958 On Aquatic Carnivorous Coleoptera or Dytiscidce. 



panied by a flattening of the joints, such as would be produced by a pi-essure 

 acting on the incrassate cylinder in a vertical direction. These two processes of 

 incrassation and flattening do not go on 'pari passu, some species with much 

 incrassation showino; but little flatteninof, and vice versa. The flatteninsf of the 

 tarsi is carried to its greatest extent in the cases where the individuals are large 

 (gen. Colyrabetes, Meladema), and then the joints are co-adapted, so as to form a 

 saucer-like surface beneath ; but this differentiation never attains the perfection of 

 these parts as seen in Dytiscus, Hydaticini and Cybistrini. The front and middle 

 claws of the males sometimes attain a truly wonderful development, and frequently 

 in such cases are very unequal {vide Lancetes, L. unguioularis ; Agabus, Dytiscus 

 bipustulatus ; Hhantus, Dytiscus exoletus, &c.) The clothing of the under-surface 

 shows great difference in its development ; it is rudimentary and inconspicuous in 

 the lower forms fMetronectes, and several Agabi, &c.), and it may even remain, 

 but littlf developed in species which in other respects have become considerably 

 perfected. In its rudimentary form the clothing is short and uniform, and has the 

 appearance as if some very fine grains of sugar were placed on the under-surface 

 of the foot : in this state I have frequentl}' spoken of it in my descriptions, as 

 " glandular pubescence;" as development becomes perfected, the clothing undergoes 

 both growth and differentiation, the middle hairs have their glandular extremities 

 developed into conspicuous palettes, while the external ones become beautifully 

 elongated into pencils of fine fringing hairs. These de\elopments do not neces- 

 sarily go on pari passu, but the palettes may be greatly developed, with the fringing 

 hairs less perfect, or vice versa ; the greatest development of the palettes may be 

 seen in some Colymbetes (Dytiscus striatus. No. 972,6.9.), while it is in some of 

 the Khanti (I'ytiscus adspersus, and Dytiscus pustulatus for example), that the 

 most perfect fringing hairs are found ; in these cases the extremities of the fringing 

 hairs are beautifully curved. This differentiation into palettes and fringing hairs 

 is not the only one that has occurred in the Colymbetides, for in some of the 

 species of Colymbetes we find that there is {vide D. striatus, &c.) coexistent with 

 these two structures, a basal patch of glandular or cilia-like pubescence ; in such 

 an insect we have a transformation similar to what has occurred in Cybistrini, whei'e 

 the male palettes are surrounded with fringing hairs, and the clothing of the 

 included part of the palette consists of fine hairs on the basal portion and of transverse 

 series of palettes on the outer part. The perfection attained by this structure in 

 the Colymbetides, is however far inferior to what we find in Cybistrini. 



Gradations in other important parts of the organization are displayed in the 

 Colymbetides ; thus the posterior stigmata, small and almost useless in some of 

 the lower forms, become in Colymbetes transverse, and are the seat of develop- 

 ment of a beautifial minute anatomical structure. 



As highly exceptional characters exhibited by certain members of the family, 

 the following are worthy of note : a dilatation of some of the intermediate joints of 



