ITCH-MITES. 51 
shape; their eight short legs, which extend but a short dis- 
tance beyond the body, are supported by bands of harder 
substance (chitin), and usually possess at their ends toes 
with suckers. Itch-mites take their food by means of 
peculiar nipper or needle-like mandibles, and as they lead a 
burrowing life they need and possess no eyes. Neither do 
they possess a respiratory apparatus, the respiration being 
cutaneous. The different kinds of itch-mites are chiefly dis- 
tinguished from each other by the number and position of 
the spines, by the hooks on the tarsi, and by the chitinous 
bands. Fig. 26 shows the structure of head and leg of a 
member of this order of parasites. 

Fig. 26.—Head and front leg of scab-mite, ventral view; a, mandibles; b, 
antenne; c, maxilla; d, membrane joining the antenn@; e, e, joints of limb; f claw; 
“g, ambulacrum or sucker. Greatly enlarged. After Curtice. 
Itch-mites are all very small, and for this reason not 
readily detected, though some, for instance the one causing 
the mange of the horse, can be seen withthenaked eye. Not- 
withstanding the small size of the mites causing the itch in 
man it was already known to be the cause of that disease as 
early as 1197, and it had already been closely observed and 
illustrated by Wichmann in 1780. Yet all real knowledge of 
this case of cause and effect seems to have been lost, and the 
Academy of Science in Paris offered in the beginning of this 
century (1812) a prize of $1500 to anybody who could dem- 
onstrate its existence. For a long time no one could pro- 
