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 Lcpidoptcra have first to remove 

 themselves from the locaHty 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 



MOTHS wrrH 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- 



