582 INSECTS AND ARACHNIDA. 



young of a species of this group, which is commonly known as 

 the "harvest bug," and which is usually designated as the 

 Acarus autumnalis ; this is very common in the autumn upon 

 grass or other herbage, and insinuates itself into the skin at the 

 roots of the hair, producing a painful irritation; like other 

 Acarida, for some time after their emersion from the egg, it 

 possesses only six legs (the other pair being only acquired after 

 the first moult), so that its resemblance to parasitic insects 

 becomes still stronger. It is probable that to this group also 

 belongs the Demodex fotticulorum, a creature which is very com- 

 monly found parasitic in the sebaceous follicles of the human 

 skin, especially in those of the nose. In order to obtain it, 

 pressure should be made upon any one of these that appears 

 enlarged and whitish, with a terminal black spot; the matter 

 forced out will consist principally of the accumulated sebaceous 

 secretion, having the parasites with their eggs and young min- 

 gled with it. These are to be separated by the addition of oil, 

 which will probably soften the sebaceous matter sufficiently to 

 set free the animals, which may be then removed with a pointed 

 brush; but if this mode should not be effectual, the fatty matter 

 may be dissolved away by digestion in a mixture of alcohol and 

 ether. The pustules in the skin of a dog affected with the 

 " mange" have been found by Mr. Topping to contain a Demo- 

 dex, which seems only to differ from that of the human sebaceous 

 follicles in its somewhat smaller size ; and M. Gruby is said to 

 have given to a dog a disease resembling the mange, if not 

 identical with it, by inoculating it with the human parasite. 

 The Acarida are best preserved as microscopic objects, by 

 mounting in glycerine. 



401. The number of objects of general interest, furnished to 

 the Microscopist by the Spider tribe, is by no means considera- 

 ble. Their eyes exhibit a condition intermediate between that 

 of Insects and Crustaceans, and that of Vertebrata ; for they are 

 single like the " stemmata" of the former, usually number from 

 six to eight, are sometimes clustered together in one mass, but 

 are sometimes disposed separately, while they present a decided 

 approach in internal structure to the type characteristic of the 

 visual organs of the latter. The structure of the Mouth is 

 always mandibulate, and is less complicated than that of the 

 mandibulate insects. The Respiratory apparatus, which, where 

 developed at all among the Acarida, is tracheary like that of 

 Insects, is here constructed upon a very different plan ; for the 

 "stigmata," which are usually four in number on each side, 

 open into a like number of respiratory sacculi, each of which 

 contains a series of leaf-like folds of its lining membrane, upon 

 which the blood is distributed so as to afford a large surface to 

 the air. In the structure of the limbs, the principal point 

 worthy of notice is the peculiar appendage with which they 

 usually terminate ; for the strong claws, with a pair of which 



