Cuap. XII. PARASITIC FLIES. 363 
were set in commotion, but the worker-minors remained behind to 
repair the damage, whilst the large-heads issued forth in a most menacing 
manner, rearing their heads and snapping their jaws with an expression 
of the fiercest rage and defiance. 
The armies of all Ecitons are accompanied by small swarms of a kind 
of two-winged fly, the females of which have a very long ovipositor, and 
which belongs to the genus Stylogaster (family Conopsidee). These 
swarms hover with rapidly vibrating wings, at a height of a foot or less 
from the soil over which the Ecitons are moving, and occasionally one 
of the flies darts with great quickness towards the ground. I found 
they were not occupied in transfixing ants, although they have a long 
needle-shaped proboscis, which suggests that conclusion, but most 
probably in depositing their eggs in the soft bodies of insects, which the 
ants were driving away from their hiding-places. These eggs would 
hatch after the ants had placed their booty in their hive as food for 
their young. If this supposition be correct, the Stylogaster would offer 
a case of parasitism of quite a novel kind. Flies of the genus Tachinus 
exhibit a similar instinct, for they lie in wait near the entrances to bees’ 
nests, and slip their eggs into the food which the deluded bees are in 
the act of conveying for their young. 
