RESPIBATOBT OROANS OF INSECTS. 321 



posterior edge of the mesothorax. There are eight abdominal 

 spiracles, the first one situated just in front of the auditojy 

 sac or tympanum (see Fig. 374), and the remaining seven are 

 small openings along the side of tlie abdomen, as indicated 

 in Fig. 281. From these spiracles air-tubes pass in a short 

 distance and connect on each side of the body with the spi- 

 racitlar trachea (Fig. 281, s, Fig. 283, s), as we may call it. 

 The air-tubes consist of two coats, in the inner of which is 

 developed the so-called spiral thread (tsenidium). These 

 spiracular tracheae begin at the posterior spiracle^ and extend 

 forward into the mesothorax, there subdividing into several 

 branches. Branches from them pass to the two main ven- 

 tral tracheae (Fig. 381, v), and to the two main dorsal tra- 

 cheae (Fig. 281, D, Fig. 282, D). The main tracheal sys- 

 tem in the abdomen, then, consists of six tubes, three on a 

 side, extending along the abdomen. The pair of ventral 

 tracheae extend along the under side of the digestive canal; 

 the dorsal ti'acheee rest on the digestive canal. These six 

 tubes are connected by anastomosing tracheae, and, with 

 their numerous subdivisions and minute twigs and the sys- 

 tem of dilated tracheae or air-sacs, an intricate network of 

 tracheae is formed. 



The system of thoracic air-tubes is quite independent of 

 the abdominal system, and not so easy to make out. The 

 tubes arising from the two thoracic stigmata are not very 

 well marked; they, however, send two well-marked tracheae 

 into the head (Fig. 281, c. Fig. 282, c), which subdivide into 

 the ocular dilated air-tube (Fig. 281, oc, Fig. 382, oc) and a 

 number of air-sacs in the front of the head. 



The series of large abdominal air-sacs, of which there are 

 five pairs (Fig. 282, 3-7), arise independently of the main 

 tracheae directly from branches originating from the spira- 

 cles, as seen in Fig. il81. They are large and easily found 

 by raising the integument of the back. There is a largo 

 pair in the mesothorax (Fig. 283, 3) and two enoi-mous sacs 

 in the prothorax (Fig. 383, 1), sometimes extending as far 

 back as the anterior edge of the mesothoiax. All these sacs 

 are superficial, lying next to the hypodermis or inner layer 

 of the integument, while the smaller ones are, in many cases, 



