190 On Aquatic Carnivorous Coleoptera or Dijtiscidie. 



structures, each of which is a record of past action (of the discharge of function 

 under curtain conditions) and thus in the structure of every organic being there is 

 written a portion of the histoiy of the world. Just as the coarser facts of that 

 great history are written 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 

 beino-s. 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. 



II. Comparative Review of the Structures of the Family.* 



Size. — The Dytiscidaj 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 among 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 hiffher forms consist of individuals of much larsrer 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 Cy bister we have the little C. dehaani only 13 mm. long, while C owas attains 

 40 mm. of length. 



In Form the Dytiscidse 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 grea,test 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 Dyiiscus cicurus. Fab. (No. 95G, Ehantus,) the length is nearly 2^ times 

 the greatest breadth ; all the intermediate grades of elongation occur. The extremely 

 ^hort forms are found only among the smaller species. Sometimes the anterior extrem- 

 lity of the body is broader than the apical, as in Matus, but more usually the reverse is 

 the case, and in Thermouectes 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. 



