IOI4 INSECTS AND ARACHNID A 



iei is a minute creature of almost circular form, the female of 

 which burrows under the epidermis, causing the disease. The mite is 

 found at the end of the burrow, not in the pustule at its commence- 

 ment. The first two pairs of legs and the third leg of the male 

 are terminated by suckers, the other legs by long bristles. The 

 male is smaller than the female. The Analgesince (Dermaleichi) are 

 a very large and curious group ; the males often differ greatly from 

 the females, and the skin is often greatly strengthened by chitinous 

 plates and structures. The species are not always parasitic on one 

 bird only ; often the same species may be found on numerous birds, 

 while several species frequently live on the same bird ; they are not 

 usually supposed to be injurious to the birds ; they are found on the 

 feathers. 



The Phytoptidce are extremely minute creatures living in galls 

 which they form on the leaves and twigs of numerous trees and 

 plants ; they are elongated in form with the two hind pairs of legs 

 abortive ; there is but little variety among them. Slightly resem- 

 bling them in general form, but very different in other respects, is 

 Demodex folliculorum, which is found in the sebaceous follicles of 

 the human skin, particularly the nose. Those follicles, which are 

 enlarged and whitish with a terminal exterior black spot, may be 

 forced out by pressure, and the Acarus will often be found within. 

 Similar parasites exist on the dog and pig. 



There are numerous other curious and interesting forms which 

 cannot be included in any of the families mentioned above. 



The number of objects furnished to the microscopist by the 

 spider tribe is very large from a biological point of view, although 

 mere objects of microscopical interest popularly are not so numerous 

 as in insects. Their eyes exhibit a condition intermediate between 

 that of insects and crustaceans and that of vertebrata, for they are 

 simple like the * stemmata ' of the former, usually number from six 

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

 frequently disposed separately ; while they present a decided ap- 

 proach 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 is not tracheal, as in insects and some Acarina, but is 

 constructed upon a very different plan, for the ' stigmata,' which 

 are usually four in number on each side, open upon 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 the last joint of the 

 foot is furnished, have their edges cut into comb-like teeth, which 

 appear to be used by the animal as cleansing instruments, and in 

 many cases for the manipulation of the silk of their snares. But a 

 feature deserving study by the microscopist is the physical cause of 

 the exquisite sensitiveness of these < feet.' By resting these upon a 



