138 
HALF HOURS WITH INSECTS. [Packard. 
Fig. 100. 
Trachea. 
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, trachea ; 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, aa), open by a slit into an inner chamber (e&), 
guarded by a muscle (m). The air thus admitted is carried 
into every part of the body by the numerous 
fine subdivisions of these tubes, which form 
a beautiful net-work of silvery threads when 
filled with air. The}' - 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, Spiracle, 
bends the end of its abdomen so as to allow the air to pass 
into the spiracles under the elytra, 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 
Fig. 101. 
