INSECTS AND ARACHNID A. 249 



associated with the wingless parasitic Insects, to which they bear a strong 

 general resemblance, save in having eight legs instead of six. The true 

 'mites' (Acarince) generally have the legs adapted for walking, and 

 some of them are of active habits. The common cheese-mite) as seen by 

 the naked-eye, is familiar to every one; yet few who have not seen it 

 under a Microscope have any idea of its real conformation and move- 

 ments; and a cluster of them, cut out of the cheese they infest, and 

 placed under a magnifying power sufficiently low to enable a large num- 

 ber to be seen at once, is one of the most amusing objects that can be 

 shown to the young. There are many other species, which closely re- 

 semble the Cheese-mite in structure and habits, but which feed upon 

 different substances; and some of these are extremely destructive. To 

 this group belongs a small species, the Sarcoptes scabiei, whose presence 

 appears to be the occasion of one of the most disgusting diseases of the 

 skin the itch and which is hence commonly termed the ' itch-insect.' 

 It is not found in the pustule itself, but in a burrow which passes-off 

 from one side of it, and which is marked by a red line on the surface; 

 and if this burrow be carefully examined, the creature will very com- 

 monly, but not always, be met-with. It is scarcely visible to the naked 

 eye; but when examined under the microscope, it is found to have an 

 oval body, a mouth of conical form, and eight feet, of which the four 

 anterior are terminated by small suckers, whilst the four posterior end 

 in very prolonged bristles. The male is only about half the size of the 

 female. The Ricinice or ' ticks ' are usually destitute of eyes, but have 

 the mouth provided with lancets, that enable them to penetrate more 

 readily the skins of animals whose blood they suck. They are usually 

 of a flattened, round, or oval form; but they often acquire a very large 

 size by suction, and become distended like a blown-bladder. Different 

 species are parasitic upon different animals; and they bury their suckers 

 (which are often furnished with minute recurved hooks) so firmly in the 

 skins of these, that they can hardly be detached without pulling away 

 the skin with them. It 4s probably the 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 Acar- 

 ida, it possesses only six legs for some time after its emersion from the 

 egg (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 Demodexfolliculorum, a creature which 

 is very commonly 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 mingled with it. These are to be separated by the addi- 

 tion 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 dis- 

 solved-away by digestion in a mixture of alcohol and ether. The pus- 

 tules in the skin of a Dog affected with the ' mange' were found by 

 Mr. Topping to contain a Demodex, 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 



