284 MR. H. M. BEENAED ON THE EELATION 



mandibles are typical biting-jaws, like those of an ordinary 

 Spider. 



Again, tbe pedipalps are equally clearly Arachnidan. Tbe 

 basal parts are fused in the middle line and prolonged into a kind 

 o£ chitinous beak, while their palps are 4-jointed, the penultimate 

 joint being prolonged into a slightly curved hook which, with the 

 terminal joint, forms a kind of chela. There is therefore in this 

 case no difficulty whatever iu homologizing the mouth-parts with 

 those of an Araneid. 



Pollowing upon these month-appendages we have the typical 

 four pairs of ambulatory legs of the thorax. These together 

 make the typical six pairs of appendages of the Arachnids. 



This certainly seems the simplest and most natural homology 

 of the Acaridan limbs. It agrees best with all we know as to 

 the development of the Acarids, which bears such striking re- 

 semblance to that of the Arachnids. It is necessary to emphasize 

 this point, which we have hitherto assumed, because of the desire 

 on the part of some to find other (? Hexapodan) homologies for 

 the mouth-parts and limbs of the Acarids. Haller, for instance, 

 claims a distinct 2nd pair of maxillae and an underlip. The 

 difficulties in the way of this interpretation of the Acaridan 

 morphology are, it seems to me, overwhelming. 



All other Arthropods, for instance, have antennae, while the 

 Acaridse agree with the Arachnids in not having such appendages. 

 Further, it is extremely doubtful whether the special structures 

 which Haller claims as 2nd maxillae and underlips can really be 

 so interpreted. They appear to be secondary modifications in 

 adaptation to the needs of the animal, and quite in accordance 

 with the extraordinary specialization exhibited by so many of the 

 Acaridae in almost every part of the body. No trace of such 

 extra mouth-parts can be found in such simple forms as Tetra- 

 nychus^. Winkler has described the underlip of Gamasus, and 

 pointed out its very probable homology with the underlip of a 

 Spider. 



It seems to me, then, that this attempted change in the clas- 

 sification of the Acarids is in a high degree strained, in com- 

 parison with the usual classification which ranks them with the 

 Arachnida, and which is based upon developmental history, upon 



* Mr. A. D. Michael, one of the first authorities on the Acarids, informs me 

 that he certainly could not claim for the Mites any appendages homologous 

 with the 2nd maxillffl and labium of the Hexapoda. 



