ACARINA 



691 



spiratory stigmata usually belong to the prosoma, and the primitive 

 segmentation of the opisthosoma has either entirely or almost 

 entirely disappeared. 



Remarks. — ^The Acarina include the mites and ticks which have 

 long been known to be human parasites, but it is only recently that 

 the latter have come into notice as carriers of disease. 



It is true that a mite, or rather its larva, has long been suspected 

 t be the cause of Tsutsugamushi disease, but it is probable that 

 at present we are not fully aware of the disease carrying and pro- 

 ducing effects of these small arachnids. 



The Acarina are cosmopolitan in their distribution, and are 

 said to be most numerous in temperate regions. They are abundant 

 at high altitudes. Banks remarks that the parasites follow the 

 distribution of the host — -i.e., of the food- — a remark the importance 

 of which has already been appreciated. The observer will find 

 that a parasitic disease is often bounded by the distribution of 

 the food of the insect, which spreads the real cause of the disease. 



Morphology. — -The morphology is described under the heading 

 Ixodoidea. 



Life-History. — The female lays eggs covered by a shell and an 

 inner jmembrane called the deutovum. A six-legged larva hatches 

 out from the egg, and after a time, during which it may be active 

 or quiescent, moults and produces the nympha, which is eight- 

 legged, and resembles the adult, except in the non-development of 

 the sexual organs and apertures. The leg which is added to the 

 nympha is the fourth, or posterior. The nymph is usually energetic, 

 and feeds on some host ; eventually passing into a quiescent stage 

 and undergoing considerable histological changes, it moults and 

 becomes the adult male or female. The males are usually smaller 

 and more active than the females. Both sexes suck fluid nourish- 

 ment from the host, whether animal or plant. Their life-history 

 will be found to vary with the different families and species. 



Pathogenicity. — The Acarina are important in the dissemination 

 of disease both among men and animals. In men they are respon- 

 sible for the spread of such fevers as Button's relapsing fever, the 

 tick fever of the Rocky Mountains, Tsutsugamushi disease, and the 

 disease associated with the Miana bug in Persia; also they are 

 responsible for a number of skin diseases, among which may be 

 mentioned scabies. In animals they are responsible for the spread 

 of the Babesiases, and of some Spirochaetiases, as well as causing 

 some forms of dermatitis. Incidentally it may be mentioned that 

 they also cause disease in plants, producing galls, etc. 



Enemies. — Hurtful as they are themselves, they are not, however, 

 free from persecution on the part of other animals. Blood-sucking 

 insects, particularly the Reduviidae, prey on some arachnids, 

 sucking their blood. Wellman has reported that he has caught 

 Phoney gates bicoloripes Stal in the act of sucking the juices of 

 Ornithodoros mouhata, the tick responsible for the spread of Button's 

 relapsing fever. 



