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Friday, July 26, 2002


Robber fly

* Japanese name: Shioya abe
* Scientific name: Promachus yesonicus
* Description: Robber flies are robust insects with powerful legs. The face of the fly is covered in a dense "beard" of bristles, and there is a deep groove between the eyes. There are many species of robber fly (almost 5,000 worldwide), but despite their strength and size (shioya abe are 23-30 mm long) they are harmless to humans.
* Where to find them: All over Japan, from June to August. Robber flies are best seen in open, sunny habitats, grassland, bushes, valleys and open woodland glades. Flies often perch where they can get a good view of passing insects -- their potential prey. They are most active in the hottest part of the day.
* Food: Other insects, usually plant-eaters. Robber flies are voracious predators and are sometimes called bee-catchers because they will attack even bees and wasps. They often capture and kill insects much larger than themselves, as can be seen in the photo, and this is why robber flies have such bristly beards: They protect the fly's eyes from the kicks of the struggling prey. On seeing a flying insect, a robber fly will swoop out from its perch and (unless the prey is much too big) catch it with its strong legs. With its pointed mouthparts, the fly then stabs the prey -- either in the "neck" (between the head and thorax), between the thorax and abdomen, in the eyes, or (ouch!) up the end of the prey's abdomen. Enzymes are injected to immobilize the prey and liquefy its tissue. The fly is soon able to suck out the juices.
* Special features: Robber flies are ecologically important insects because their appetites keep the numbers of other insects in check. But the larvae can be as voracious as the adults. In China, they are used as bio-control agents, to kill the larvae of scarab beetles that damage wheat crops. Adult male robber flies have an interesting courtship display: They hover in front of the female, displaying a fringe of hairs on their hind feet. They also raise their abdomen and reveal white-haired genitalia.

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