HENRY J. FRANKLIN. 239 



tion. It seems possible that the fly is enabled to more easily 

 approach and capture its bee prey because of this similarity. 

 Moreover, the larvae of many of the Syrphidae, which re- 

 semble bumble-bees in their coloration, are reared in the 

 nests and fed on the larvae of these bees. It is supposed 

 that, because of their resemblance, these flies are better able 

 to enter the nests of the bees and lay their eggs in them. 

 It seems probable, therefore, that with the families Asilidae 

 and Syrphidae, two factors have worked together in guiding 

 natural selection in bringing about the color resemblances 

 of certain of their species to bumble-bee species. 



These factors are : 



(a). The advantage oi protective tnimicry already discussed, and 



(b). The advantage of aggressive mitnicry. 



The bumble-bee hawk moth, He^naris diffinis Boisduval, 

 resembles bumble-bees a great deal in general appearance 

 and may be a case of protective mimicry. 



Many of the species of Psithyrus appear to imitate, in 

 their coloration, the species of Bombus in the nests of which 

 they are inquilines. This mimicry is probably entirely ag- 

 gressive. 



Species and Subspecies Limits. 



Many of the New World species of Bombidae, as they are 

 considered in this paper, are so distinct from all others that 

 they can hardly, at least with our present knowledge of their 

 variations, be considered by any one as being closely enough 

 related to any other forms to raise any question as to their 

 complete and separate species rank. With many other spe- 

 cies, however, the relationships are evidently very close, the 

 differences, while apparently constant, being so small that 

 it must be entirely a matter of personal opinion whether they 

 should be given full species rank or be considered as only 

 subspecies. A list of species showing several such doubtful 

 relationships is here included : 



1. Bombtts appositus and B. distinguendjis Morawitz. 



2. " ca/ifornicus, B.fervidus a.n.d. B. sonontcs. 



3. " pennsylvanicus and -5. sonorus. 



4. " terricola and B. occidentalis. 



TRANS. AM. ENT. SCO. , XXXVHI. 



