112 



EVENINGS AT THE MICBOSCOPE. 



or accurately closed ; and in many soft-skinned insects, 

 such as the silkworm, and most larvae, they are set 

 in a horny ring, by which means they are prevented 

 from collapsing, through the unresisting character 

 of the general- integument. The opening and shut- 

 ting 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 Btructure 

 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 ad- 

 mit many floating particles 

 of dust, soot, and other extra- 

 neous 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. 



Yarious and beautiful are the modes in which this 

 common purpose is effected, but I can show you only 

 two or three. This is one of the. breathing orifices of 

 the common House-fiy, in which, as you see, minute 

 processes grow from the margin all round, which ex- 

 tend partly across the open area, branching and rami- 

 fying again and again, and spreading and interlacing 

 with those of the opposite side, so as to form a perfect 



8PIKACLE OF FLY. 



