INSECTS : THEIE BEEATHING ORGANS. 97 



You must not suppose, however, that the whole of one 

 tube is formed out of a single wire. Just as in a piece of 

 human wire-work the structure is made out of a certain 

 number of pieces of limited length, and joinings or inter- 

 lacings occur where new lengths are introduced, so, 

 strange to say, it seems to be here. It is strange, I say, 

 that it should be so, when there can be no limit to the 

 resources, either of material, or skill to use it ; but so it 

 is, as you may see in this specimen, which has been dis- 

 sected out of the body of a silkworm. The spiral is 

 much looser here than in the air-tube of the fly, the turns 

 of the wire being wider apart ; and hence its structure is 

 much more easily traced. Here you see in many places 

 the introduction of a new wire, always commencing with 

 the most fine-drawn point, but presently taking its place 

 with the rest so as to be undistinguishable from them. 

 In some cases certainly (perhaps this may be the expla- 

 nation of the phenomenon in all) the wire so introduced 

 may be found to terminate with the like attenuation before 

 it has made a single turn, and seems to be inserted when 

 the permanent 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 supernumerary 

 wire. 



I told you that the air enters these tubes through cer- 

 tain " 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 orifice 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 dis- 

 cover to be little oval disks sunken into little pits, of 



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