238 APPENDIX. 



ited a specimen. Some points about it, and especially the position 

 of the spiracles, being yet rather obscure in his mind, he requested 

 me to examine my material, which I have thus been led to do. 



As confirmatory of the affinities of Platypsyllus, as here proved, 

 it may be mentioned that Leptinus testaceus Miill. , the only species 

 of its genus, is known to be parasitic on mice, as it has been found 

 upon them in Philadelphia by Dr. John A. Ryder, and I have taken 

 it in the nests of a common field mouse near Washington ; but still 

 more interesting is the fact that Leptinillus validus Horn (also the 

 only species of its genus) is an associate parasite of Platypsyllus on 

 the beaver, a number of both having been taken by one of my 

 agents, Mr. A. Koebele, in San Francisco, from beaver skins brought 

 from Alaska. 



Platypsyllus, therefore, is a good Coleopteron, and in all the 

 characters in which it so strongly approaches the MaUophaga it 

 offers merely an illustration of modification due to food habit and 

 environment. In this particular it is, however, of very great interest 

 as one of the most striking illustrations we have of variation in 

 similar lines through the influence of purely external or dynamical 

 conditions, and where genetic connection and heredity play no part 

 whatever. It is at the same time interesting because of its synthetic 

 characteristics, being evidently an ancient type from which we get a 

 very good idea of the connection in the past of some of the present 

 well-defined orders of insects. 



FINIS. 



