72 SECTIONAL ADDRESSES 
Coleoptera. Whether or not it has a special function in other families, 
we can only say that it may have arisen in the Hydrophilidz in response 
to physiological activities. 
If we now concentrate our attention upon the Dytiscidz and examine 
the characters upon which the subdivisions are based, we find that the 
majority of those upon which the tribes, genera and species are separated, 
show different stages of development and, since each stage is obviously 
a survival stage, the progress in evolution can scarcely have been due to 
natural selection or the elimination of the unfit. The Dytiscidz include 
about 1,700 species grouped in about 80 genera and the modern classi- 
fication is based upon that published by Sharp ?° in 1882 in his classical 
volume on the family. First there are two series known as the Fragmentati 
and the Complicati, distinguished by the fact that in the former the 
metathoracic episternum is cut off from the mid-coxal cavity by the 
mesothoracic epimeron, whereas in the Complicati the latter sclerite or 
plate does not intervene. The Fragmentati are divided into two on the 
shape of the post-coxe which are more like those of the land relations in 
the Noterine and more like those of the Complicati in the Laccophilina, 
we find that the latter are, so to speak, approaching the boundary line 
between the /ragmentati and the Complicati. 
In the Complicati there are three tribes: Hydroporine, Colymbetine, 
and Dytiscine, the first being separated from the others because the 
prosternal process, which projects backwards between the bases of the 
front legs and is a definite feature of the whole family, is bent and does 
not lie in the same plane as the rest of the prosternum, whereas in the 
other two this structure is flat. The Colymbetine are separated from the 
Dytiscine because the eye is notched in the former and smoothly rounded 
in the latter. 
These are characters upon which the main divisions of the Dytiscidz 
are based and our classification is founded upon what we believe to be 
relationships. The ancestral Dytiscid gave rise two to forms, fragmentatus 
and complicatus; the former again divided and produced the Noterine 
and Laccophiline ancestors, while complicatus first gave off the Hydroporine 
and then split to form the Colymbetine and the Dytiscine. But examina- 
tion shows that, very frequently, the tendency to vary in the same direction 
exists on both sides of a division. As already mentioned, the Laccophiline 
tend to become Complicati and, in the Hydroporine, there are variations 
in the bend of the prosternal process, some having the structure almost 
as flat as it is in the Colymbetine and Dytiscine. 
Within the Hydroporine, the largest group of the family, with about 
34 genera and 870 species, another interesting tendency is recognisable. 
In all other Dytiscids we find that the ventral plates of the middle and 
posterior thoracic segments (meso- and meta-sterna) are connected in the 
mid-ventral line. In most of the Hydroporine, however, this is not the 
case, these parts being separated. In the genus Hydroporus, usually 
20“ The Aquatic Carnivorous Coleoptera or Dytischide,’ Tvans. Roy. Dublin 
Soc., vol. ii, series 2, 1882. 
