ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 775 



Sense-hairs of the Hydrachnida.* — Dr. G. Haller has obtained 

 the material for his investigations from the Lake of Geneva, a locality 

 which has already t added considerably to our knowledge of the 

 systematic zoology of the group. 



Olfactory hairs. — In Atax the long hairs of the first pair of legs 

 occur chiefly on the lower and outer surfaces of the 2nd, 3rd, 4th, 

 and 5th joints, and diminish in length towards the latter joint. 

 They are sword-shaped. Almost all of them are inserted within 

 certain excavated eminences of considerable size, which protect the 

 base of the hair, while allowing of its free movement, and contain 

 the ganglion from which proceeds the nervous twig which supplies 

 the hair. The hair is traversed by a central cavity which opens to 

 the exterior near the point by an extremely attenuated canal; a num- 

 ber of similar fine canals leave the central cavity, and open at the 

 extremities of some fine teeth which fringe the posterior side of the 

 hair. The function is probably olfactory, and thus the first pair of 

 legs in Atax is equivalent physiologically to an antenna. Similar 

 hairs occur in Axona, and on the two hind pairs of legs, as well as 

 on the first, in Atax itself. 



Scales and tactile hairs. — On the hinder and interior surfaces of 

 the palps, where these hairs are absent, they are replaced by certain 

 scales and tactile hairs. The former are very widely distributed 

 among the Acaridai, and in some Oribatidce occur over the whole body 

 as well as the extremities, or they may be confined to the body or 

 to certain parts of it, or, as in Atax, to the extremities. In Atax 

 crassipes they occur only in the first pair of legs, at considerable 

 intervals, on the 2nd to the 5th joints ; they have a lancet-like shape ; 

 the cavity branches and opens in the same way as that of the olfactory 

 hairs, although in Atax the margin does not present the same thorn- 

 like points for the canals to open into. Nerves have been observed in 

 connection with the canals. The function of the scales is probably 

 also olfactory. 



Of the tactile hairs, already described by Haller elsewhere, two 

 new forms are described. The hook-shaped form ends in a fine head, 

 and is now stated to be connected with a nerve-fibre. The length and 

 thickness remain constant in the same species, but vary enormously 

 in different Acarids, the long and stout bristles of the ultimate and 

 penultimate joints in the two anterior limbs of the Dermaleichi and 

 Atax, and the short and weak hairs of the penultimate joint of the 

 maxillary palps, being referable to the same type ; a ring of very 

 short hairs surrounds the margin of the body of Uropoda clavus. 



The second form of tactile hair occurs only in a representative of 

 a new genus, Forelia, from the Lake of Geneva ; it occurs exclusively 

 in the male, and is aggregated in large quantities, covering consider- 

 able areas ; it may be either a short, slightly curved hair on a small 

 chitinous eminence which is penetrated by a nerve-fibre, or such a 

 hair may be accompanied by another of about half its length ; locality, 

 the end of the foot of Atax. 



* Archiv f. Naturges., xlviii. (1882) pp. 32-46 (1 pl.)c 

 t See Lebert's researches, this Journal, iii. (1S80) p. 69. 



