THE RESPIHATOKY SYSTEM. 



17 



There are also present in flying insects (although not in larvae) 

 air-sacs connected with the trachea?. It has been supposed that 

 the use of these sacs is to lighten the weight, but this is erroneous, 

 for, as pointed out by A. A. Packard, it is evident that the wings 

 have to support just as much weight when the insect is flying, 

 whether the tracheae and sacs are filled with air or not ; the case, of 

 course, would be different were they filled with hydrogen gas. The 

 real use of the sacs, some of which are very large, is to afford a 

 greater supply of air, and therefore of oxygen, than that contained 

 in the air-tubes alone, and thus to afford a greater breathing 

 capacity. This is further proved by the fact that the sacs are largest 

 in the more swiftly flying insects, such as moths, flies and bees, 

 whose greater exertions create a demand for a more abundant 

 amount of air. 



Fig. 9. — Tracheal sacs connected with the third abdominal segment of 

 Geotrupes sylvaticus. st±, fourth stigma or spiracle ; st 5 , fifth stigma or 

 - spiracle ; tr, branches of the trachese ; s, air-sacs. The thread-like parts 

 represent fat-bodies. (After Kolbe.) 



The stigmata or spiracles, as a rule, can be opened or closed at 

 will by means of muscles, but in some cases are only protected 

 by short hairs or hairy tufts. In the Coleoptera each segment of 

 the body (except the head and, as a rule, the last segment) has a 

 spiracle, or, more correctly, there is a spiracle on the boundaries of 

 each of the segments ; the shape and position of these organs 

 sometimes afford a good character for classification (as in the 

 DYTisoiDiE and SoarabjeidtE). Gills or branchiae are rarely found 

 in the order, so far as the perfect insects are concerned ; they 

 occur, however, in many larvae (e. g., Gyrinus, Hydrous, Berosus, 

 etc.), in the form of processes arising from the sides of the 

 segments. All water insects which are not provided with gills or 

 corresponding organs have to rise more or less frequently to the 



o 



