorite insects - the honey bee. Needless to say, that doesn't set well with beekeepers when they discover a pile of bee bodies under a tree branch near a hive. The mound of bee carcasses marks the spot where a robber fly repeatedly perched to dine upon a bee captured while flying in or out of the hive.

The robber flies are such consummate predators of other insects that they will capture and make a meal of their own species. And at times, female robber flies behave very much like female praying mantids. Females of both of these predatory species sometimes make a meal of potential mates right in the middle of a romantic advance.

Immature robber flies are found in decaying wood, leaf litter or in high organic soil. Like their parents, juvenile robber flies are predators. The immature robber flies primarily feed on immatures of other insects, including grubs of beetles. This means that robber fly kids and their parents both help keep other insect populations in check.

While many robber fly adults are colored a rather drab grey, a few species resemble the species upon which they prey. For instance, one robber fly species looks very much like a wasp. Another resembles a bumble bee. Such a color pattern is called aggressive mimicry. Similar to a Trojan horse, these predators are able to get close to prey that they resemble without frightening the prey.

Just like their famous human namesakes, robber flies are able to steal what they need. They are accomplished thieves. But unlike Jesse James and Robin Hood, robber flies are interested in food, not filthy lucre.

 

Writer: Tom Turpin
Editor: Olivia Maddox