422 THE CANADIAN NATURALIST. [Dec. 



complete turning upside down of the general plan. Here, in the 

 Insect body, we have blood, it is true, and a pulsating organ 

 (termed the dorsal vessel), which appears to give this blood a 

 somewhat definite and uniform motion through different parts of 

 the animal's frame. Bat no blood-vessels are any where to be 

 seen, nor can we discover any one organ set apart for the special 

 aeration of the vital fluid. But we do find something no less 

 wonderful and interesting ; nay, I would rather say, immeasur- 

 ably more interesting and instructive, because illustrative of the 

 limitless resources of that Infinite Mind which thus condenses and 

 concentrates within the small dimensions of a point, such an ex- 

 quisitely perfect and marvelously elaborate vital mechanism ! 



What is there then, in the anatomy of an insect, which claims 

 the special and careful attention of a modern physiologist 1 Not 

 only (I humbly think) the mere structural difference, which I 

 will now briefly describe, but the physiological inference which 

 may possibly be deduced therefrom, as to the true nature and im- 

 mense importance of the respiratory function in the animal 

 economy. As this paper will be accompanied by a series of 

 microscopic preparations, illustrative of some of the structural 

 peculiarities here alluded to, it will not be necessary to give any 

 lengthened verbal description. I will merely remark, therefore, 

 that instead of the blood (which flows in grooved channels or 

 canals through the body of an insect) being forced to one spot to 

 receive oxygenation, the air is conveyed to it, by means of a most 

 elaborately arranged system of external breathing mouths, termed 

 spiracles, and internal air tubes, termed trachea. Although the 

 plan of respiration is the same essentially in all insects, the modi- 

 fications of these breathing organs is as wonderfully varied as the 

 external appearance and peculiar habits of the creatures them- 

 selves. When it is remembered that insects pass through a series 

 of metamorphosis, some living in water at one period of their 

 existence, and then assuming an aerial life ; others burying in the 

 earth, during their early days, and then coming forth to roam 

 abroad amid the forest trees ; and when we recollect that almost 

 all exist under very different external conditions, at different 

 periods of their changeful history, and that in each of these 

 states respiration is an indispensible function, we need not be 

 surprised to find striking and important modifications in the phy- 

 sical structure of their breathing organs, suited in each case to 

 the peculiar exigencies of the individual. It will be impossible, 



