In most insects the first pair of legs is the shortest, and the 
hinder pair the longest, the length being given by the tibiae 
and tarsi; in Gyrinus, however, the anterior pair is the longest, 
in Parnus the legs are very similar, and in Hydrochus they 
are all perfectly alike, which gives the insect a peculiar cha- 
racter : to those genera Hydrochus appears to be related, as 
well as to Helophorus and Hydrsena (pi. 307). 
These insects inhabit ponds and ditches; they are found on 
mud at the sides, and on plants in the w r ater, on which some- 
times they are seen floating. 
1. H. elongatus Fab. Curt. Brit. Ent.pl. 359. 
April and May, sides of ponds, Battersea, and other places 
round London; also in Norfolk, Suffolk, and the West of 
England. Taken at Nottingham by Dr. Howitt, and at Col- 
chester by Mr. Davis. 
2. H. crenatus Fab. E. S. 1. 205. 6. — Oliv. 3. No. 38.pl. 1. 
/■ 
Narrower and shorter than II. elongatus : beautiful glitter- 
ing green, sometimes cupreous or black. Trophi and legs 
castaneous, tips of palpi, antennae and tarsi black. Head and 
thorax deeply punctured, the foveae on the latter shallow, with 
a large deep puncture near each posterior angle. Elytra with 
10 regularly and deeply punctured lines on each, giving the 
spaces between, which are very narrow and convex, a crenated 
appearance. 
The above species have been frequently confounded, by Na- 
turalists paying too much regard to colour: it is difficult to 
decide with certainty to which Panzer’s and Olivier’s figures 
belong; but as that of the latter author does not show the 
elevated striae on the elytra, and is too small for II. elongatus , 
I have referred it to this species. 
Taken in the same situations as the former insect, near 
London, and Glanville’s Wootton Dorsetshire, and in Nor- 
folk. 
3. H. brevis Herbst. Paylc . — Gyll . 1. 132. 8. 
1^ line long: dull black, antennae and legs pitchy, middle 
of the tibiae and tarsi castaneous : head and thorax rugose, 
the latter with 5 deep foveae similar to H. elongatus : elytra 
convex and ovate, the suture and 4 sharp ridges on each 
crenated, with two rows of large punctures between each. 
May, June, and July. Ditches in Norfolk, at Whittlesea 
Mere, and near Bristol. 
The plant is Potamogeton pectinatum (Fennel-leaved Pond- 
weed). 
