July, 1889. 
WILD BEES. 
153 
their colours and general appearance, closely resemble the 
species on which they are parasitic, and so are enabled to enter 
the nest, and deposit their eggs with greater safety. But, 
besides these flies, they have other parasites in the extra¬ 
ordinary genus Gonops, several species of which are usually 
abundant. 
Of bees, the genus Psithyrus (— Apathus ) is parasitic on 
the bumble bees. In general appearance they closely 
resemble Bombi, but of course they have no workers, and no 
pollinigerous apparatus. The females hibernate and deposit 
their eggs in the spring and early summer, when the Bombi 
have begun to provide for their larvas, and amongst these they 
are developed. 
And here, in conclusion, it will be appropriate to say some¬ 
thing about parasitic bees in general. Clearly parasites 
belong to two distinct classes : (i.) those which are nearly 
related to their hosts, and (ii.) those which are totally 
dissimilar. 
Of the first class the most striking instance is found 
in these Psithyri. There cannot be the faintest doubt 
that they are degenerate forms of Bombi which, through 
parasitic habits, have lost their means of collecting pollen 
and their workers. But other parasites belong to this class 
which do not at first sight resemble their hosts ; such are 
Melecta and Anthophora , Megachile and Coelioxys. Osmia and 
Stelis. For it is highly probable that the differences between 
these parasitic genera and their hosts have merely been 
brought about by their parasitism. Take Melecta and 
Anthophora, for instance, what is more likely than that some 
ancestral species of the latter formed large colonies as 
Anthophorapilipes and others now do, and that some individual 
took to depositing its eggs in its neighbours’ burrows, instead 
of collecting its own pollen, and that this was continued from 
generation to generation. Naturally the loss of pollinigerous 
apparatus would follow (and in a social bee the loss of the 
worker as well), and other changes would take place in the 
parasite best suited for the accomplishment of its purpose. 
I should rather wonder myself if this had not sometimes 
happened. I have seen a female of the Fossor, Agenia* 
variegata, which preys on spiders, enter the burrow of another 
female, probably either to rob or dispossess it, as a furious 
fight ensued; and I have seen two females of Pompilus gilbus 
fight for a spider which one of them had captured. I have 
* Cf. M. Fabre's observations on Tachytes; also a Fossor, mentioned 
by Darwin (“Origin of Species,” p. 216). 
