392 PHYSIOLOGY. 



the surface of the water when they wish for fresh air, and bring that 

 portion of their body provided with these apertures in communication 

 with the air above the surface. Among the beetles there are two 

 families especially which live in the water, namely, the Hydrocantharides 

 and Hi/drophilux. The mechanism of respiration differs in both. The 

 Dytici, when they wish to breathe, bring the posterior extremity of 

 their body to the surface of the water, and they then separate the last 

 segment of the abdomen from the elytra, and thus admit air beneath 

 the elytra within the space between them and the abdomen ; they then 

 close it by pressing the last segment firmly to the abdomen, and 

 return with their fresh supply to the bottom of the water. Here 

 this air is so long inspired by the spiracles, which are situated also 

 within this cavity between the elytra and the abdomen, as it is fit 

 for respiration, after which the insect returns to the surface of the 

 water, again to renew its supply. We thus observe in these insects 

 the same process as we find in those which live in the air. The 

 Hydrophili breathe differently. These, as Nitzsch * has observed and 

 described in detail, do not bring the apex of the abdomen, but the head, 

 to the surface of the water, and then project one of their clavate 

 antennae, the whole clava of which is covered with fine hair, until it 

 comes into contact with the air. But they so twist the clava that its 

 base is exposed to the air and the apex touches the breast, which, as 

 well as the whole underside of the insect, is clothed with short silky 

 pubescence. By this means a communication is made with the external 

 air and that beneath the water covering both the clava of the antennae 

 and the whole under surface of the insect to which it adheres by means 

 of the coating of down, and by means of this communication fresh air 

 is transmitted to the venter of the insect, and by the same means the 

 expired air is also removed, and the air is likewise transmitted from 

 the ventral surface beneath the elytra, where it is in- and expired by 

 the spiracles there situated. It is to the air thus adhering to the venter 

 that the Hydrophili are indebted for their lightness. It is with diffi- 

 culty that the majority can keep themselves at the bottom of the water 

 by clinging to substances there, and, when once at the surface, only by 

 the help of other bodies, for example, the stem of a plant, down which 

 they creep, can they recover their situation beneath. The great 

 Hydrophilus piceus alone, by means of its stronger muscular power, 



Rcil's Archiv. fur Physiologic, t. .\. p. 440. 



