On Aquatic Carnivorous Coleoptera or Dytiscidce. 221 



bister scutellaris (No. 1 1 U 1) : tkese facts suggest that it is j ust possible that the remote 

 and less perfected ancestors of Cybister scutellaris may have possessed a prothorax 

 somewhat similar to that of Pelobius. The prothorax of the Haliplidte is likewise 

 intermediate between the Carabidte and Dytiscidse, its general shape being 

 Carabideous, while it possesses a large prosternal process and the coxal cavities are 

 closed behind, by what may be called a horny membrane so as to be in these 

 respects nearer to the Dytiscida^ ; it shows however characters extremely difierent 

 from Pelobius, the front coxae being short and globular, instead of elongate as in 

 Pelobius, the anterior margins destitute of ciliae, and the prosternal process broad, 

 and quite truncate behind instead of acuminate. 



In the Carabidte the peculiar genus Omophron has been supposed to be in respect 

 of its prothorax an approximation to the Dytiscidce, but this is not really the case, 

 for the coxal cavities are closed behind in a manner thoroughly Carabideous, 

 and quite different fi'om what we find in Dytiscidas, Haliplidoe, Pelobius, and 

 those Carabidaj which ajDproximate them (Trachypachys and Systolosoma). 

 The prothorax of Silphomorpha (Cai-abidee) in some respects approaches that 

 of the Dytiscidae, but in others it is very different ; the broad roof-like sides, and 

 the existence of a prosternal process, and the absence of ciliation on the upper 

 front margin are the points of approximation, while the form of the prosternal 

 process, the mode of closure of the coxal cavities, and the very short coxae, are very 

 different from what we find in the Dytiscidse. If, however, the prothorax of a 

 Carabus, of an Agabus and of a Silphomorpha were taken for comparison, I think 

 we might quite as justly conclude that the thorax of the water beetles was 

 intermediate between those of the two Carabidte, as that any other linear arrange- 

 ment of the three was correct. 



In concluding this slight sketch of the structure of the prothorax in the Dytiscidse 

 and some of the forms that approximate to the family, I may be pardoned for pointing 

 out that any attempt to account for the resemblances we find to exist by a theory of 

 heredity or community of descent only leads one into a maze of jDerplexities, which 

 becomes the more distressing the farther we pursue it. If, however, we attempt to 

 account for the resemblances as being adaptations, either by evolution or special 

 creation, to the conditions of existence, or the environment, we find them qmte 

 comprehensible ; in the beetles inhabiting water, viz., Dytiscidse, Amphizoa and 

 Pelobius, it has been necessary to exclude the water from the interior of the body, 

 and to fix the prothorax solidly so that it shall not be moved from its place during 

 locomotion ; tlms we find perfect coadaptation of the parts to suit those of the 

 afterbody, and the locking of the parts together by a prosternal process reaching 

 to a part behind the articulation required to be rendered secure ; moreover in the 

 higher forms which move much and rapidly in the water we find all projections and 

 prominences that would impede forward motion diminished or removed, and it js 

 possibly for this reason that there is an absence of the ciliae of the anterior margin. 



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