76 SECTIONAL ADDRESSES 
other, Dytiscus, one of the highest, has two equal or subequal claws. 
Only in Cybister has one claw disappeared, and in the related genus, 
Megadytus, the male has two equal claws, although the outer is greatly 
reduced in thickness, while in the female the outer is not only reduced 
in solidity but also in length. All through Agabus, one of the lower 
genera, the claws are similar in both male and female, but in J/ybius, 
with unequal claws, the male outer one is more or less triangular and 
blunted, while in the female it still retains the normal curved and pointed 
appearance. Thus while in I/ybius the male seems to be in advance of 
the female, in Megadytes the female is nearer to the one-claw stage. 
The foregut of the Dytiscid ends posteriorly in a structure known as 
the proventriculus, which is usually very muscular and armed with spines 
and hairs on its inner wall. Its functions are to crush solid particles of 
food and to filter fluids passing forward from the mid-gut. It is tubular 
and consists of eight lobes, four of which, the primary lobes, alternating 
with the others, project farther into the lumen and overlap the secondaries. 
On making a comparative examination of this structure about a year ago, 
I was struck by the fact that the four most conspicuous lobes in a number 
of species were oval in shape, while in others they were triangular, and 
an examination of more than a hundred species showed that, except in 
one genus, the same type of proventriculus was constant throughout 
every genus. Further, it showed that, whereas all the Hydroporine came 
into the ‘ round’ group with the Noterine, the Laccophiline were in the 
‘triangular’ group with the Dytiscine, the Colymbetine being in both 
groups, the single genus Copelatus having one subgenus in one and the other 
subgenus in the other. This proventricular character, therefore, seemed 
to be inexplicable on any theory, until I discovered that the round lobes 
so prominent in the one group are the secondary lobes, not the primaries, 
as I had described them,” so that the change which has taken place has 
not been from round to triangular but has been a development of the 
secondaries so as to squeeze out the primaries which have been greatly 
reduced. Moreover, whereas the prominent primaries were very 
obviously crushers, the improved secondaries are much less crushers and 
more efficient filters, and this fits in with the thesis I have already put 
forward with regard to various external characters that definite evolu- 
tionary lines run through the family, always from a more simple to a more 
complex stage. But here again it is difficult to recognise any part played 
by a struggle for existence or elimination of less efficient types, seeing 
that a whole series of these exists. 
One other character to which considerable importance is attached by 
systematists is the male and, to a less extent, the female sexual armature, 
the apical part of the male armature consists of a median aedeagus and 
a pair of lateral lobes or parameres, the organ of intromission being the 
zedeagus. These three parts are frequently of extreme importance in 
separating species, but they also retain a constancy of type in each genus. 
22 “The Proventriculus in the Dytiscide# as a Taxonomic Character,’ Stylops, 
vol. iii, 1934. 
