CLASSIFICATION OF ACARINA. 33 



absence of the tracheal system ; but it does not appear 

 to result necessarily from an aquatic life. 



It seems somewhat dubious whether the Limno- 

 caringe can properly be called terrestrial. Trouessart's 

 own description of their habitat is " Acari living in the 

 mud or walking at the bottom of the water; larvae 

 commensal on aquatic insects." The Hydrachnidse 

 are not invariably inhabitants of fresh water,* but 

 these exceptional cases may possibly be neglected. 



I scarcely like the expression " falcate " (en crochets) 

 as opposed to " chelate " for the mandibles of the 

 Ixodidae. Trouessart, however, describes these organs 

 much more exactly in his fuller definition of the family, 

 where he calls them " mandibles with two branches, 

 pseudo-chelicerse, not forming a didactile forceps but 

 a harpoon with a double dart." Even this does not 

 quite give me the idea of the organ, which does not 

 consist of a rigid spear of one joint with a double 

 distal end ; it is a piece of apparatus which would be a 

 chela if it were not that the teeth of the moveable arm 

 are on its outward edge, and are turned in the same 

 direction as the teeth, or curved point, of the fixed arm. 

 Neumann f calls them " Chelicerse." Supino { adopts 

 the same expression as Trouessart in the latter's fuller 

 definition, viz. " Pseudo-chelicerae." I should be in- 

 clined to regard them as consisting of a fixed spear 

 and a moveable cutter. 



I do not quite like the name " Octopoda," as applied 

 to the Demodicidas, because it would seem to be 

 indicative of the whole of the Acari except the 

 Phytoptidaa (Eriophyidas) , or even of a larger group. 

 Trouessart adopts Canestrini's subdivision of the 

 Oribatidae, which I have already spoken of (page 28). 



* Philippi, "Zoologische Betrachtungen VI," " Pontarachna, eine 

 Hydrachnide des Meeres," in ' Arch. Naturg.,' Bd. i (1840), p. 181, von 

 Sohaub. " Ueber marine Hydrachniden nebst einigen Berrnerkungen 

 iiber Midea. Bruz." S. B. Ak. Wien, 1889, Bd. xcviii, Abth. 1, pp. 163 

 179. 



f " Revision de la famille des Ixodides," in ' Mem. Soc. Zool. France,' 

 t. ix (1896), p. 2. 



J ' Nuovi Ixodides della Birmania,' Padova, 1897. 



3 



