A ROBBER FLY 289 



flying. Again, when a bumble bee entered the bell 

 of a flower to find a sweet store in its nectary, he 

 emitted a shrill sustained sound while drinking it 

 without any movement of the wings. The sound 

 expressed its pleasure and doubtless the pleasure 

 was all the greater for the sound. 



Much has been written on the anatomy of the fly 

 with regard to its sound-producing mechanism, and 

 it is known that the sound is partly produced by the 

 vibrations of the wings and partly by other means, 

 also that it can be varied at will. Authorities differ 

 as to the exact process, but we are not concerned 

 with these details. 



The leaf-locust, CEcanthus, has been described as 

 an example of the best kind of music in the most 

 musical insect order. Let me now describe the 

 highest performance known to me in the lower kind 

 of music the fly and other humming insects are 

 capable of. 



One of the most notable families in the Dipterous 

 or Fly Order is the Asilidse, our common Robber 

 Flies; large rapacious flies that have a world-wide 

 distribution and vary greatly in form, size and colour- 

 ing. In the typical genus they are of a crane-fly grey 

 in colour with long hairy legs and under parts. The 

 fly to be described is of this genus, but I am in doubt 

 as to the species, which is either Asilus rufiventris 

 or a species closely allied. It is one of the commonest 

 robber flies in Argentina and its habits are similar 

 to those of the order generally. Thus, it places itself 

 with head up, in a vertical position on a stem or 



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