134 LEAVES FROM MY NOTE-BOOK. 



mechanical one, is shown by the fact that the air-bladder of a fish, 

 when punctured, will refill with a gas containing as much as eighty 

 per cent, of oxygen ; but if the branches of the vagus nerve 

 which supply the air-bladder are cut, no more gas is formed. Nor 

 could oxygen be made to diffuse into the air-bladder of a Pike 

 which was filled with atmospheric air and surrounded by pure 

 oxygen, //// the epitheHu7ti had been killed by maceration in distilled 

 water. 



The fierce larva of the Dytiscus beetle is a kind of sabre- 

 toothed tiger among insects ; its sharp curved mandibles are 

 grooved like the poison-fangs of a serpent, and are formed into a 

 tube by the closure of the maxillae. Whilst the blood of the living 

 prey is being sucked by pharyngeal action through the tube so 

 formed, the maxillae at the same time act as a mouth-lock. 

 When the Dytiscus wishes to swallow a solid morsel, the maxillae 

 are relaxed and leave the orifice of the mouth open ; but the 

 ordinary mode of feeding of this ferocious creature is through its 

 blood-pump. Snails, worms, insects, tadpoles, and fishes, are 

 victims in turn. 



The larva of Hydrophilus piceus, another water beetle, has an 

 extraordinary provision for keeping itself from undue pressure in 

 its pupa stage, which it passes in damp earth (Fig. 3). On each 

 side of the head (on the forepart of the pro-thorax) it has three 

 strong brown hooks, and two similar hooks are found at the hinder 

 end of the body. These hooks, being solid, could contain no 

 part of the future insect , and the problem was " of what use 

 could these hooks be to a pupa buried in the earth, and left 

 behind when the beetle emerges ? " But investigation showed 

 these organs to be essential to the proper development of the 

 insect. The skin of the pupa is very delicate. Buried in damp 

 earth it could hardly escape injury, and the weight pressing on 

 the pupa might distort its frame ; but the pupa protects itself from 

 these dangers by assuming an unusual attitude. It extends itself 

 back downwards in a horizontal position, and supports thet&eight of 

 its body by the three sets of hooks, as up07i a tripod. In this attitude, 

 though surrounded on all sides by moist earth, it keeps its body 

 from actual contact with any object until it has assumed its final 

 shape. The pupa of Hydrobius supports itself upon the floor of 



