10 1 2 INSECTS AND ARACHNIDA 



eggs are matured ; the oviducts both terminate in an unpaired 

 vagina, whence the eggs pass into a long, membranous, extensible 

 ovipositor, often wrinkled or striated with singular fineness and 

 beauty. The external aperture is closed by chitinous folding doors. 

 A more or less similar arrangement may be found in most Gamasidce, 

 Hydrachnidce, &c., but without the ovipositor. Spermathecse are 

 often found in the Gamasidce, Tyroglyphidce, &c., and accessory 

 glands frequently accompany the vagina in almost all families. The 

 male system varies greatly, but is frequently constructed on similar 

 lines, preserving somewhat of the ' ring ' form. 



The principal families into which the Acarina are divided are as 

 follows : 



The Gamasidce, which in the adult stage are mostly pro- 

 vided with a hard chitinous cuticle in all parts of the body. They 

 are mostly predatory, but the females and young are often parasitic. 

 Pteroptus and Dermanyssus, however, are more leathery in texture, 

 and are parasitic during their whole lives, the former on bats, the 

 latter on birds. This family have the true stigmata, one on each 

 side of the ventral surface, usually between the second and third 

 pairs of legs ; these do not communicate directly with the external 

 air, but have a long tubular peritreme in the chitin of the ventral 

 surface, often very elaborate in form, and emerging to the air usually 

 between the first and second legs. This is highly characteristic of 

 the family. 



The Ixodidce, or ticks, most of which are probably primarily 

 vegetable feeders, but will, when opportunity offers, attach them- 

 selves to animals by sinking their long serrated rostral projection 

 into the skin, have a single ventral stigma on each side, com- 

 municating directly with the air by a large cullender-plate, which 

 is an interesting microscopical object. The males have the dorsal 

 surface of the abdomen almost entirely covered by a chitinous plate, 

 which is much smaller in the females; but the leathery portion of 

 the abdomen in that sex is capable of great distension for the pur- 

 pose of permitting the suction of animal juices. The Argasidce 

 must be included in this group ; their tenacity of life and power of 

 existing without food are marvellous ; their bite is severe, but the 

 terrible stories told of the results of the bite of the Persian Argas 

 have not been supported on investigation. 



The Oribatidce are mostly wholly chitinised, the chitin being 

 very hard and brittle. The stigmata are in the acetabula of the 

 legs. The pseudo-stigmata (hearing organs) of this family have 

 been before referred to. Oribatidce are vegetable feeders, living in 

 moss, lichen, fungus, dead wood, under bark of trees, &c., and some 

 few species on aquatic plants. They are widely distributed from 

 the arctic regions to the equatorial. Hoplophora has the power of 

 withdrawing the legs wholly within the carapace, and then shutting 

 down the cephalo-thorax against the abdomen, so as to close the 

 opening, when it appears like a chitinous ball ; from this power it 

 has been called the ' box-mite.' The sexes have not any external 

 difference. 



The Trombidiidce are a large and varied group, mostly predatory 



