274 ELEMENTARY STUDIES IN INSECT LIFE 



and extend to every organ in the system. These 

 trachea do not depend upon the month for their supply 

 of air, but are connected with the body-wall direct, the 

 outer portion of this connection being known as spira- 

 cles. (Figs. 206, 210.) These spiracles have valves and 

 openings which close and open at intervals, allowing 

 free interchange of air. The trachea? which run from 

 these spiracles are membranous tubes, which do not 

 collapse, because they are kept open by continuous 

 rings of cartilage, similar, though on a smaller scale, 

 to the cartilage in the windpipe of those animals pos- 

 sessing lungs. This distribution of air within the 

 body tends to make the insect lighter and more cupahlr 

 of flight. In addition to these trachea, however, there 

 are organs especially made to assist in buoying the in- 

 sect when on the wing. These are commonly known as 

 air-sacs, and connect with the spiracles as .shown in 

 the figure. (Fig. 210.) 



In a live insect, notice under the lens the action of 

 the spiracle situated just in front and dorsad of the 

 base of the mesothoracic leg. If the student will for 

 a time watch this spiracle it will be seen to have two 

 lips, and that these open and close in unison with the 

 expansion and contraction of the body-wall. This 

 movement of the body is more manifest in the abdo- 

 men. Figure 207 shows this spiracle much enlarged. 



Take out one or two of the largest trachea? found, 

 and study their structure by tearing them apart on 

 the dissecting-table under the lens. When the trachea 

 are pulled apart with the needles, do they sometimes 

 appear still to be connected by a thread which ravels 



