RESPIRATORY ORGANS OF INSECTS. 321° 
posterior edge of the mesothorax. There are eight abdominal 
spiracles, the first one situated just in front of the auditory 
sac or tympanum (see Fig. 274), and the remaining seven are 
small openings along the side of the 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- 
racular trachea (Fig. 281, s, Fig. 282, 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 (tenidium). These 
spiracular trachez 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 trachee (Fig. 281, v), and to the two main dorsal tra- 
chee (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 
trachee extend along the under side of the digestive canal; 
the dorsal trachee rest on the digestive canal. These six 
tubes are connected by anastomosing trachew, and, with 
their numerous subdivisions and minute twigs and the sys- 
tem of dilated trachee or air-sacs, an intricate network of 
tracheee is formed. 
The system of thoracic air-tubes is quite fudependent 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 trachee 
into the head (Fig. 281, c, Fig. 282, c), which subdivide into 
the ocular dilated air-tube (Fig. 281, oc, Fig. 282, oc) and a 
number of air-sacs in the front of the head. 
The series of Jarge abdominal air-sacs, of which there are 
five pairs (Fig. 282, 3-7), arise independently of the main 
trachesw directly from branches originating from the spira- 
cles, as seen in Fig, 281. They are large and easily found 
by raising the integument of the back. There is a large 
pair in the mesothorax (Fig. 282, 2) and two enormous sacs 
in the prothorax (Fig. 282, 1), sometimes extending as far 
back as the anterior edge of the mesothorax. All these sacs 
are superficial, lying next to the hypodermis or inner layer 
of the integument, while the smaller ones are, in many cases, 
