66 TRANSFORMATIONS OF INSECTS. 
into nymphs in the water—the caddis-flies, for instance—acquire 
a power of locomotion as the period of their full development 
approaches, to enable them to creep up the stems of plants, and 
leave that medium in which it is impossible for them to exist as 
perfect insects. In every instance the assumption of the perfect 
state is accompanied by a ‘slipping off of the external covering. 
Before this can be effected many Lepidoptera have first to remove 
themselves from the locality in which they have undergone their 
previous metamorphosis. When this happens to be in the interior 
of trunks of trees, or in other situations from which it is difficult 
to escape, the abdominal segments of the pupa are often beset 
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MOTHS WITH WINGS INCOMPLETE, JUST AFTER EMERGENCE, 
with minute hooks, similar to those on the feet of the larva. By 
means of these, by alternately contracting and extending its abdo- 
minal segments, the pupa is enabled to force an opening through 
its silken cocoon, or to move itself along until it has overcome 
the obstacles which might oppose its escape as a perfect insect. 
“Immediately after the insect has burst from the pupa case 
it suspends itself in a vertical position, with its new organs, the 
wings, somewhat depending, and makes several powerful respiratory 
efforts. At each respiration the wings become more and more 
enlarged by the expansion and extension of the tracheal vessels 
within them, accompanied by the circulatory fluids. When these 
organs have acquired their full development, the insect remains at 
rest for a few hours, and gains strength, and the exterior of the 
body becomes hardened and consolidated, and forms the dermo- 
