INSECTS: THEIR BREATHING ORGANS 117 



point, but presently taking its place with the rest so as to 

 be indistinguishable from them. In some cases certainly 

 (perhaps this may be the explanation of the phenomenon 

 in all) the wire so introduced may be found to terminate 

 with the like attenuation before it has made a single volu- 

 tion, and seems to be inserted when the permanent curva- 

 ture 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 physi- 

 ologist employs, certainly: he calls them spiracles. In 

 our own bodies the air enters only at one spiracle, a curi- 

 ously 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-gray 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. There are nine of 

 these organs on each side, a pair to each segment or divi- 

 sion 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 spiracles. 



Essentially, these organs, under whatever modifications 

 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, or accurately 

 closed; and in many soft- skinned insects, such as the silk* 

 worm, and most larvae, they are set in a horny ring, by 



