Field Studies of the Non-Social Wasps 435 
One often finds nests, portions of which, and especially 
the middle portions, are two, three or even four tiers 
high, built one on top of the other, as shown in fig. 61, 
where there were two layers and a third was being built. 
Whether one mother constructs tier upon tier, or whether 
a second mother builds her nest on top of the first, I do 
not know. This condition occurs almost too frequently 
to be attributed to mere accident. Nevertheless the oc- 
currence of nests in this form brings out an interesting 
point in the instinct of the emerging wasp. Fabre‘ car- 
ried out some experiments with the mason bee, Chalico- 
doma muraria, in which he found that instinctively the 
insect could bore out of only one earthen covering, and 
while apparently it had the physical ability to emerge 
from an extra covering it would rather die in its prison- 
like house than make the extra exertion to escape. But 
how fares it with these wasps whose mothers or aunts 
build tier upon tier, making it necessary for the emerg- 
ing wasp to bite through more than one wall before gain- 
ing its freedom? It is interesting here to note that the 
insects seem to have some faculty of discerning the front 
side of their cells, just as the larva of the blue wasp 
(Chalybion) or the yellow-legs (Sceliphron) has the 
faculty of righting itself in its cell as it reaches maturity 
so that it always pupates with its head toward the exit. 
In T. politwm nests of only one layer of cells, one seldom 
sees an error in choosing the proper side for exit. It has 
been my good fortune to obtain a few nests which were 
two or more layers deep, and to study their contents to 
learn whether these wasps did plod faithfully on until 
they gained their freedom, or gave up at the first defeat. 
T have little doubt that there is sufficient strength in those 
mandibles to penetrate several walls, so the question is 
4The Mason—Bees. Tr. by A. T. DeMattos. Chap. II. 
