454 



PHYSIOLOGY. 



Chironomus plumosus, it is produced by means of a lateral convolution 

 of the anterior portion of the body, whereby its posterior end strikes 

 forwards. 



But all perfect insects, as well as the larvae of those with an imperfect 

 metamorphosis, which live in water, swim by means of their legs. 

 Among the Coleoptera it is the family of Hydrocantharides which 

 possess this faculty, for example, Hydrophilus, Elophorus, and the 

 whirlwigs (Gyrinus) ; other insects which live in water, as many Cur- 

 culios, Helodcs Phellandrii, Donacia Zoslerce, Elmis, Potamophilus, 

 Parnus, cannot swim, but creep about, clinging to different objects. 

 In the other orders we find but a single family of the Hemiptera whose 

 limbs are adapted to swim, namely, the genera Notonecta, Sigara, 

 Naucoris, and also in an imperfect degree the genus Nepa. They all 

 are, like the majority of insects, from the quantity of air contained in 

 their bodies, as well as from the lightness of their constituents, of less 

 specific gravity than water, and consequently float upon the surface 

 without any exertion of their own, when they contract their limbs. 

 The respiration of atmospheric air, to which all these genera are 

 restricted, is thereby facilitated to them. It is thence also that many 

 water-beetles cannot quit the surface when they have remained for 

 some time in the air ; the air then exercises so great an attraction 

 upon them that their swimming power is not able to counteract it, 

 and they consequently remain in this condition until they succeed in 

 overcoming it. This may be observed in any Gyrinus; it first whirls 

 itself about upon the surface before it can dive. Other beetles have so 

 little specific gravity, that even with all their endeavours they cannot 

 get beneath the water when any accident has removed them from their 

 places of concealment to the surface. This is the case in many of the 

 smaller Hydrophili, for instance, in Hydrophilus orbicular is and the 

 Elophori. These, therefore, never swim, but creep about, clinging to 

 objects beneath the water ; if they quit their hold they immediately 

 rise to the surface, and struggle here until they meet with a reed, that 

 serves them as a ladder to descend by. Even the powerful Hydro- 

 philus piceus swims very awkwardly, and has great difficulty to 

 continue beneath the water. 



The great Dytici are the best swimmers, namely, Dyticus dispar, 

 or Ilceselii. The whole form of its body is flat compared with its 

 size, much narrowed anteriorly, and laterally has a sharp edge, which 

 gradually increases in bulk to where its posterior legs are placed. 



