HOUSEHOLD INSECTS 97 



tionary development, man has retained a small number of 

 epizoic or external body parasites, belonging to several groups 

 of insects. The number of such insects affecting the human 

 species appears to be generally less than that of those affect- 

 ing other mammals and this may perhaps be due to the 

 more cleanly personal habits which he has been able to 

 acquire. If we carry the matter further back, however, 

 to the stage of our as yet undetermined forebears, we find 

 that the monkeys and anthropoid apes are also compara- 

 tively free from bodily insect parasites. It is possible, there- 

 fore, that we may have our much despised anthropoid ances- 

 tors to thank for this at least. It is known that monkeys 

 actively catch and destroy fleas and other parasites, but, if 

 the whole tale must be told they are eaten to insure their 

 destruction. 



Several of the groups of insects which include human par- 

 asites have already been mentioned as carriers of certain 

 diseases. From this account it will be remembered that it is 

 rather difficult closely to draw the line between true human 

 parasites and other blood-sucking species that only rarely 

 attack man. There are all sorts of intergrades between 

 species like the stable-fly or the common green-head fly of our 

 seashore resorts, to whom human blood serves only for an 

 occasional meal, to the body louse which passes all stages of 

 its existence on the body upon which it is entirely dependent 

 for food. Thus the females of certain mosquitoes suck human 

 blood almost exclusively but are free-living creatures and dur- 

 ing their preparatory stages are aquatic. The human flea 

 (Pulex irritans) is generally restricted to man as a host dur- 

 ing its adult life, but its preparatory stages occur in accumu- 

 lations of dry organic matter of other kinds. While this is the 

 only flea peculiar to the human species, our domesticated dog 

 and cat have fleas peculiar to themselves, which have been 

 widely distributed with these animals, and are common house- 

 hold pests in most parts of the world. Indeed, the fleas most 

 annoying to persons are usually of the kinds regularly present 

 on dogs, cats, or even rats, rather than the human flea. 



