The Robber-Flies 



spondents sent in an interesting account of the damage done to 

 his apiary. The robber-fly captured bees by making rapid 

 dashes catching them on the wing, then wrapping its legs about 

 the bee, and pressing it tightly to its own body it immediately 

 sought a bush or tall weed upon which to alight and devour its 

 prey, piercing a hole in the body and sucking out the fluids and 

 soft internal viscera, leaving only the hard outer skin. Upon the 

 ground beneath some favorable perch for the fly near the apiary 

 hundreds of these shells of bees were found accumulated in a 

 single day. The correspondent thought that it was through the 

 work of the robber flies that during certain seasons in a bee rais- 

 ing region in New York not a single 

 hive threw off a swarm. The beak 

 of a robber-fly is so strong that it 

 can pierce the skin of a human being, 

 but fortunately none of these creatures 

 has yet acquired the habit of feeding 

 upon warm-blooded animals. 



Some robber-flies are very delicate 



Fig. 81. — Erax apicalis. , , J 



(After Comstock.) and slender, as in Leptogaster, some of 

 them being almost as slender as midges, 

 upon which they probably feed, and looking half starved, in 

 spite of their voracity. The colors of the robber flies are variable, 

 but nearly all are very hairy or bristly or spiny. In this family 

 some good cases of what is called " aggressive mimicry " are seen, 

 aggressive mimicry meaning a resemblance of a predatory insect 

 to the insects upon which it feeds, thus facilitating the capture 

 of its prey. We should not fear grizzly bears if they looked like 

 harmless, peaceable human beings. Thus the robber-fly known 

 as Deromyia annulata Bigot, looks like the common wasp 

 Polistes metricus Say; while some of the flies of the genera Dasyl- 

 lis and Mallophora resemble bumblebees. In some of the latter 

 the hind shanks are modified so as to look very much like the 

 pollen-bearing hind legs of the bumblebees. This curious struc- 

 tural modification can be of no service to the fly except in in- 

 creasing its resemblance to the bees. Then also, as another 

 illustration, one of the robber-flies of the genus Laphria resem- 

 bles a big wasp of the genus Vespa. 



The larvse of the robber-flies much resemble the larvae of the 

 gad-flies, although the adults are so widely different. They live 



142 



