240 On Aquatic Carnivorous Coleoptera or Dytiscide. 
ment and are at their maximum in Dytiscus, where all are large, and those placed 
on the two apical segments are, contrary to the usual rule, larger than the others, and 
are of a transversely elongate, elliptical form. In Cybister and Megadytes the 
abdominal stigmata are all small, and the quite small apical one, passes through the 
segment in the form of a tunnel, instead of opening directly through it as it does 
in others of the family ; moreover in the two genera just named, the terminal 
segment appears to be much more retractile and mobile than itis in other forms. 
The side pieces of the hind body consist of a hard membrane, becoming externally 
harder and corneous, and attached to the side of the turned up edge of the ventral 
plates; the side piece of the basal segment is marked in the Colymbetini and 
Dytiscini with transverse rugee, which do not occur in any other Dytiscide, and are 
probably of some assistance in the process of respiration. 
The ventral plates or segments, are six in number, and are hard and corneous ; 
they are transversely arched and each one has its outer margin turned upwards, 
and even somewhat inwards, and this part is marked off from the rest of the surface 
by a raised margin, so that the upper edges of the ventral plates look at first sight 
as if they did not belong to this part of the body, but were rather the abdominal 
side pieces; the margin giving them this appearance is however nothing but a 
raised carina for the accommodation of the edges of the elytra, a similar margin 
being developed, on the outside of the hind coxa and even along the side pieces of 
the metasternum. The basal segment is very much modified in form to accurately 
adapt it to the hind coxa, and for this purpose its middle appears to be completely 
cut away, so that in an undivided insect, the first visible ventral segment appears 
to be separated into two pieces placed one on each side and separated in the middle 
by a considerable interval; but on dissecting off the hind body it is seen that the 
two pieces are connected together by a slender isthmus, but that this middle piece 
or isthmus is concealed by being turned upwards at right-angles to the rest of the 
plate. The basal segment carries on its upper face (and therefore as it were in the 
interior of the body) a transverse corneous partition, extending all across the hind 
body, and adapted to the reflexed posterior portion of the hind coxa ; this concealed 
transverse partition is the true first ventral plate, the basal plate just spoken of 
being really a portion of the second segment : it results from this arrangement that 
the first dorsal plate is attached to what is truly the second ventral plate, the 
second dorsal plate to the third ventral, and so on; but this nomenclature is not in 
use and the first visible ventral segment or true second plate is called the first plate. 
The second ventral plate is more or less slightly emarginate in the middle in front, 
by being adapted to the projecting internal laminz of the coxz ; in some cases this 
emargination is but slight (Cybister, Dytiscus) but im others Dytiscus duodecim-pus- 
tulatus No. 462 e.g.) it extends nearly to the hind margin of the segment, so that 
in such a case if the ventral plates are counted along the middle line they appear 
to be only four in number. The sutures between the first and second, and 
