190 On Aquatic Carnworous Coleoptera or Dytiscide. 
structures, each of which is a record of past action (of the discharge of function 
under certain conditions) and thus in the structure of every organic being there is 
written a portion of the history of the world. Just as the coarser facts of that 
great history are wricten in the forms of the hills and valleys, the shores and the 
abysses, so are the minute details recorded in the forms and structures of organic 
beings. To read that history and understand it, is the task of many generations, 
it is but recently we have learned that such a history exists to be read, and we 
have as yet scarcely mastered its alphabet. 
IJ. Comparative Review or tHE Structures or THE FAMILy.* 
S1zk.—The Dytiscide are of very variable size, Notomicrus brevicornis being 
only 1 mm. in length, while Megadytes ducalis attains nearly 50 mm. The most 
minute forms are found amony the Noterides, Hydrovatini, Bidessini, and Hydro- 
porides ; the most massive amongst the Cybistrini and Dytiscini. The Noterides, 
Laccophilini, and Hydroporides, do not comprise within their limits any individuals 
so large as the smallest of the Cybistrini and Dytiscini; and in respect of this point 
the Colymbetides and Hydaticides stand intermediate between the small insects and 
the large ones. The linear classification I have adopted agrees, as a whole, in a 
remarkable manner with the development in size of the individuals composing the 
groups : all the small forms come in the earlier half of the arrangement, while the 
massive Cybistrini terminate it. It may be truly said, in a generalized manner 
that the higher forms consist of individuals of much larger size than do the lower 
forms ; no doubt many exceptions to this generalization may be pointed out ; and 
much variety exists in respect of size within the limits of a single genus ; for instance, 
in Cybister we have the little C. dehaani only 13 mm. long, while C. owas attains 
40 mm. of length. 
In Form the Dytiscidze show comparatively but little variety, in fact there is probably 
no other family of Coleoptera of equal extent so uniform in this respect. The outline 
is oval or oblong-oval, the greatest breadth being usually attained a little behind the 
middle of the length of the body : considerable differences are found in the amount 
of elongation, sometimes the oval is so short that the outline approaches to the 
circular form, as is the case in many species of Hydrovatus and in Colpius, while in 
the case of Dytiscus cicurus, Fab. (No. 956, Rhantus,) the length is nearly 23 times 
the greatest breadth ; all the ntermediategrades of elongation occur. The extremely 
short forms are foundonly among the smaller species. Sometimes the anterior extrem- 
ity of the body is broader than the apical, asin Matus, but more usually the reverse is 
the case, and in Thermonectes the hinder portion of the body is very much broader 
than the anterior half ; in certain rare cases the extremity of the body and the wing- 
* This sketch should form part of the fifth synthesis, but is placed in this position in conformity with 
the usual custom. 
