98 



EVENINGS AT THE MICROSCOPE. 



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 division 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 

 silkworm, and most lame, they are set in a horny ring, 

 by which means they are prevented from collapsing, 

 through the unresisting character of the general integu- 

 ment. 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 con- 

 trivance which induced me 

 to call them trap - doors. 

 Small as are their openings, 

 they are still large enough 

 to admit many floating par- 

 ticles 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 im- 

 portant functions. Hence 

 they need to be guarded 

 with some sort of sieve, or 

 filter, which, while admitting the air, shall exclude the dust. 



SPIRACLE OF FLY. 



