38 



THE STUDY OF INSECTS 



Fig. 72. — Tracheal 

 system of cockroach. 

 The alimentary canal re- 

 moved to show the ven- 

 tral tracheal communica- 

 tions. (After Miall and 

 Denny.) 



These spiracles are either simple openings into the respiratory system, 

 or are provided with valves, sieves, or fringes of hair for the exclusion of 

 dirt. They lead into a system of air-tubes termed trachea. The accom- 

 panying figure will indicate the distribution of the main trunks of these 

 trachea? in a cockroach (Fig. 72). There is a short trunk 

 arising from each spiracle; these are all connected to- 

 gether by a large longitudinal trunk on each side of the 

 body, and by numerous transverse trunks. From these 

 large tracheae there arise a great number of smaller ones, 

 not shown in the figure, which branch and subdivide 

 and extend to all parts of the body. Connected to the 

 tips and sides of these small tracheae are minute tubes 

 called tracheoles. The tracheoles appear to be ultimately 

 connected with the various tissues and it is through these 

 that air reaches the tissues of the body. 



Although insects are, strictly speaking, air-breathing 

 animals, many of them, as is well known, live in the 

 water. The study of the ways in which aquatic insects 

 breathe is a very interesting one ; it presents to us many 

 wonderful modifications of structure. Some of the 

 more common of these are described in subsequent pages 

 of this book; in this place we can only make a few 

 generalizations. 



The various modes of respiration of aquatic insects may be classified 

 under two heads: first, those in which the insects obtain air from above 

 the surface of the water; second, those in which the insects breathe the 

 air that is mechanically mixed with the water. 



With many aquatic in- 

 sects the spiracles open be- 

 neath the wings, which are 

 folded upon the abdomen. 

 The insect, by coming to the 

 surface of the water and 

 lifting the tips of its wings, 

 forms a cavity beneath them, 

 into which the air rushes. 

 The insect can then swim 

 through the water, carrying 

 this air with it in a position 

 where it can be respired. 

 When the air becomes im- 

 pure, the insect rises to the 

 surface, forces out the air 

 from beneath its wings, and 

 takes in a new supply. Wa- 

 ter beetles and aquatic bugs 

 afford familiar examples of this mode of respiration. 



Some insects are provided with long tubes connected with their spira- 

 cles, by means of which they can draw their supply of air from above 

 the surface of the water while they crawl upon the bottom of shallow 

 ponds. Our most common illustrations of this are bugs of the family 

 Nepidce; but the most remarkable development of this kind is exhibited 



R 



Fig. 7.?. — Tracheal gill of a damselfly: A, en- 

 tire gill showing the tracheae; B, part of gill more 

 magnified, showing both tracheae (T) and tra- 

 cheoles (I). 



