THE METAMORPHOSES OF THE RESPIRATORY SYSTEM. 51 
period when the larva ceases to feed, preparatory to changing into 
the pupa state. Newport examined their growth in the same 
chrysalis which yielded such interesting details concerning the 
alterations in the nervous system during metamorphosis. The 
dilatation of the tracheze 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 tracheze 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 trachee 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 
trachez 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 trachez 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 
tracheee 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 trachez 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 trachee through the spiracles. 
When quiet, and especially if the temperature is low, insects 
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