The Ecology of a Sheltered Clay Bank 201 
While these wasps, as well as the Monobia mud wasp, 
were to be regarded as secondary occupants of the car- 
penter-bees’ burrows, the relationship was more com- 
plex, since by occupying these burrows they took just 
that much space from the carpenter-bee, because each 
generation of Xylocopa used the old domiciles, in so 
far as they were adequate, and if the tunnels had been 
usurped by other species, the mother carpenter-bees 
were obliged to spend their time and energy at the slow 
process of hewing wood instead of being fruitful and re- 
plenishing the earth. Therefore, the only direct bearing 
that these two wasp occupants had on the inter-relations 
of life in the unit as a whole, was to deprive the car- 
penter-bee of some of her rightful nesting-places and 
to serve as a host for the parasite, Argyromoeba tigrina, 
for they both gathered their food elsewhere. 
The blue mud wasp, Chalybion caeruleum Linnaeus. 
This wasp was heretofore regarded as the mud- 
dauber wasp making nests very similar to those of the 
“tyellow-legs.’? On the contrary, I have shown else- 
where that this wasp does not make nests of her own, 
but occupies the nests of Sceliphron caementarium, 
either by breaking into a ‘‘live’’ cell and destroying the 
Prey or by using the abandoned cells. In so far as the 
bank was concerned, however, this species was occa- 
Sionally seen foraging for spiders among the burrows. 
On several occasions when one was foraging among the 
Spider webs, she broke through or became entangled, 
whereupon, in a very skillful manner, she quickly dis- 
entangled the web from her person. Elsewhere, how- 
ever, I have often found dead specimens that had been 
caught in spiders’ webs. 
