ITCH MITES. 299 



CASE killed before they have had time to begin to lay eggs, and as soon 

 as the old crop of eggs is exhausted the cure is complete. 



It is not alone on man that this species establishes itself. It 

 has been found on the lion, on the dog, the lama, the sheep, the 

 ox, the horse, and the sow, and although it is very possible that 

 some of the observations on which this statement is made are 

 erroneous, and that some allied species peculiar to the animals in 

 question may have been mistaken for the S. scabiei, still there is 

 no reason to doubt that it is the species which is most universally 

 distributed, and which is found on the greatest number of mam- 

 mals. We hear of old mangey lions in a wild state, but it is most 

 probable that it is only in captivity that they are really attacked 

 by the itch, which no doubt they owe to the dirty attendants that 

 wait upon them. They succumb rapidly to its attacks, dying in a 

 few months, and become miserable objects before they die. The 

 head is the part chiefly attacked. It becomes covered with a 

 thick crust, the nostrils closed, the skin of the head and neck 

 swollen into hard folds, and the whole animal in the last stage of 

 debility. 



There remains one point for consideration regarding these mites, 

 not less curious than any of the preceding, viz. : — How they 

 produce the physiological effects that characterize the malady. It 

 is a problem that will often recur to us as we go along through 

 the other classes of insects. We shall find the larva of one kind 

 of insect feeding in some part of a plant without giving rise to any 

 symptoms that can be called inflammatory or envenomed, while 

 others of the same size, of a different kind, feeding alongside of 

 them produce immense growths or galls. In speculating upon 

 the cause of the itching sensation and inflammatory symptoms 

 of the skin in attacks of the Sarcoptidse, the first and most natural 

 supposition is that they are caused by the incessant minute nib- 

 bling going on just at the termination of the smallest ramifications 

 of the sentient nerves terminating in the skin. Undoubtedly a 

 constant gnawing of this kind must not only produce irritation, 



