AND THE LOWER ANIMALS. 67 



pillars, of two great lateral trunks, reaching from end 

 to end of the body, and giving off hundreds of branches 

 and ramifications, which travel over the whole frame, 

 penetrate the smallest cavities, and supply the most 

 delicate organs. In all insects with the power of 

 flight, and consequently in the Pieris, this apparatus 

 develops a pair of pouches destined to contain air, 

 and thus, of course, diminishes the specific gravity of 

 the body. According to Newport,* it is only in the 

 chrysalis that these organs are formed with rapidity. 

 In Pieris, they first appear some time in autumn, and 

 are half- formed before the winter; they remain at a 

 standstill during this season, and assume their perfect 

 form a short time after the last metamorphosis. 



The function of all the organs we have already 

 described is that of preserving or maintaining the 

 life of the individual. They are all, moreover, in the 

 full exercise of their duties from the moment that the 

 creature emerges from the egg. Those organs con- 

 nected with the perpetuation of the species are very 

 differently situate. These are so slightly developed, 

 and so imperceptible whilst the insect is in the cater- 

 pillar stage, that the penetrating researches of Herold 

 could not demonstrate their existence. These organs 

 are quite rudimentary, even at a period of five months 

 after the conversion of the caterpillar into a chrysalis. 

 It is only at the last moment, and just as the butterfly 

 is about to make its escape, that they are to be seen in 

 process of completion, whilst they undergo their entire 

 development only in the perfect insect. 



* " On the Eespiration of Insects." — Philosophical Transactions. 

 1836. 



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