FORM, GROWTH, AND CHANGE 19 



thoracic pleura ; the second pair may be found on the similar 

 delicate connexion between the mesothorax and the meta- 

 thorax close to the bases of the intermediate legs. Thus there 

 are two pairs of spiracles (Fig. 9 sp. i, ii) on the thorax. The 

 abdomen carries eight pairs of these openings, situated on the 

 successive segments from the first to the eighth inclusive ; 

 each spiracle is close to the lower margin of the tergum and 

 towards the front edge of the segment (Fig. 9 sp. 2, 8). 



The system of air-tubes (or tracheal system) to which these 

 spiracles lead is among the most wonderful and characteristic 

 features of an insect's body (Fig. 10). An air-tube has a deli- 

 cate cellular wall (Fig. n ep), lined with what is really an exten- 

 sion of the outer cuticle, thickened spirally (Fig. n ct) so as to 

 strengthen the tube and prevent it from collapsing. The 

 tubes branch repeatedly and their fine endings (tracheoles) are 

 distributed (Fig. n tl) so as to be in close contact with the 

 various tissues. 



Two main trunks extend lengthwise, one along either side 

 of the insect, connected with the successive spiracles by short 

 lateral tubes, and giving off branches ventralwards and dorsal- 

 wards ; the latter series lead into paired longitudinal dorsal 

 trunks which run one on either side of the tubular heart ; 

 they are also connected with large bladder-like air-sacs (Fig. 

 10 a. s) arranged segmentally in pairs. The branches of the 

 main longitudinal trunks fork repeatedly, and finally give 

 rise to the minute thin-walled tubes or tracheoles (Fig. n tl) 

 through which the living tissues of the insect obtain the oxygen 

 necessary to support the combustion-process constantly going 

 on, and give up the carbon dioxide which passes with the 

 expired air out of the spiracles. 



To the student of anatomy, the most striking feature about 

 the air-tubes of an insect is that they are lined with an exten- 

 sion of the creature's chitinous outer cuticle. The cellular 

 walls (Fig. ii ep) of the air-tubes, which make the chitinous 

 lining must be regarded therefore as inpushings of the skin, and 

 this view is confirmed, not only by the origin of the tracheal 

 system in development, but also by the fact that at each moult 

 the chitinous linings of the air-tubes are cast off with the exo- 

 skeleton and re-made. 



It has been mentioned how the longitudinal air-tubes of 



