INSECTS : TIIEIK BREATHING ORGAlfS. Ill 



but presently takiii"; its place ^vitli the rest so as to he 

 undistinguishable from them. In some cases certainl}' 

 (perhaps this may be the explanation of the phenome- 

 non in all) the wire so introduced may be found to ter- 

 minate with the like attenuation before it has made a 

 single volution, and seems to be inserted when the per- 

 manent curvature of the pipe would leave the wires on 

 the outer side of the curve too far apart, half a turn, or 

 even much less, then being inserted of supeiTiumerary 

 wire. 



I told you that the air enters these tubes through 

 certain " trap-doors." This is not the term which the 

 physiologist employs, certainly : he calls them spiracles. 

 In our own bodies the air enters only at one spiracle, 

 a curiously defended oritice opening just in front of the 

 gullet at the back of the mouth. But in the class of 

 animals we are now considering there are a good many 

 such breathing orifices. You may see them to great 

 advantage in any large caterpillar, the silkworm for 

 example, where all along the sides of the pearl-grey 

 body you perceive a row of dots, which with a lens 

 you discover to be little oval disks sunken into little 

 pits, of a black hue with a white centre, through 

 which is a very slender slit. Tliere are nine of these 

 organs on each side, a pair to each segment or divis- 

 ion of the body, with the exception of the first, which 

 is the head, and of the third and fourth, which are 

 destined to bear the wings ; these are destitute of spir- 

 acles. 



Essentially, these organs, under whatever modifica- 

 tions of form and position they may appear, have the 

 same structure. They are narrow orifices, with two 

 lips capable of being opened at the will of the animal, 



