OF RESPIRATION. 393 



can work itself beneath the water, and swim about in it, although but 

 slowly, if unassisted, whereas the Dytici swim with the greatest facility 

 on all sides. A third type of water beetles, the Gyrinus or whirl wig, 

 also conveys an air bladder with it when it dives, which he can accom- 

 plish only with difficulty and the greatest exertion, or by means of 

 other assistance ; he, however, receives the air posteriorly between the 

 abdomen and the elytra, which is the easier to him as he swims freely 

 about in circles upon the surface. The larvae of the Dytici and 

 Hydrophili likewise breathe through spiracles which are situated at 

 the anal extremity ; they therefore only require to bring the end of the 

 tail to the surface of the water when they wish to respire. They are, 

 therefore, seen with a raised tail and pendent head hanging to the 

 surface by means of their plumose anal leaves. As soon as an enemy 

 approaches they hastily seek the bottom, but in the course of a few 

 seconds resume their former position. The perfect insect, however, 

 can remain longer beneath the water, as it conveys a supply of un- 

 decomposed atmospheric air with it. 



The majority of the remaining insects which dwell in water breathe 

 through tubes, with the exception of those which breathe by means of 

 gills. The mechanism of this mode of respiration scarcely differs from 

 that of the general mechanism of respiration. By raising the air tube 

 to the surface of the water, the influx of fresh air is admitted to the 

 tracheae, and this ensues upon each expansion of the cavities of the body, 

 whereas by means of each contraction the previously inspired air is 

 again rejected. But it appears probable to me that expiration is 

 effected not solely by the posterior tubes, but also through an aperture 

 immediately behind the head in the first segment of the body. I have 

 indicated these apertures in the description given above of the respiratory 

 apparatus of the rat-tailed maggot ; they are also found in the majority 

 of the larvae of the Diptera which do not live in water, for instance, in 

 the maggots of the Muscce, and also probably in the larvae of the gnats, 

 and in these they then develope themselves to the subsequent air tube 

 in the thorax of the pupa. As now these anterior apertures remain 

 constantly in the water, they cannot serve for inspiration, but being 

 present they cannot be superfluous in the organisation of the larva ; 

 besides, nothing appears more probable than that the inspired air is 

 again expired through these anterior apertures. 



