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 jDossess 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, bj- 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; 6, 

 antennae; c, maxillEe; d, membrane joining the antennae; 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 with the naked eye. Not. 

 ^thstanding 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- 



