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UNITED STATES NATIONAL MUSEUM BULLETIN 224 



evergreen forest of the northern hemisphere. James 

 (1938) gives a good discussion of asilid habitat prefer- 

 ences for the State of Colorado. 



It is well known that these flies have certain resting 

 or observation sites and greatly preferred ones. I re- 

 cently made use of such a post to capture a series of 

 Atomosia puella Wiedemann. The site was the black 

 trunk of a wild-cherry tree some 50 yards from a house, 

 and with other trees nearby. Over a 2-week period I 

 found from three to six flies on that part of the tree 

 trunk directly in the sunlight and extending from a foot 

 above ground to some seven or eight feet above. As 

 fast as the immediate population was captured, others 

 within the space of a few hours would move in from an 

 unknown source ; 54 flies were caught there. These flies 

 were not present on nearby trees or outbuildings. Melin 

 (1923) records an instance of what he considered the 

 ability of a robber fly to modify an instinctive behavior 

 pattern to suit circumstances. 



We do not know the limiting factors in population 

 size in asilids. Lack of available insect prey, as far as 

 the adult insects are concerned, is perhaps infrequently, 

 maybe never, a factor. It is possible that the larvae, 

 about which we know little, may find food scarcity a 

 limiting factor. A rather strong contributing factor 

 may be inept behavior or low ability of the individuals 

 in some genera to capture prey. Melin (1923) and 

 Efflatoun (1937) both comment interestingly on the 

 numerous, unsuccessful attempts of genera such as 

 EpitHptus Loew and Cerdistus Loew, to capture prey. 



Melin (1923) describes the mating habits of a num- 

 ber of species in this family. He found that prenup- 

 tial behavior is rather limited and this coincides with 

 my own observations. I once watched Promachus 

 bastardi Macquart hover behind a female selectively 

 for a brief period before mating. The most highly 

 developed courtship reported in the family is the spe- 

 cies Heteropogon lautus Loew, as described by Brom- 

 ley (1933), and of species of Cyrtopogon Loew, as 

 reported by Wilcox (1936). In the species of Hetero- 

 pogon Loew the front tarsus is decorated, and many of 

 the American species of Cyrtopogon Loew, in contrast 

 to European species, have elaborate, tarsal ornaments. 

 Waving of the ornamented tarsi, accompanied by wing 

 movement and at times a low buzzing sound are the 

 chief movements, says Wilcox (1936d), who adds that 

 the process may be accompanied by the male lashing 

 the abdomen or striking the head and thorax of the 

 female with the forelegs. When mated, pairs often 

 fly without uncopulating, in which case the female 

 usually takes the lead in flight, the male with vibrating 

 wings, but facing the opposite way. The hum of 

 mated couples in flight is of a distinctly lower pitch. 



Most robber flies emit a characteristic hum, some- 

 times of low, sometimes of high pitch, according to 

 the size and character of the species. Many species 

 are exceptionally wary ; of none is this more true than 

 the giant Australian species of Phellus Walker, which 

 are difficult to catch. 



The recorded enemies of asilids consist of spiders, ac- 

 cording to Bromley (1914), and wasps are recorded as 

 enemies of asilids by both Bromley and by Melin ( 1923) . 

 Csiki has recorded one instance of birds feeding on a 

 robber fly. Quantitatively, one of the principal enemies 

 must be asilids themselves. Possibly lizards may feed 

 on robber flies. Mantids capture an occasional asilid. 



PREY 



Asilid prey has been given considerable study by 

 Hobby, Poulton, Bromley, Carrera and many others. 

 Out of the numerous publications on food selection 

 by these flies, a few patterns emerge. Certain species 

 in Europe and North America eat hive bees and do 

 great damage at times to apiaries. They have been 

 blamed for the failure of hives of bees in New York 

 state to throw off a swarm. The bee killer of that state 

 is Promachus fitchii Osten Sacken. In Texas the cor- 

 responding species are Saropogon dispar Coquillett and 

 Diogmites symmachus Loew and in southern Europe 

 Dasypogon diadema Fabricius is widely known as a 

 bee catcher. According to Bromley (1934) Parkes 

 records the killing of over 700 Saropogon dispar Co- 

 quillett in one bee yard in a 3-day period. Besides 

 these named, many other species have been captured 

 and found with occasional bees. Very few if any in- 

 sects completely escape the attraction of the predatory 

 robber flies. In England even dragon flies have been 

 listed among their prey. Some species are compara- 

 tively unselective. 



Melin (1923) states that Dioctria Meigen species 

 catch mainly Hymenoptera. Hobby (1932b) found 

 that Dioctria rufipes De Geer captured 40 percent Hy- 

 menoptera with a predilection for Ichneumonidae ; 55 

 percent of the remaining prey consisted of Diptera. 

 Linsley (1944) noted that of 22 prey records of Gallini- 

 cus Loew, all were Osmia and Andrena bees. Hobby 

 (1931a) in a lengthy and commendable study of asilid 

 prey and behavior analyzes the prey captures of 23 

 British species. He notes that the capture darts of 

 species like Laphria Meigen may range up to 6 meters 

 and that Leptogastrinae characteristically hunt on the 

 wing rather than by sudden sallies from an observa- 

 tion post. I have frequently found Leptogaster Mei- 

 gen species dangling by a foreleg while sucking their 

 captured prey. The time required to consume the prey 

 varies with its size, but Edwards (1883) watched a 

 robber fly of the group Asilinae capture eight geomet- 

 rid moths in 20 minutes. Melin (1923) hazards some 

 estimates of feeding time. 



Mated asilids are not infrequently seen with the fe- 

 male holding prey of some kind. I have not infre- 

 quently found Megaphones Bigot mating with a 

 female and in turn holding prey. I have found Proma- 

 chus bastardi Macquart feeding on Megaphorus clausi- 

 cella Macquart. Several other instances of asilids 

 feeding on asilids have been recorded, even those of the 

 same species. I have seen cannibalism in Megaphorus 

 Macquart. Poulton (1906), records cannibalism in 



