XVIII STRUCTURE 457 
three to nine weeks, and the development of the egg from eleven 
weeks to six months. At least a year is occupied in the whole 
eyele. These ticks, and many others, communicate disease ' by 
inoculation, conveying it from one animal to another. 
No poison glands have been demonstrated in the Acari, the 
funetion of the salivary glands of the Ticks being probably to 
prevent the coagulation of the blood of their victims. 
It is an important point in the mode of life of the Ticks that 
they can live for a long time without food. Mégnin®* states 
that he kept an <Argas alive for four years, entirely without 
nutriment. 
In the Tetranychinae (see p. 472), glands apparently homo- 
logous with the salivary glands of the Ticks have taken on the 
function of spinning organs. According to Donnadieu,’ these 
glands, which resemble bunches of grapes, and are possessed by 
both sexes, open into the buccal cavity at the base of the 
chelicerae. The gummy fluid exudes from the mouth, and is 
combed into threads by the pedipalps. The legs of these mites 
are furnished terminally with curious hairs ending in a round 
knob, which are supposed to have some relation to their spinning 
habits. 
The males are the busiest spinners, the time of the females 
being largely occupied in laying eggs among the excessively fine 
threads of silk with which the Mites cover the under surface of 
leaves. In the Eriophyidae (see p. 464) corresponding glands 
are thought to furnish an irritant fluid which causes abnormal 
growths or galls upon vegetable tissues. 
External Structure—It is often stated, but erroneously, 
that there is no distinction between cephalothorax and abdomen 
in the Mites. Certainly no such division can be made out in 
the Hydrachnidae (see p. 472) or in some other forms, but in 
the majority of Acari the cephalothorax is clearly marked off by 
a transverse groove or suture. In some cases the anterior 
portion of the cephalothorax is movably articulated with the 
rest, and forms a sort of false head called a “capitulum.” In 
most Mites the chitinous integument is soft and non-resistant, 
but it is otherwise with the Oribatidae or “ Beetle-mites ” (see 
1 For the Protozoa to which these and similar diseases are due, cf. vol. i. pp. 120 f. 
2 0. R. Soc. Biol. Paris (7), iv., 1882, p. 305. 
3 Ann. Soc. Linn. Lyon, xxii., 1876, p. 29. 
