KNOWLEDGE OF THE HYDRACHNIDAE. 267 



irritating effect of the bites of certain Acari, eminently that of 

 the so-called Harv-est-bug {Leptus or Tetrarychus atitumnalis), 

 would certainly justify an anticipation of the existence of such 

 , structure. According to the more recent researches of C. J. 

 Neuman,* the mandibles in certain Hydrachnidae, such as 

 Zebertia, Midea and Aiiurania, is supplemented by a second 

 more minute opposing stylet, which consequently imparts to the 

 4ippendage the aspect of a true chela. 



The second pair of pointed appendages or palpi of the Hydrach- 

 nidae, stationed immediately in advance of the four pairs of 

 locomotive limbs and homologous with maxillary palps or pedi- 

 palpi of the typical spiders and with the great claws or chelae 

 of the scorpions, vary somewhat in structure among the different 

 members of the group, such divergences as exist yielding con- 

 venient correlative characters for generic diagnosis. With the 

 majority of forms these palpi, which consist of five joints, are 

 -simply oval-shaped or denticulate at their distal extremity, while 

 in a few others, such as the genera Arrhenuras and Hydry- 

 2)hantes the penultimate joint of each palpus is so prolonged 

 at one of its distal angles as to constitute, in conjunction 

 with the short fifth or terminal joint, a prehensile claw^ or 

 chela like that of a scorpion or lobster. The bases of the 

 palpi are in all instances united by a movable joint to the 

 -anterior or lateral angles of a chitinous lower lip-like structure 

 or labium, which embraces and conceals the mouth and man- 

 dibles in a sheath-like fashion and is not infrequently produced 

 anteriorly in such a manner as to constitute a distinct pro- 

 boscis or rostrum. The composition of this labium through the 

 coalescence of two lateral pieces is evident in many species, 

 und there would therefore appear to be justifiable grounds for 

 anticipating that these labial elements are the homologues of 

 the extended basal plates or epimera of the succeeding locomotive 

 appendages. 



The locomotive limbs, to the number of four pairs, which 

 follow upon the oral appendages exhibit no marked structural 

 differences, all consisting of six distinct articulations, and the 

 proximal point of each being united to a more or less extensive 

 ^dnate chitinous induration of the ventral surface of the 



* Svei'iges Hydraclinider. 



