204 On Aquatic Carnivorous Coleoptera or DytisGid<B. 



Dytiscus lanio) it cau also be traced nearly across the head ; on the other hand 

 in Noterini, Laccophilini, and the Hydi'oporides, the clypeal suture is totally 

 obliterated : in the genus Dytiscus where this suture, as already remarked, is 

 unusually distinct it is in some species greatly deeper in one sex than in the other 

 {vide Dytiscus dauricus. No. 998). The clypeus itself terminates over the labrum 

 as a thin edge, so that the labrum continues the plane of the surface of the 

 clypeus ; but in Hyphydrus we find an exception to this, the anterior part of the 

 clypeus being deflexed at right angles, so that the labrum is placed on a different 

 plane to that of the front of the head ; in this case a slightly raised line passes in 

 a cui-ve from eye to eye, and marks oft' the small deflexed anterior portion of the 

 clypeus from the rest ot the surface ; there are other members of the Hydroporides 

 in which the deflexed anterior portion of the clypeus exists in a less distinct 

 manner, {vide Dytiscus insequalis and allies in the genus Coelambus), and this is 

 also the case with the raised margin, which exists in various degrees of obliteration ; 

 or partial development, the middle part of this line is frequently absent, even 

 when the lateral part remains distinct on each side, {vide species of Herophydrus 

 and Hydrovatus) ; I have spoken of this margin as existing in various degrees of 

 obliteration, but it is doubtful whether this is really the case, and perhaps it may 

 rather be that it is in some cases partially developed ; this question can scarcely 

 be determined without the aid of embryological research, but as the deflexed and 

 margined clypeus is accompanied by a change in position of the parts of the 

 mouth, inasmuch as they are in such case placed more on the undersurface, and 

 as it seems to be an advantage to these predaceous beetles to have the parts of 

 the mouth, more especially the mandibles and maxillse, quite in the anterior part 

 of the head, I think the process of evolution is to bring about the diminution of 

 the deflexed anterior portion of the head, and so permit the labrum, and the 

 mandibles and maxillEB which this covers to be brought quite to the front of the 

 head ; but from other considerations, which I need not here enter into, the reverse 

 of this might be argued. 



The depressed line, which I have described as existing on each side of the head 

 close to the front angle of the clypeus, exhibits much variation, and is in the Hydro- 

 porini frequently so indistinct as to be nearly absent ; in other cases these lines 

 assume a greater extension so as to nearly join in the middle, and form a nearly con- 

 tinuous line parallel with the front of the clj'peus (Dytiscus maculosus, No. 92, e.y.)-, 

 in those species where the front of the clypeus is deflexed, there may be detected 

 at its very front edge a transverse depression, which seems to have arisen from the 

 extension of these Unes {vide Hyphydrus major) ; in the Noterides these clypeal 

 depressions are usually placed nearer together, and so farther from the angles of 

 the clypeus, while in Colpius they appear only to be represented by a very vague 

 impression at a rather greater distance from the front edge. In the larger Dytiscida;, 

 these depressions assume more the form of foveas than of lines, and are of remarkably 



