43 THE BIOLOGY OF INSECTS 



of surrounding tissues so that the calibre of the tube must 

 be reduced when the insect's body contracts, while there 

 is elasticity in the Uning which ensures expansion of the 

 tube again when the pressure is withdrawn. The fine 

 air- tubes lead into the minute tracheoles, through whose 

 exceedingly thin walls the gaseous exchanges between air 

 and tissues are carried on. Each tracheole (Fig. 46, tr) arises 

 as a cavity in a cell of the tracheal epithelium, ultimately 

 uniting with the cavity of the tube, the cell growing out as 

 an elongate hollow thread. The insect's breathing system 

 is adapted for bringing air-currents alternately into and out 

 of these tracheoles, and the mechanism of the process is of 

 great interest. 



The air-tube system may be regarded as growing inwards 

 from the paired series of openings (spiracles or stigmata, 

 Figs. 14, 15) that are found on the sides of most of the body- 

 segments in the great majority of insects ; these spiracles 

 indicate indeed where the air-tubes began to grow into the 

 body from the outer skin (ectoderm) during the embryonic 

 growth of the insect (see p. 154). Spiracles are commonly 

 present on one or two of the thoracic and eight of the 

 abdominal segments. In some primitive wingless insects — 

 bristle-tails — the branching system of air-tubes arising 

 from each spiracle remains distinct from all the others, but 

 usually large longitudinal trunks run along the body connect- 

 ing the successive spiracular tubes ; more slender longi- 

 tudinal trunks run dorsally along either side of the heart and 

 ventrally along either side of the nerve-cord, while transverse 

 commissures link up the right and left trunks so that the 

 whole tracheal system forms a complex network. Each 

 spiracle is surrounded by a rim of strong thick chitin, and 

 the aperture is often guarded by a series of inwardly directed 

 hair-like or spine-like processes which hinder the access of 

 foreign bodies to the system. Just within the spiracle is a 

 valve wherein by means of a specially thickened chitinous 

 *' bow," or a lever operated by suitable muscles the cavity of 

 the spiracular tube can be closed (Fig. 15). The breath- 

 ing of insects was well described by F. Plateau (1884) 



