( cxliii ) 



This agreement in certain characters may be likened to the 

 colour-similarity found in cave insects, or to the resemblance 

 of certain marine animals to fish, or of subterranean lizards 

 (Amphisbaena) to earthworms and snakes. In fact, parasites 

 show that a similarity of surroundings is frequently ac- 

 companied by, or associated with, a certain amount of agree- 

 ment in structure and colour. I hope that a short survey 

 of the chief points of resemblance exhibited by those ecto- 

 parasitic insects which live on birds and mammals may be 

 of some interest to you. 



Insects Ectoparasitic on warm-blooded 

 Vertebrates. 



a. With sucking mouth-parts. b. With biting mouth-parts. 

 Anoplura Mallophaga 



Cimicidae Hemimeridae 



Polyctenidae Platypsyllidae 



Siphonaptera 

 Hippoboscidae 

 Nycteribiidae 

 Streblidae 



There are some other insects which may be called semi- 

 parasitic, such as certain Staphylinids found on mammals 

 in South America, and the blind Silphid beetle Leptinus 

 testaceus of Europe, frequently observed in the burrows of 

 mice. These I mention only in passing. 



The skin of the host covered with pelt or feathers offers 

 opportunities of life more uniform as to temperature and 

 moisture than that enjoyed by the insect struggling against 

 changing atmospheric conditions. True parasites are not 

 much affected by climate, the conditions remaining more 

 or less uniform whether the host is sub-tropical or sub-arctic 

 in its distribution, or dwells in a moist or a dry climate. In- 

 sects that are free are much more affected by climatic con- 

 ditions. Again, the forces of evolution are much less active in, 

 and affect the parasite far less than, the host. Evolution in 

 the case of the parasite is retarded by the uniformity of 

 conditions. It is for this reason that parasites not in- 

 frequently illustrate the phylogenetic connection of the 



