68G PAEASITIC AKACHNIDA. 



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 

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

 infest, and placed under a magnifying power sufficiently low to 

 enable a large number 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 resemble 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 commonly, 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 is 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 Acarida, for 

 some time after its 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 follicidorum, 

 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 



