Hydaticus is distinguished from Dyticus by the length of the 
2nd joint of the antennae, by the terminal joint of the palpi, 
and many other minuter distinctions ; from Acilius it is still 
further removed by the convexity of the abdomen and the di- 
lated tarsi of the middle pair of legs, which certainly exhibit 
a nearer affinity to Dyticus ; and from both it may be distin- 
guished by the smooth elytra of both sexes. 
Fabricius in his Entomologia Systematica , in describing Dy- 
tiscus cinereus evidently confounded it with the male of D. sul- 
catus , referring to Linnaeus for the characters, and to Schaeffer’s 
figure of D. sulcatus to identify it, thereby giving rise to an 
error, which has been extended by Mr. Marsham in his Ento- 
mologia Britannica • and Olivier has unluckily assisted in the 
confusion by referring to two figures of his own, in his description 
of D. cinereus , one of which (tab. 4. fig. 32 b) is the female ; 
but the other (fig. 32 a) is an Acilius with patelliform tarsi; 
although it is clear from his description that he was unac- 
quainted with the male, for he there says the anterior tarsi are 
simple. Schooner however, in his invaluable Synonyms, has 
cleared up the point, and his friend Gyllenhal has confirmed 
his opinion in the Insecta Suecica : and as the Acilii have the 
entire margin of the thorax yellow, and Linnaeus in his de- 
scription of D. cinereus expressly says 46 Thorax flavus, mar- 
gine anteriore et posterior e (non lateribus) nigro” there can be 
no doubt of our insect being the one described. 
The scarcity of this species has contributed greatly to the 
confusion that has happened : upon the continent it is very 
rare, and in this country was unknown till Mr. Chant and 
Mr. Bentley took the sexes at Whittlesea Meer, the end of 
last July. We can now enumerate the following as British, 
of this rare and beautiful genus. 
1. Hydaticus cinereus Linn., Nobis. 
2. transversalis Fab., Panz.fasc. 86. pi. 6. mas. 
m. July, near Yaxley, Huntingdonshire. 
3. stagnalis Fab., Panz. fasc. 91. pi. 7. fern. 
Ponds, Wiltshire. 
4. Hybneri Fab., Oliv. tab. k.f 33. — parapleu- 
rus Marsham. m. July, Whittlesea Meer. 
They all inhabit ponds and ditches, and are found in June 
and July. 
The plant is Ranunculus aquatilis (Water Crowfoot). 
