THE METAMORPHOSES OP THE RESPIRATORY SYSTEM. 5t 



period when the larva ceases to feed, preparatory to changing into 

 the pupa state. Newport examined their growth in the same 

 chrysaHs which yielded such interesting details concerning the 

 alterations in the nervous system during metamorphosis. The 

 dilatation of the tracheje of the larva of the Small Tortoise-shell 

 butterfly begins as soon as it suspends itself in its silken hangings. 

 During its suspension it makes several powerful respiratory efforts, 

 accompanied by much muscular exertion, and these efforts are con- 

 tinued at intervals, until the old skin is fissured and thrown off 

 It is at this period that the tracheae become much enlarged, and 

 Newport found them so about two hours after the insect had 

 suspended itself. At about half an hour before the insect becomes 

 a pupa, the whole of the trachc?e become more distended, par- 

 ticularly those on the under surface of the thorax, from which 

 branches are given to the legs, so that the enlargement of these 

 tracheae is probably connected with the subsequent rapid develop- 

 ment and extension of those locomotive organs. It is at the actual 

 period of transformation that the changes take place most rapidly. 

 At that time the laborious respiratory efforts made by the insect 

 appear to affect the condition of all the organs greatly. When 

 the skin is thrown off these efforts cease for a few minutes ; after 

 which the abdominal segments become shortened, and the wings, 

 then scarcely as large as hemp-seeds, are gradually distended at 

 their base, and are perceptibly enlarged at each respiration. It 

 seems probable that the dilatation of the tracheae is partly occa- 

 sioned by the closing of the spiracles and the expansion of the 

 air within the tubes, due to muscular efforts and the alteration in 

 the bulk of the insect. Half an hour after the transformation the 

 tracheae which ramified over the gullet are enlarged to double 

 their former size, and in forty-eight hours the vesicular structures 

 are all complete and ready for use. 



The side vesicles of the abdomen of the bee may be noticed in 

 the engraving on the next page, and the tracheae issuing from them. 



The respiration of the bee can be observed when it has 

 ceased flying, and is at rest ; then the muscles of the abdomen 

 shorten the segments, and by relaxing allow them to lengthen 

 again. Air is thus pumped into the tracheae through the spiracles. 

 When quiet, and especially if the temperature is low, insects 



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