20 
Psyche 
[March 
THE MIMETIC RESEMBLANCE OF FLIES OF THE 
GENUS SYSTROPUS TO WASPS 
By Charles T. Brues 
Biological Laboratories, Harvard University 
Together with a few relatives the genus Systropus is of 
very different appearance from the other members of the 
family Bombyliidse to which it belongs. The body is ex- 
tremely elongated and the dense pile that generally covers 
the body in the other, more or less thick-set bombyliids is so 
greatly reduced that the flies appear almost bare. 
During a recent visit to the Dutch East Indies, I was for- 
tunate in taking with the net two species of this genus on 
the island of Sumatra. Having previously observed and 
collected one of our common North American species, 
Systropus macer Loew, I was at once struck by the very 
different color pattern of its Sumatran relatives. Also the 
latter appear to mimic entirely different types of wasps, 
suggesting that two independent lines of evolution have been 
active in developing widely divergent, but equally fine cases 
of mimetic resemblance in the two hemispheres. This situa- 
tion appeared so remarkable, that I attempted to follow it 
further by an inquiry into the color patterns of the other 
numerous members of the genus which is known to be almost 
world-wide in distribution. On account of their striking 
appearance and easy recognition a great many species have 
been described by a considerable number of entomologists. 
Bezzi, in his taxonomic review of Systropus has already 
commented on the general color pattern which prevails 
among the species inhabiting two of the great zoological 
regions. He refers to the Oriental forms as having the 
aspect of vespids and the Nearctic ones as ammophiloid, i.e., 
like the sphecoid genus “Ammophila” now called Sphex by 
taxonomists. The comparison is particularly apt in the case 
of the North American species as the resemblance to these 
digger-wasps is very great. Indeed, when in flight our com- 
