On Aquatic Carnivorous Coleoptera or Dytiscidce, 927 



II. 8. — Group Hyphidrini. [Vide p, 370.) 



About thirty species, of which no less than twenty-four form the genus 

 Hyphydrus, while the others are all autogenera, form this secondary aggregate. 

 The size of the individuals is always small but never minute, the greatest length 

 being 6 m.m., the least 3 or 4 m.m. In respect of form, colour and sculpture there 

 is however much variety. The positive characters distinctive of the group are 

 found in the hind coxa3 and their articular cavities ; these latter are never contiguous} 

 and are quite exposed, being unprotected by the coxal processes ; these are adpressed 

 to the level of the ventral segments and thus separate the coxal cavities, their 

 postero-external angles are obtuse ; there is no coxal border, and the coxal lines 

 are not curved outwards near the extremity. The hind, coxae themselves have an 

 extreme development, and extend forwards so as to reduce the side-wing of the 

 metasternum to a very slender band. All the components moreover have a largely 

 developed prominent ligula on the inner face of the elytra near the apex. The hind 

 coxa is free and not soldered to the ventral segments. 



Although these insects appear to be approximated by the Pachydrini, by virtue 

 of the form of the hind coxse and their exposed articular cavities, yet they remain 

 very distinct by the unsoldered coxae. In Pachydrini, where the adpressed coxal 

 processes are soldered with the ventral segment, the process of evolution can only 

 bringabout approximation of the trochanters by diminishing the intervening coxal pro- 

 cesses, but in Hyphydrini where the processes are not soldered to the ventral segment, 

 approximation of the trochanters becomes possible, by means of freeing the extremity 

 of the coxal process from the level of the ventral segments, and the playing of the 

 trochanter above the liberated portion if extension inwards of the articular cavity 

 accompanies the change of level of the process ; now if Hyphydrus major be ex- 

 amined it is seen that the outer angle of the coxal process is really somewhat 

 detached so as to form as it were the beginning of such a process of evolution.* 



As regards the structure of the articulations of the swimming legs, the Hyphydrini 

 must be I think regarded as an imperfect form, the coxal processes serve in no 

 respect to protect the articular cavities, and the pyxal processes project beyond the 

 coxal processes : but in other respects the group has attained a high degree of evolu- 

 tion. The hind coxae are extremely large, and in Hyphydrus have actually attained 

 about the maximum of possible extension. There are no forms known in which the 

 coxae are small, and the lower forms of the aggregate are in this respect very 

 highly developed. We may I think conclude that we have here then highly evoluted 

 forms of an inferior type of structure ; and also that the surviving species have 

 been able to maintain their existence by means of extreme evolution of certain 



* The process of evolution in the Hyphidrini may actually have been the reverse of what is sketched 

 above, this being one of the cases where we can readily see the transitions that may have occurred, but 

 not so readily determine which marks the stai-ting, which the terminal point of the series. 



