188 



THE STUDY OF FRESH-WATER ANIMALS. 



of the abdomen by means of its hairs entangles air at the 

 surface of the water, which is shut into an air-tight chamber 

 on the insect's back by the closing of the wing covers. 

 The spiracles are situated on the back of the abdomen, and 

 thus the insect whilst in the water continues to breathe 

 from this supply. The beetle may be seen from time to 

 time rising to renew its supply. Small beetles may often 

 be seen carrying down a bubble of air at the tips of their 

 tails for the same purpose. 



Water Bugs. Nepa, the water scorpion, has a long tube 

 made up of two halves uniting longitudinally, which is 

 pushed to the surface so as to take in an air supply 

 (Fig. 74). 



The breathing of the water boatman, Notonecta, is 

 interesting. It may be seen to float 

 upward, pushing the tip of the ab- 

 domen above the water, thus bringing 

 the hind spiracles into direct com- 

 munication with the air. But further 

 it may be noticed that the under side 

 of the body which is nearest to the 

 surface is keeled and fringed with 

 hairs arranged in parallel rows, so that 

 air is entangled here. The oar-like legs 

 may be seen sometimes brushing the 

 entangled air forward towards the 

 spiracles upon the thorax. 



It will be noted that all of the fore- 

 going use oxygen obtained directly 

 from the atmosphere. Other creatures 

 use the oxygen dissolved in the water. 

 The adaptations of such creatures as 

 caddis flies, which cause the water to 

 flow through their tubes over the tuft or hair-like fila- 

 ments upon their bodies ; of the alder flies (abdominal 

 filaments) ; may flies or stone flies (gill tufts or flattened 

 gill plates) , should all be looked for, and their action as 

 far as possible understood. 



Fig. 74. The Water 

 Scorpion (Nepa). 



