the walking legs provided with bristles, claws or sucking discs for 

 attachment. 



Among the forms retaining more or less a free-living habit the 

 more characteristic are the species of Tyroglyphiis, especially T. 

 farinae, known as cheese mites and living upon cheese, flour or 

 fruit. Nearly all forms of acarines possess six-legged or hexapod 

 larvae, the larva sometimes being the significant parasitic stage. 

 Thus the harvest mite or "jigger", to be distinguished from the 

 penetrating flea to which the name is more properly applicable, is 

 the hexapod larva of Trombidium, and was formerly recognized 

 as a distinct form to which in America the name Leptus autumnalis 

 was applied. The larva becomes prevalent in the early autumn 

 on low vegetation, from which it gains access to the human skin. 

 It burrows into the skin, prodducing small red spots with a lighter 

 centre, and bringing about considerable itching and irritation of 

 the part affected. 



The true itch-mites, Sarcoptidae, permanent parasites of the 

 skin of birds and mammals, include the human itch-mite, Sarcoptes 

 scabiei. The body is whitish, rounded or oval, considerably smaller 

 in the male, the chelicerae pincer-like, and the pedipalpi jointed. 

 The males are less commonly seen since they die off after copula- 

 tion. The females burrow in the surface layer of the skin, forming 

 tunnels, in the course of which the eggs are deposited. Demodex 

 folliculorum, the mite of the hair-follicles, is an elongated worm-like 

 form, showing, however, the typical structure of the acarines. It 

 lives in the hair follicles and sebaceous glands of man, similar 

 species being found in the lower mammals. 



The Ixodidae or ticks are blood-sucking acarines, of larger size 

 and with a somewhat leathery skin. They are common surface 

 parasites of the mammals (e.g., bush and cattle ticks). In the 

 European dog-tick, Ixodes ricinus, the eggs produce a hexapod larva 

 which attaches itself temporarily to a mammal host. Dropping 

 off after a time it is metamorphosed into a nymph or pupal stage 

 which repeats the performance. The nymphs finally give rise to 

 adults, males and females, which gain access to the mammal host, 

 where the former wander over the surface, while the latter attach 

 themselves by the anterior end in the surface layer of the skin, the 

 free part of the body becoming greatly distended. When fully 

 mature the female becomes detached. 



The parasitic habit of the ticks is responsible for their impor- 

 tance as carriers. Thus in the case of the African tick fever, the 

 causative organism is a protozoan spirochaete (S. duttoni) but the 

 transmitting organism is the tick, Ornithodorus moubata. In America 

 a conspicuous example is afforded by the Rocky Mountain 



16 



