INSECTS ; THEIE BEEATHING OEGANS. Ill 



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

 iindistinguishable from them. In some cases certainly 

 (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 supernumerary 

 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 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 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 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, 



