SPIRACLES OF INSECTS. 359 



All entomologists know that the spiracles act as sieves, pre- 

 venting any extraneous objects from gaining admission into 

 the breathing-tubes. But, unless they have had personal expe- 

 rience, they cannot appreciate the efficacy of the spiracle 

 when acting as a respirator. Even the microscope, though it 

 may magnify the object to any extent, does not show the 

 wonderful filtering power of the spiracle. The figure in the 

 illustration represents a spiracle of the common " blue-bottle " 

 fly, and any one who wishes to examine such an object for 

 himself can have but little difficulty in doing so, especially in 

 the warm season of the year. 



How effectual is the barrier thus interposed by Nature 

 between the external world and the interior of the insect may 

 be inferred from the following narrative : — 



Many years ago, while absorbed in the comparative anatomy 

 of insect structure, I believed myself to have hit upon a plan 

 for injecting the minutest of tubes with mercury. So I took 

 a male cockroach, placed a vessel of mercury in the receiver 

 of an air-pump, and suspended the cockroach exactly over it. 

 As the reader will fully have surmised, my idea was, first to 

 exhaust the air from the inside of the insect, then to plunge it 

 into the mercury, and then to admit the air, which, at a pres- 

 sure of fifteen pounds to the square inch, was likely to drive 

 the mercury into the smallest of tubes. Such a plan was very 

 successful with ordinary tissues, and might succeed with insects. 



Accordingly, I exhausted the air from the vessel in which 

 the cockroach was placed, and kept it in a state of exhaustion 

 for a whole day, so as to prove that every particle of air was 

 withdrawn from the insect. I then plunged the cockroach 

 deeply beneath the mercury, and admitted the air, hoping that 

 the severe pressure would drive the mercury into the respira- 

 tory vessels. But not one particle of the mercury could pass 

 through the wonderful filter with which the cockroach had 

 been provided, and, except that I had learned the power of 

 the spiracle, I might have saved both the time and trouble. 



It is worthy of notice that, almost countless as are the species 

 of insects, no two of them possess exactly the same structure of 

 the spiracles, the individuality being marked as clearly in these 

 tiny organs as in the entire insect. 



