66 



WILD LIFE IN CALIFORNIA 



Diptera, thereby showing its relation to the 

 common house fly as well as to other flies. 



The Robber fly is equipped by nature with 

 a strong projecting beak which it thrusts 

 into its victim. Thus it is used as a wea- 

 pon and at the same time affords the means 

 of sucking the life juice of its captive. Its 

 feet bear strong claws to aid it in holding 

 securely the insects that it may capture. 

 Authorities say that these flies are among 

 the most voracious of all insects and that they 

 prey on all other forms of insects, fearing 

 none, not even wasps or other forms armed 

 with defensive weapons. David Sharp, the 

 well-known English entomologist, in writing 

 about the Robber flies says: "As is the case 

 with so many other insects that prey on liv- 

 ing insects, the appetite of the Asilidae 

 seems insatiable; a single individual has been 

 observed to kill eight moths in twenty 

 minutes. They have been said to suck the 

 blood of verterbrates, but this appears to be 

 erroneous." Therefore, if the fears of the 

 timid excursionist have been aroused by the 

 story of the ferocious actions of the Robber 

 fly in Diablo Canyon they may be put aside, 

 for the fly does not attack members of the 

 human family. Its presence there in no way 

 detracts from the charms of the beauties of 

 the canyon, and so far as mankind is con- 

 cerned is not even an annoyance. It is a 

 terror to other members of the insect world 

 only. 



Since writing the foregoing I discovered a 

 very much smaller species of the Robber fly 

 on the roads about the foot of the moun- 

 tain. These insects averaged less than a 

 quarter of an inch in length. They were 

 more numerous than their relatives of the 

 mountain side, but their conduct was the 

 same in congregating in numbers and flying 

 and circling about certain portions of the 

 road, with from two to three of the Robber 

 flies attached to one victim, which I uni- 

 versally found to be a small gnat. Undoubt- 

 edly the little Robbers congregated on the 

 lower roads because there was where the vic- 

 tims that constituted their food were to be 

 found, just as the circumstances appeared on 

 the upper road, in the case with the larger 

 flies. 



A few weeks later when near the summit 

 of the mountain I saw representatives of an- 

 other specie of these insects, some of the 

 very largest of the Robber flies. They were 

 about an inch in length, with the same 

 characteristic slender and frail structure. 

 The big flies were not present in any great 

 number; nor could I find any with captive in- 

 sects or feeding otherwise. Nor were they 

 flying about as were both of the smaller kinds 

 I have described, but seemed to be resting on 

 the brush, rocks and grass, and took to the 

 wing only when disturbed. 



SKETCH FROM LIFE OF ONE OF THE LARGER SPECIES OF ROBBER FLIES 



(Enlarged four times) 



