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CLASS ARACHNIDA, AND THE ORDER ACARINA 267 



horizon ; but the other families are of no importance, further 

 than that some of them include numerous species which are 

 predaceous on other mites. 



Synopsis of Principal Families of Acarina. 

 (Based mainly on Banks.) 



A. Body not vermiform, always with four pairs of legs in adult = i. 



B. Body vermiform, with four or two pairs of legs = 2. 



{Abdomen composed of 10 distinct segments, each of the first 4 with 

 a pair of dorsal spiracles (stigmata) Opiliocarida. 



Abdomen not segmented = 3. 



Two pairs of stigmata, one above the third pair of legs, the other 

 placed quite dorsally above the fourth pair of legs 



HolothyreidcB. 



A pair of stigmata, placed one on either side behind the coxag of 



the fourth, third, or second legs = 4. 



^No stigmata in the positions aforesaid = 5. 



Cheliceras ending in a series of strong, laterally-directed, hook-like 



teeth IxodidcB. 



Chelicerse not ending in hooks, usually pincer-like and exsertile 



= 6. 



'Stigmata behind the third pair of legs Gamasidce. 



Stigmata behind the second pair of legs ; the nymphs attach 



themselves to insects by means of a long viscid stalk secreted 



by adanal glands Uropodida. 



pair of (usually club-shaped) bristles, projecting each from a 



pore, or pseudostigma, placed near each posterior corner of 



the cephalothorax ; stigmata concealed in the leg-sockets ; 



integument hard and shiny = OribatidcB. 



No pseudostigma and bristle in this position ; integument softer 



= 7- 

 /Aquatic mites (fresh water and marine) = 8. 



I Not fitted for an aquatic life = 9. 



(A large movable beak or head. Marine Halacarida. 



Beak small ; legs fringed with hairs for swimming ; mostly fresh 

 water Hydrachnida. 



Pedipalps usually of 4 or 5 segments ; the tarsi usually end in 

 claws, never in a sucker ; stigmata (when present) near bases 

 of cheliceras = 10. 



Pedipalps of 3 segments, more or less adherent to lip ; the two 

 anterior and two posterior pairs widely separated ; the tarsi, 

 or some of them, usually end in a sucker ; no special breathing 

 organs as a rule =11. 



("Pedipalps with an additional, subterminal finger-like appendix 

 (Fig. 127) TrombidiidcB. 



[ Pedipalps with simple tip Bdellida. 



6. 



