448 Trans. Acad. Sci. of St. Louis 
are spiders, and all are good food, whether of her own 
choosing or not. How long will it be before she learns 
to retain the spiders and save herself the trouble of 
gathering a new lot, and thereby gradually acquire 
habits that are more or less parasitic? It is indeed 
puzzling that she has not already learned it, since she 
has proved herself ingenious in adopting other labor- 
saving devices. 
When we again ask ourselves why, we find the ques- 
tion unanswerable until we consider just what the posi- 
tion of the egg of her predecessor is. When we know 
that S. caementarium lays her egg on the first spider 
that she brings in, then it dawns upon us that there 
is good reason for C. caeruleum to carry out and throw 
away every spider to the very lowermost one in the 
cell, since only by that means can she be sure of 
eliminating her enemy’s egg on the last one. Rather 
would she destroy perfectly good food, and labor to re- 
place it with the same kind, than risk her egg in com- 
petition with the young of the original owner. In this 
she is wise—or at least she acts wisely—because if the 
Sceliphron egg were allowed to remain in the cell the 
larva therefrom would not only be present in the cell, 
but would always be a little the older and larger of the 
two, and hence would win in the majority of cases oD 
the fateful day when it was decided which should eat 
the other.* Thus we see that while it makes very little 
difference to Sceliphron where her egg reposes, it does 
make a vast difference to Chalybion, and the latter has 
regulated her habits to conform in astonishing detail to 
the ways of the first. 
*For an account of the fg age nga of €. caeruleum, see 
article in Ann. Ent. Soc. Amer. March, 1 
