PARASITIC ACARIDA. 581 



hatched in the succeeding spring, give origin to a new brood of 

 larvae that repeat the curious life-history of their predecessors. 1 

 The non-sexual multiplication of the larvae is obviously a pro- 

 cess of gemmation, analogous to the multiplication of cells by 

 subdivision ; whilst the true Generative process, analogous to 

 the conjugation of cells, is only performed when perfect Aphides 

 of distinct sexes have been evolved. 



400. Arachnida.^- The general remarks which have been made 

 in regard to Insects, are equally applicable to this class ; which 

 includes, along with the Spiders and Scorpions, the tribe of 

 Acarida, which consists of the Mites and Ticks. Many of these 

 last are parasitic, and are popularly associated with the wingless 

 parasitic Insects, to which they bear a strong general resem- 

 blance, 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 resem- 

 ble the cheese-mite in structure and habits, but which feed upon 

 different substances ; and some of these are extremely destruc- 

 tive. 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 



1 For a careful examination and philos6phical appreciation of the real nature of this 

 process, see Dr. Waldo J. Burnett's Memoir in the " Transactions of the American 

 Academy of Arts and Sciences." 1853. 



