28o HYMENOPTERA ACULEATA. 



inquiline genera the species closely resemble their hosts, 

 but the species of Nomada are as unlike those of Andrena, 

 Euccra, &c., with which they live, as could well be imagined, 

 and yet they seem to pass in and out of the burrows of their 

 hosts without molestation. It has been suggested that 

 their wasp-like colours make them objects of terror to 

 their hosts, but why this colour terrorism should be 

 more useful here than in the cases of Meleda, Coclioatjs, 

 PsUhyrus, &c., is hard to see; the sting is feeble, 

 as in Andrena, nor have I ever known one make itself 

 felt, and if such a bee as Eucera were to turn and 

 show fight to a Nomada the result would be almost certainly 

 against the intruder. In such a case as this wasp-like 

 colours might be protective, but Eucera is so much larger 

 than its parasite, that one can hardly imagine but that 

 some plucky individual in the course of ages would have 

 tried its powers even against a presumed wasp, and I feel 

 fully convinced that if a real wasp tried to enter a Eucera 

 burrow, there would be a battle royal at once. It seems 

 therefore evident that the inqiiiline life which these bees 

 live is not disagreeable to those they live with, but how 

 such a state of life has been attained is a problem the 

 solution of which seems very doubtful. It has been 

 suggested by several that the inquiliues have once been 

 industrious, but finding that they could live without 

 working have gradually become differentiated from the 

 species they originally belonged to, have lost their pollinig- 

 erous organs and have assumed distinctive characters 

 according to the various circumstances of each. This looks 

 very reasonable in the case of Psithyrus which so closely 

 resembles its host, Bovihus, but in Nomada it is harder to 

 conceive^ for although nearly all its essential characters 

 appear to be Andreniform its general aspect is so unlike 

 that genus that if those views are correct, which is far from 

 improbable, it must have left the parent stem in very 

 ancient days. There is a great deal to bo learnt yet about 



