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 

 cycle. 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 

 function 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. Megnin ^ 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 



^ For the Protozoa to which these and similar diseases are due, cf. vol. i. pp. 120 f. 



2 a. n. Soc. Biol. Paris (7), iv., 1882, p. 305, 



^ Ann. Soc. Linn. Lyon, xxii., 1876, p. 29. 



