I AQUATIC BEETLES 47 



paraffin, the vapour of the alcohol escapes by the tips 

 of the mandibles and by every other opening, as trains 

 of minute bubbles, but none appear at the mouth. 



I have lately verified by dissection the chief points 

 of Burgess's description. While doing so it occurred 

 to me to try whether the mouth-lock acted auto- 

 matically, opening when the mandibles opened, and 

 closing when they close. Accordingly two larvae 

 were taken, one having the mandibles closed, while 

 in the other they were forcibly extended by a piece 

 of pith. Longitudinal median sections of each were 

 cut and mounted, when it appeared that in the larva 

 with closed mandibles the mouth was completely 

 closed, while in the larva with open mandibles a 

 passage about '05 mm. deep in the narrowest part 

 was immediately apparent. There has not been time 

 to repeat the trial, but I believe that reliance may 

 be placed upon this test case. 



It seems that the mouth-apparatus of the Dytiscus- 

 larva is employed in the following way. When a 

 living victim has been seized by the mandibles, they 

 close upon it and pierce it, the base of the mandible 

 with the hinder orifice of the canal being at the same 

 time brought into the corner of the mouth. The 

 mouth-lock is by the same action closed, and then the 

 pharyngeal pump can be employed to suck the blood. 

 But if the larva should require to swallow solid 

 morsels, as it occasionally, though rarely, seems to 

 do, the separation of the mandibles would immediately 

 relax the mouth-lock and give them passage into the 

 stomach. 



Almost all kinds of aquatic animals, Snails, Worms, 



