958 On Aquatic Carnivorous Coleoptera or Dytiscide. 
q Pp 7] 
panied by a flattening of the joints, such as would be produced by a pressure 
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 showing but little flattening, and vice versd. The flattening of the 
tarsi is carried to its greatest extent in the cases where the individuals are large 
(gen. Colymbetes, 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. unguicularis; Agabus, Dytiscus 
bipustulatus ; Rhantus, Dytiscus exoletus, kc.) The clothing of the under-surface 
shows great difference in its development ; it is rudimentary and inconspicuous in 
the lower forms (Metronectes, and several Agabi, &c.), and it may even remain, 
but little 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 frequently spoken of it in my descriptions, as 
‘olandular 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 developments do not neces- 
sarily go on pari passu, but the palettes may be greatly developed, with the fringing 
hairs less perfect, or vice versd; the greatest development of the palettes may be 
seen in some Colymbetes (Dytiscus striatus, No. 972, ¢.g.), while it is in some of 
the Rhanti (Mytiscus 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 (wde D. striatus, &e.) 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, where - 
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 uscless in some of 
the lower forms, become in Colymbetes transverse, and are the seat of develop- 
ment of a beautiful minute anatomical structure, 
As highly exceptional characters exhibited by certain members of the tamily, 
the following are worthy of note: a dilatation of some of the intermediate joints of 
