118 EVENINGS AT THE MICROSCOPE 



which means they are prevented from collapsing, through 

 the unresisting character of the general integument. The 

 opening and shutting of them is performed by an internal 

 apparatus of muscles, which is sometimes strengthened by 

 being attached to two horny plates, which project inwardly. 

 But the most curious thing to be noted in the structure 

 of these spiracles is the contrivance which induced me to 

 call them trap- doors. Small as are their openings, they 

 are still large enough to admit many floating particles of 



dust, soot, and other extraneous 

 matters, which would tend to clog 

 up the delicate air-passages, and 

 to impede the right performance of 

 their important functions. Hence 

 they need to be guarded with some 

 sort of sieve, or filter, which, while 

 admitting the air, shall exclude 

 the dust. 



Various and beautiful are the 

 modes in which this common pur- 

 pose is effected, but I can show 

 you only two or three. This is 

 SPIRACLE OF FLY one of ^ ^e^ing orifices of the 



common House-fly, in which, as you see, minute processes 

 grow from the margin all round, which extend partly 

 across the open area, branching and ramifying again and 

 again, and spreading and interlacing with those of the 

 opposite side, so as to form a perfect sieve, which the 

 finest atoms of dust cannot penetrate. 



The same end is attained, in another way, in the dirty 

 cylindrical grub, which is found so abundantly at the roots 

 of grass in pasture lands, and which country folk call, 



