Sept., 1912.] Matheson : Haliplid.e of North America. 181 



thoracic spiracle is small and probably functions to some extent as 

 air can reach it. Thus these beetles carry their air supply under 

 the elytra but obtain it by way of the coxal plates and lateral grooves 

 leading under the elytra. These grooves are lined with fine short 

 set?e, all pointing towards the entrance and prevent the ingress of 

 water. 



Structural Adaptations. — The Haliplidx are undoubtedly closely 

 related to the Carabidse on the one hand and to the Dytiscid^e on the 

 other. Whether they can be considered as a transition group is a 

 question I do not care to discuss at this point. The adaptations of 

 the diving beetles have been well discussed by Needham and William- 

 son (1907). The same kinds of adaptations there pointed out are 

 found in this family though not carried to such perfection of de- 

 velopment. The demands for aquatic life are: (i) Rigidity of 

 structure. (2) Diminished resistance. (3) Efficiency in swimming. 

 The rigidity of bodily form is brought about by the close fitting and 

 adjusting" of the parts of the external skeleton. As in the Dytiscidae 

 the head fits closely into the prothorax, the parts of the thorax are 

 quite solidly joined together, and the elytra are closely applied to the 

 sides of the body and to each other on the mid-dorsal line. 



The close-fitting of the head into the prothorax has been brought 

 about not by the flattening of the head as in the Dytiscid?e but by the 

 development of the prosternum to form a deep ridge into which the 

 bead fits (Fig. 12). The prosternum is firmly fused and almost com- 

 pletely conceals the mesosternum. The metasternum is prolonged 

 forward between the middle coxc-e and fits firmly into a socket-like 

 depression in the prosternum (Fig. 11). The elytra fit closely to the 

 sides of the body and their bases meet the sinuate margin of the 

 pronotum closely and firmly, making a water-tight and smooth junc- 

 ture. The scutellum is wanting. Posteriorly the elytra are held in 

 place by the knob-like prolongations of the posterior coxse (Fig. 12). 

 This kind of an adaptation is, so far as I know, not met with in any 

 other of the families of the Coleoptera. 



The necessity for a diminished resistance has been met to some 

 extent and along the same lines as in the Dytiscidx. The antennae 

 are filiform, reversed, and each lies in a small groove directly below 

 the eye ; the eyes are not so prominent as in the Carabidae ; the con- 

 tours of the body are rounded; and there is a lessening of sculpturing 



