INSECTS. 235 



cumstances. The converse of this experiment equally succeed- 

 ed ; for by keeping pupae in an ice-house during the whole sum- 

 mer, the production of the fly was retarded a full year beyond 

 the ordinary period. And it is a fact well ascertained, that the 

 pupa state sometimes continues for years, thus providing for the 

 continuance of the species, should adverse seasons threaten to 

 destroy the inclosed animals before they had carried through the 

 great purpose of nature by reproduction. 



The mode in which insects break through their prison-house 

 or cocoons and assume the perfect form is various. Previous to 

 this period the colour of the pupa undergoes an alteration ; the 

 golden or silver tinge of many vanishes, and those which are 

 transparent usually permit the form and colours of the insect 

 within and the motions of their limbs to be seen. In the objected 

 pupa the struggles of the included butterfly or moth first effect 

 a longitudinal slit down the middle of the thorax, where there 

 is usually a suture for the purpose, and the insect gradually 

 withdraws itself from its case. The members are also with- 

 drawn from a series of inner membranous sheaths which sepa- 

 rately include them like a glove. In the coarctate pupa, where the 

 outer case is generally more rigid and destitute of sutures, a lid 

 or operculum is found at the anterior end, which the animal is 

 enabled to push off; and the Coleopterous insects, whose tem- 

 porary dwelling is under ground, await the progress of the de- 

 velopement and hardening of their elytra, before mining up- 

 wards to the open air. In other families the cocoon is ruptured 

 by the inclosed insect ; or in cases where the portions of the 

 case have been glued together, that glue is dissolved by a sol- 

 vent fluid and the animal left free to escape ; and among the 

 ants the working class not only feed the young previously, but 

 at their period of transformation cut the minute threads of the 

 cocoons when the perfect insect is ready to appear. In the 

 gnat, which undergoes its change on the surface of the water, 

 the pupa case splits like a little boat, and the animal raises it- 

 self from the horizontal to the vertical position, extricates its 

 members from their confinement., rests for a moment on the wa- 

 ter till its wings are unfolded, and flies away. 



The last stage of the life of insects is termed the imago or 

 perfect state. In this state all their parts are fully developed, 

 and it is only in this stage that they are qualified for the great 



