52 [Ju'y- 
OX THE BRITISH GYRrS'ID^. 
BY D. SHAEP. M.B. 
The GyrinidcB must be considered as one of the most peculiar and 
interesting of all the groups of beetles which are found in this country. 
The family, though it contains very few genera and species, is among 
the most sharply defined : indeed, though it possesses points of resem- 
blance on the one hand with the Di/tiscida, and on the other with the 
Farnidce, it is so distinct as to forbid the idea of its being descended 
(in a Darwinian sense) from either of them, unless we suppose that an 
extremely free disappearance of connecting links, of which we can now 
find no trace, has taken place. It is also interesting to notice that a 
genus of Carabid(B, Adehtopus of Hope, more resembles the GyrinidcB in 
general appearance than do any insects of either of the two families to 
which it is allied : not only is the facies of Adehtopus that of Gyrinus^ 
but both possess two separate eyes on each side the head, a pecioliarity 
of structure almost, I believe, without parallel in the rest of the 
Coleoptera : the dLnieun^n, too, oi Adehtopus are very short and com- 
pressed, so as to show a great resemblance to those of Gyrinus ; indeed 
the similarities between Adehtopus and the Gyrinid<B appear to be 
exactly of the character that has been called mimicry ; and it is also 
worthy of note that the Gyrinid<s, or the insects mimicked, exhale a 
peculiar nasty-smelling fluid when handled. As the Gyrinidw inhabit 
exclusively the surface of the water, and Adehtopus lives under the 
bark of trees, no theory of protection founded on natural selection can 
account, I should imagine, for this remarkable reproduction of peculiar 
characters in very distinct groups. 
Thomson (SkandinaAiens Coleoptera, Vol. II. ) places the Gyrinidce 
along with Parnus. Seterocerus, and others in a group which he calls 
AmpJiibii ; but they are now generally considered a distinct family, and, 
along with the Dytiscidce, form the group called HydradepTiaga. The 
characters by which the Gyrini4<B are distinguished from the other 
HydradepTiaga are so very peculiar, that, though my object at present 
is only to call attention to the characters of our British species, it is 
impossible to pass over these interesting points without some short 
notice of them. 
1st. The structure of the trophi is difierent from what holds in any of 
the Dytiscid<e, though not very peculiarlv or decidedlv. 
2nd. The Gyrinidce possess a pair of eyes on each side of the head, and 
these are placed so that the upper ones enable the insect to see 
