A SYNOPSIS OF AISTHALIAX ACARINA — RAINBOW. 149 



fourth cuxHj or a little behind ; palpi free ; skin 

 often coriaceous or leathery ; tarsi often with a 

 sucker 3 



No such distinct spiracle in a stigmal plate on tliis 

 part of bod}- 4. 



3. Hyperstome large, furnished below with many recurved 



teeth ; ventor with furrows ; skin leathery ; large 

 forms, usually parasitic Ixodoidea 



Hyperstome small, without teeth ; venter without 

 furrows ; body often with coriaceous shields, pos- 

 terior margin never crenulate ; no eyes. Gamasoidea. 



4. Body usually coriaceous, with few hairs ; with a 



specialised seta arising from a poi-e near each posterior 

 corner of the cephalothorax ; no eyes; mouth parts and 

 palpi very small ; ventral openings of abdomen large ; 

 ne verparasitic ; tarsi never with a sucker. Orihatoidca. 



Body softer, without such specialised seta. ... 5. 



5. Living in water Hydradinoidea. 



Not living in water . 6. 



G. Palpi small, three-jointed, adhering for some distance 

 to the lip ; ventral suckers at genital opening or near 

 anal opening usually present; no eyes; tarsi often end 

 in suckers ; beneath the skin on the venter are seen 

 rod-like epimera that support the legs ; body often 

 entire ; adult frequently parasitic. . Sarcoptoidpa. 



Palpi usually of four or five joints, free : rarely with 

 ventral suckers near genital or anal openings ; eyes 

 often present ; tarsi never end in suckers ; body 

 usually divided into cephalothorax and abdomen ; rod- 

 like epimera rarely visible ; adults rarely parasitic. 7. 



7. Last joint of palpi never forms a " thumb '' to the 

 preceding joint ; palpi simple, or rarely formed to 

 hold prey ; body with few hairs . . Eupodoidea. 



Last joint of palpi forms a " thumb " to the 

 preceding, which ends in a claw (a few exceptions) ; 

 body often with many hairs. . . . Tromhoidea. 



