74 SECTIONAL ADDRESSES 
Hydroporine genera, but it is remarkable that in the higher genera it only 
appears in sections, suggesting that it is a characteristic of the lower 
forms and is disappearing in the higher ones. Further, in a number of 
the Hydroporines, even in those with double punctuation, there are 
frequently other markings, and these are often similar to those found in 
the higher Dytiscids, suggesting that, if the higher forms have lost their 
punctuation and developed reticulation, so the lower ones are, so to speak, 
preparing to follow along the same line. 
Among the secondary sexual characters displayed by the Dytiscidz is 
a widening in the males of the anterior and sometimes also of the middle 
pair of feet, and this occurs in varying degrees throughout the family. 
The sexual importance of these structures is in connection with céitus 
when the front feet rest upon the pronotum of the female while the middle 
feet lie along her wing-cases towards the apex. 
The under side of each of the three basal segments of the foot is covered 
with a brush of hairs of two kinds, simple hairs tapering to a point and 
sucker-hairs, open at the apex and capable of being applied to a smooth 
surface and of adhering to it. These sucker-hairs occur in every species 
of Dytiscid I have examined, from the Noterines upwards, but they 
show considerable variety, from what appears to be a mere widening tube 
with an open end to a pedicel carrying a very highly-developed sucker- 
disc; but the sucker-hairs are, apparently, definite in number and 
position in each genus and, certainly in the higher forms, in each species. 
Here we have a structure which performs a definite function and which 
Chataney 21 has shown has evolved from a simple hair, and whose stages 
of growing complexity can be traced through the Dytiscide from the 
lowest, in which they are simple wide tubes and few in number, to the 
highest, in which elaborate and often very numerous suckers are to be 
found. Moreover, not only can this gradually improving structure be 
traced as described, but in the Colymbetine, with about twenty-seven 
genera and 560 species, each stage can again be recognised, suggesting 
that the evolution of sucker-hairs, which appeared in the Hydroporina, 
appeared again independently in the Colymbetine. With regard to the 
Dytiscinz, there seem to be no early or intermediate stages, all the forms 
possessing highly complex structures but, within the group, a further 
variation has taken place. All the Hydroporine and Colymbetine have 
the apex of the sucker elongate and parallel-sided, not rounded, but in 
only one group of the Dytiscine, the Cybistrini, containing about one 
hundred species, does this type occur, all the other sections, with about 
two hundred species, having circular suckers. Chataney, who has studied 
these suckers, has shown that in the Cybistrini every species can be 
identified by the arrangement of the suckers and, if this is not strictly 
true for other groups, there is very little overlapping from one species 
to another. 
In all this wonderful evolution no change whatever has taken place in 
the female pronotum and wing-cases which, apparently, have been equally 
21 “Sur le Tarse des Dytiscides,’ Ann. Soc. Ent. France, vol. 1xxix, Igto. 
