1923] Observations on the Feeding Habits of Robber Flies 43 
From this may be seen that over 50% of the prey taken from 
the flies consisted of honey-bee workers. The explanation of the 
presence of so many bees in such an uninviting environment as 
these sand areas is this: beyond the western edges of the fields 
where I made these observations were banks of sumac to and from 
which the bees were continually streaming, many taking their 
course over the sand plains not more than ten or fifteen feet 
from the ground. It was from these that the robber-flies took 
heavy toll. Where the flies were most abundant I often saw 
one sweep upward and grasp a bee, only to fly so far with its 
prey as I approached that I was unable to locate it and obtain the 
record. 
Several honey-bees that were taken from the flies were after- 
wards dissected and examined by Mr. R. E. Snodgrass of the 
Bureau of Entomology, with whom I was staying at the time, 
and myself. A specimen that the fly had dropped because it 
had consumed all parts possible was found to be nothing but 
an empty shell. Nearly all of the digestive, nervous, and mus- 
cular systems had been dissolved, probably by the introduction 
of an enzyme, and sucked out. The poison sac was intact, as 
was the tracheal system and all chitinized portions. A bit of 
the small intestine and unrecognizable pieces of other organs 
remained in the body cavity, but most of the internal struc- 
tures were gone. Examination of others in different stages 
of consumption showed the muscle tissue of the thorax reduced 
to a shredded mass and the muscles and brain in the head-capsule 
disintegrated. Whether or not this was accomplished by an 
enzyme secreted by the salivary glands, I do not know. Further 
work might determine this point. 
An interesting point noted in watching the flies was the 
fact that although P sammocharids were the predominating and 
most conspicuous hymenopterous insects flying over the plains, 
the flies seemed to have trouble in capturing these. I watched 
them dart at these wasps on several occasions but the wasps 
eluded them by dropping to the ground and running through 
clumps of bunch-grass. They are occasionally captured, how- 
ever, and I was able to take the flies preying upon Psammo- 
