THE ACAEINA OR MITES. 



91 



between the pseudostigmata are the superior bristles; the pair toward the tip and 



often at ends of the lamellae are the inferior bristles. There is also a pair of smaller 



bristles at the apex of the cephalothorax. Around the sides of the abdomen there is 



a line separating the dorsum from the venter. In 



the Hoplodermidse this line is often far down on 



the under side of the body. The dorsum of the 



abdomen is often devoid of hairs, but sometimes 



there are a few, usually arranged in rows. In many 



common species there is a membranous expansion 



each side at base of the abdomen, called ptero- 



morphae or wings. On the venter are two openings, 



the basal the genital one as usual. These openings 



are circular, elliptical, or rhomboidal, and are closed 



by folding doors opening outward, and hinged to 



the outer margins of the apertures. In many forms 



these apertures are very large and occupy the greater 



part of the venter ; in other and more highly organized 



species the openings are much smaller and quite 



remote from each other. 

 The mouth parts (fig. 179) of the Oribati'dse are 



obscure. The palpi are very small, five-jointed, 



and arise from the labium. They are usually in 



motion while the mite is walking. The mandibles 



are chelate in all save the genus Serrarius. The 



limbs of the chelae are commonly toothed on the 



inner sides. In Serrarius the mandibles are elon- 

 gated and rodlike, and there is no movable limb at 



tip; the edges being serrate, so that the mandibles 



act as a saw. The maxillae have their basal joints 



united into one transverse piece, the labium, 



which partly and sometimes wholly closes the 



mouth orifice. The maxillae incline slightly toward each other; their tips are broad 



and toothed. The Oribatidae deposit their eggs in crevices of wood, moss, or fungi, 



or on the ground. Their eggs are often ellipti- 

 cal, or cylindrical, ^\'ith rounded ends. Some- 

 times the surface is roughened and granulated. 

 The six-legged larva remains for a few weeks 

 in this stage, when, by a molt, it becomes 

 an eight-legged nymph. The nymph passes 

 through three molts, increasing in size at each, 

 the third molt bringing it to the adult condi- 

 tion. In some cases the eggs are not de- 

 posited, but ripen in the body of the parent 

 mite; the mother then dies and dries up, her old 

 shell remaining as a protection for the eggs till 

 they hatch. Other species have eggs with a 

 dark brown or chitinized shell, which is perfo- 

 rated with very minute pores. As the inclosed 

 larva develops the outer shell splits, showing the 



white vitelline membrane encircling the egg; this stage is the deutovum. The eggs of 



some genera bear lateral processes, and are deeply sculptured in elaborate patterns. 



The larvae are all monodactyle, and have a soft skin with but little color. The nymph 



(fig. 181) is also monodactyle; its skin is soft or leathery, or sometimes chitinized toward 



Fig. 178.— Leg, mandible, and pseudo- 

 stigmatic organ of an oribatid. 

 (Author's illustration.) 



Fig. 17 



-Mouth parts of an oribatid from 

 below. (Original.) 



