FOSSORIAL HYMENOPTERA 
458 
there ; often literally tumbling head over heels in its hurry 
to do its duty to the race before the return of the formidable 
creature whose young it is going to supplant with its own. 
Should the burrow be left open — for Bembex sometimes is 
careless — Idia’s chance has come, and in the fly scrambles, 
very quickly hurrying out again when the eggs have been laid. 
More often Bembex has temporarily closed her burrow, and 
in her absence the fly feverishly seeks an entrance — obviously 
working against time, for often Bembex will return suddenly, 
and the fly has to withdraw. It must be said, however, that 
I have never seen the Bembex take any notice of this insignifi- 
cant little intruder. Nevertheless, one often finds that Idia has 
been captured and paid the penalty — itself to be stung and 
stored up as food for the Bembex larvae. On the other hand, 
small larvae of a two-winged fly — presumably Idia — are often 
found, several at a time, in the burrow of Bembex ; but it is 
possible that they are really only scavengers, devouring the 
debris left by the Bembex larvae, and do not attack the lawful 
owner of the nest. 
Fossors at work are quite the most active insects I know. 
The nests of ants or bees do not compare in feverish activity 
with such a scene as may he witnessed on an open sandy space 
where numbers of Bembex are working together, as I have often 
found on the shore of Lake Victoria. On first arrival, you will 
perhaps see nothing very much but a number of pale yellow 
and black, waspish-looking insects, sitting on the sand or 
restlessly flying about with a remarkably dashing flight, so that 
the eye cannot follow them. These are males — idle fellows 
who have nothing to do but find their mate ; hence, probably, 
the way in which they dash about to make the acquaintance 
of a member of the opposite sex. Suppose — as is often the 
case — there are numerous Tsetse buzzing around you or 
settled on your puttees : you will see a very interesting thing. 
A female Bembex comes and apparently investigates you very 
closely, flying round and round at about the level of your knees. 
But there is no cause for alarm : it is not you, but a full-fed 
Tsetse, bloated with a heavy meal of your blood, that she 
wants. Failing this, she will select the next fattest she can 
find, which will be a gravid female, with abdomen swollen by 
