452 
DISCURSIVE NOTES ON THE 
What, then, are the enemies that keep down the numbers 
of Pompilidce and other Eossors ? The answer is : Other 
insects, either as before mentioned — such as Mutillidce, which 
are directly 4 parasitic,’ or sundry other 4 indirect parasites,’ 
some Eossors, some two-winged flies — which appropriate to 
their own use the prey stored up by the Eossor. 
Indeed, it seems, in some cases, that the Fossor is aware of 
the danger. The long journey which the helpless prey has 
to make to the burrow, during which it is often put down 
while the bearer rests, or is perhaps frightened away, provides 
ample opportunity for an alert pirate to dart down and lay 
her own egg upon the prey. An especially favourable oppor- 
tunity is provided when, having arrived at the journey’s end, 
the rightful owner of the prey puts it down at the mouth of 
the burrow and goes down to see that all is well below. 
I saw a very astonishing thing once. An Ammophila had 
just come up from laying her egg on the caterpillar bestowed 
underground when I frightened her aw T ay, got out the caterpillar 
and laid it on the ground. 'When the Sphegid returned, she 
very carefully examined the caterpillar, discovered upon it 
the egg which she had herself deposited, and, to my great 
amazement, sucked it dry and deposited another ! There 
are many very puzzling points about this, however one tries 
to explain it : one can only suppose that the custom is thus 
to destroy any egg found affixed. For many caterpillars 
are attacked by Tachinid flies, which deposit eggs upon their 
skin, and such would be useless as food for a Fossor larva 
as well as that of the fly. 
A very common small Sphex, that feeds her young on 
grasshoppers, is intensely suspicious of the approach of any 
other insect while she is engaged with her task, and charges 
fiercely with open mandibles at even the smallest ant which 
draws near by accident — much to the alarm and confusion of 
the ant, be it said. ( Vide Frontispiece, Fig. 7, Sphex 
luteipennis, Mocs.) 
In the case of Bembecidce, it is almost the exception not to 
find in attendance near the mouth of the burrow a quiet 
little brown fly — an undescribed species of Idia — waiting its 
chance. The moment Bembex leaves her burrow, Idia hurries 
