Robber Fly - Laphria grossa
Family Asilidae - Robber Flies

Live adult fly photographed at DuPage County Illinois. Size: 30mm.  Leaf beetle prey size: 10mm
Robber Fly - Laphria grossa
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This lucky gal has caught a clay-colored leaf beetle, and is busy sucking down its liquified insides through its sabre-like proboscis.  When she flew by me carrying this large beetle, she was making one very loud, low-pitched buzz. It's a marvellous sound, and these large robbers fly slow enough they are easily followed. Sometimes they'll even sit still and pose, if you approach slowly and carefully you can get within a few inches. And that's a thrill for me. I lose all track of time while stalking a big fly like this!
Robber Fly - Laphria grossa
HULL (1962): "The ROBBER FLIES, or ASILIDAE, comprise one of the largest and most abundant families of present-day insects. Distributed through all parts of the world, over 400 genera [now 530] and subgenera have been proposed and about 4,761 species are known [now 7,003]. In addition, 18 genera and 39 species have been described from Eocene, Oligocene, and Miocene; 15 of these genera are also recent [1] .

"Because of their predatory habit of feeding on other insects and their voracious appetites, they contribute to the maintenance of the natural balance among insect populations. To some extent, parasitic wasps and flies are taken by them, but much of their prey consists of plant-feeding insects. Certain species are known to seriously deplete the populations of apiaries. The adults are, with few exceptions, active flies, of considerable size and readily attract attention." [1]

Male Bumble Bee
Male Bumble Bee
Robber Fly - Laphria thoracica
Female Laphria grossa
Some robber flies are convincing bumble bee mimics. This evolutionary adaptation allows them to reap the benefits of aposematic (warning) coloration without the high metabolic cost of constructing and maintaining a venom-delivery system.

References:
1.  Fritz Geller-Grimm, "Asilidae"
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Flies of North America - Order Diptera. Flies are prevalent in virtually all habitats, with over 16,000 species in North America. Flies can be distinguished from all other insects in that they only have one pair of normal wings. The other pair has evolved into small ball-like structures called halteres. Most flies have compound eyes and mouthparts adapted for piercing, lapping or sucking fluids.
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