Field Studies of the Non-Social Wasps 447 
on the outside. Foremost, one must carefully consider 
whether or not the opening has been resealed. The 
Peckhams opened some cells (number not stated) and 
found the egg deposited on a spider which had been 
brought in just before the cell was sealed; this led them 
to say that it was ‘‘only after the nest was completely 
provisioned that the egg was laid.’’ There they made 
the error of not knowing that when they found the egg 
on a spider near the doorway, it was the egg of the 
cow-bird-wasp, C. caerulewm, and not that of the orig- 
inal builder of the nest. 
Carefully studying the cell to see which was the back 
and scrutinizing the closure to see whether the nest had 
been resealed by Chalybion (it is easy when one knows 
how), I have concluded, after long investigation, that 
S. caementarium always lays the egg on the first spider 
brought in, and that C. caeruleum always deposits its 
egg on the last one. The fact that Sceliphron deposits 
her egg on the first spider has a direct bearing on (a), 
the nesting behavior of C. caeruleum, and (b), the work 
done to test for the American wasps the experiments 
carried on by Fabre on the French Pelopoeus. 
For the first part, I have concluded that Chalybion, 
once a mud-nest builder, has so far changed her habits 
that now she is a water carrier; that instead of carry- 
ing mud to build her house she carries water in her 
gullet wherewith to soften the walls of another wasp’s 
nest so she can enter it, house-breaker fashion, and then 
carries out the spiders placed there by the rightful 
owner and brings in her own. Since the provisions of 
both wasps are the same, a general mixture of various 
kinds of spiders, one wonders how long it will be before 
C. caerulewm, already versatile and shrewd enough to 
be a house-breaker, will also learn to know that spiders 
