166 BULLETINS' OF WISCONSIN NATURAL HISTOKY SOCIETY. VOL. 3, NO. 4. 
CONCLUSION. 
A consideration of the facts now in our possession from dif- 
ferent sources leads to the following conclusions, regarding the 
larval habits of those bees that deposit their eggs in the nests of 
other bees. 
In some cases we are probably not dealing" with parasitism 
at all but with commensalism, where the larvae of the host-bee 
and the intruder live quietly together, and reach their final de- 
velopment. Such seems to be the case with the species of 
Xomada in the nests of Andrena and Halictus as reported by 
Packard. The larvae of Stelis nasuta, several of which according 
to Fabre live together in the same cell of Cholicodoma without at- 
tacking each other also seem to be commensals although we do 
not know whether they finally attack the host-larva or are indi- 
rectly the cause of its death by starvation. Stelis minuta observed 
by Yerhoeff in the nest of Osmia leucomehma and Stelis sex- 
maculata observed in our surroundings in the nest of Alcidamea 
produrta are both decidedly parasitic, Stelis sexmaculata attack- 
ing with equally destructive results the host-larva as well as a 
larva of its own kind. 
Parasitism among bees produces some very important 
changes in structure (hypermetamorphosis) and habits of the 
larvae as demonstrated by the species of Coelioxys and Triepeolas 
treated in this paper. The extraordinary development of the 
mandibles of these larvae would seem to be quite out of propor- 
tion to their needs if the destruction of the harmless host-larva 
were the only thing to be accomplished. But when we remember 
the fact that these parasites are frequently present to the number 
of two or more in one cell, and have to fight with each other for 
the undisputed ownership of the cell, then the question suggests 
itself whether these weapons of attack, as also the peculiar organs 
of locomotion in the larva of Triepeolus have not been developed 
as a result of the struggle of these larvae among themselves. 
To my knowledge several writers have attempted to explain 
the origin of parasitism among bees. A more extended acquain- 
tance with the larval life of guest-bees, as well as of host-bees 
from different regions of the globe might furnish a more solid 
foundation for a theory of this kind. The various degrees of tol- 
erance exhibited by the larvae of different species of host-bees 
might have to be considered in this respect, and for this reason 
I have laid especial stress on the pronounced tolerance of the 
larva of the host-bee Ceratina dupla, the lesser degree of tolerance 
