102 HOW INSECTS BREATHE. 



from the mouth, and wherefrom it issues in a fine 

 thread. 



There are two of these silk-making organs, and 

 both unite in the spinneret. Consequently, if silk 

 is examined in the microscope, the double thread 

 can clearly be made out, both threads adhering to 

 each other, but still distinguishable. If the threads 

 lie parallel to each other, the silk is good ; if not so, 

 it is of an inferior quality, and liable to snap. 



Most caterpillars possess this silk-factory, but 

 some have it much more largely developed than 

 others — the silk- worm, for instance. It is of con- 

 siderable size in the larva which we are examining, 

 because the Woolly Bear has to spin for itself a 

 silken hammock in which to swing while it is in the 

 sleep of its pupal state. Just before it begins to spin, 

 the organ is of very large size, and distended with 

 the liquid silk ; but after the hammock is completed, 

 the organ diminishes to a mere thread, and is soon 

 altogether absorbed. 



At the left hand of the drawing may be seen a 

 curious structure, marked b b. This is the chief 

 portion of the respiratory system, and may be at 

 once recognised by the ringed structure of the tube. 

 Indeed it is quite analogous to that of the windpipe 

 in animals. 



The mode in which insects breathe differs much 

 from that of the higher animals. In them the 

 breathing apparatus is gathered into one mass, called 



