A. D. MICHAEL ON TWO SPECIES OF ACARINA. 109 



body of the adult Lahidostomma will confine it to tliree families, 

 viz., the OribatidcB, the Gamasince, and the Halicaridce. 



It is not one of the Oiibatidce, because, firstly, the front pair of 

 legs in that family is invariably a true pair of walking legs ; 

 whereas in Lahidostomma they are hardly walking organs at all, 

 but mere feeling organs, as in Gam,asin(je, Cheyletus, &c. Secondly, 

 the stigmata of the Oribatidce are always one on each side of the 

 upper surface of the cephalothorax, lead into an air sac below, and 

 are protected by short external tubes or rings, from each of which 

 issues what is called a stigmatic hau* ; this arrangement is absent in 

 Lahidostomma, the stigmata being placed more as in Gamasime. 

 Thirdly, all known Orihatidce have five joints to each leg ; Lahi- 

 dostomma has seven. Fourthly, most of the Orihatidce are sluggish 

 creatm*es, and are all vegetable feeders ; Lahidostomma is very swift 

 and apparently predatory. Fifthly, the claws are dift'erent. The 

 resemblance to the Ojibatidce is chiefly in the chelate non-retractile 

 mandibles, and in the different parts of the sternum being united by 

 apodemes.* 



The Halicaridce approach Lahidostomma in the position of the 

 genital organs and one or two other particulars, but are utterly 

 different from it in the formation of the mouth organs, the form 

 and mode of attachment of the legs, the formation of the feet, the 

 breathing apparatus, &c. ; and, moreover, they are entirely aquatic, 

 whereas Lahidostomma is terrestrial. 



There remain the Gamasince, and this family seems to offer the 

 greatest resemblance, the mode of arrangement of the external 

 stigmata, the general effect of the creature, the chelate mandibles, 

 the mode of using the front legs, the position of the other legs, the 

 number of joints in the leg, the development from nymph to perfect 

 creature, the fact that the dorsal plate turns over and embraces the 

 ventral, but in the female allows it to escape when the abdomen is 



* In order that my meaning may not be mistaken, I annex a shortened 

 translation of Professor Robin's very clear definition of the word (" Journal 

 de I'Anat. et de la Physiol.," 1867, 523) : — " Epidemes are parts of the 

 dermal skeleton of articulata, which arise from the internal face of certain 

 of the pieces and project into the interior of the body, but they always 

 arise from a single piece, and are consequently simple. Apodeynes (in the 

 articulata) are internal, or occasionally also external, blades, of the same 

 nature as the dermal skeleton, but placed at the lines of attachment of two 

 segments or pieces of the edges of which they form prolongations ; thus 

 they are always formed of two blades pressed against each other and 



soldered (anchylosed) together " This character distinguishes 



them from epidemes, which are formed of a single blade. 



