THE MITES 95 



Due to parasitism, the mites have undergone considerable modifica- 

 tion, the scab mites are without ej-es or organs of respiration, and gener- 

 ally the tips of the feet are terminated by suckers or bristles. 



In the larval stage, the Acarina have but three pairs of legs, the fourth 

 pair appearing behind the third after a molt. 



The mouth parts are modified into a beak-like structure for piercing 

 and sucking. 



The sexes are separate and reproduction is by eggs which are extruded 

 from the genital pore situated, as in other Arachnida, anteriorly on the 

 ventral surface of the abdomen. In the scab mites (Sarcoptidse) the fe- 

 males are provided with a second genital opening, the copulating vagina, 



G J M 



Fig. 62. — Diagram of the anatomy of a spider: a, anus; b, cecum of 

 mesenteron; b', its anterior end; b", branches of cecum extending into 

 legs; c, cerebral ganglion connected with ventral ganglionic mass; d, 

 mesenteron; e, poison glands; g, heart; h, chelicerae; i, pedipalpi; j, liver; 

 k, hepatic duct; 1, lung sac; m, Malpighian tubules; n, dilation of rectum 

 into which Malpighian tubules open; o, eyes; ov, ovaries; p, female 

 genital pore; q, large and small silk glands; r, opening of tracheal system; 

 s, spinnerettes (after Boas by Kirkaldy & Pollard). 



which is located posteriorly just in front of the anus. It is at this open- 

 ing that spermatozoa are received during copulation, the anterior open- 

 ing serving as the pore of the oviduct. 



In their development the Acarina undergo a succession of stages. The 

 larvae are usually provided with but three pairs of legs (hexapodal), and 

 have no definite sexual characters. After molting a fourth pair of legs 

 appears, and the acarus enters upon its njanphal stage. Following 

 further molting the genital organs are acquired, and it has then reached 

 the pubescent stage, or stage of sexual maturity. After copulation the 

 female undergoes a further transformation, l^ecoming an egg-bearing, 

 or ovigerous female. 



Parasitism. — The majority' of the Acarina are parasitic, though, as 

 to this habit, there is much diversity, some being semiparasitic, others 

 essentially so and restricted to definite hosts. 



The order contains a number of families of which the following are 

 here considered: 



