138 



HALF HOURS WITH INSECTS. [Packard. 



Fig. 100. 



Trachcti. 



shall endeavor to show how insects primarily adapted for 

 breathing atmospheric air are enabled to breathe in the 

 water ; then I shall notice the various modes of swimming 

 in aquatic insects. 



In the first place, how are insects fitted to live under 

 water? It will be remembered that all insects breathe by 

 means of air tubes called tracheae (Fig. 

 100, ti-achea ; Fig. 101, section of spiracle). 

 These are tubes composed of three coats ; 

 the inner, a tube of mucous membrane sur- 

 rounded by a spiral thread, formed orig- 

 inally out of a homogeneous membrane 

 which ultimately splits up into these spiral 

 threads, giving rigidity and toughness to 

 the tube. There is a third loose investing 

 membrane, the so-called peritoneal coat. 

 A trachea originates from a spiracle or 

 breathing hole, of which there are usually 

 nine on each side of the body. These spiracles, or stigmata 

 (Fig. 101, act), open by a slit into an inner chamber (eh), 

 guarded by a muscle (m). The air thus admitted is carried 

 into every part of the body by the numerous fig. loi. 

 fine subdivisions of these tubes, which form 

 a beautiful net-work of silvery threads when 

 filled with air. They are bathed by the blood 

 which is everywhere oxygenated by the air 

 in these fine tubes. 



Numerous beetles and water bugs have 

 no special apparatus for breathing in the 

 water. The Dytiscus when it wishes to 

 breathe rises to the surface, tail foremost, 

 bends the end of its abdomen so as to allow the air to pass 

 into the spiracles under the elj'tra, and scuttles down to the 

 bottom in great apparent haste, with a bubble of air attached 

 to the tip of the body. When its supply of air is exhausted 



10 



Spiracle. 



