146 NATURAL HISTORY. 



find subsistence on any that offer. Their maxillary 

 organs are fitted for gnawing vegetable substances, and 

 their voracity is so great that they will in one day devour 

 an amount of food double the weight of their own bodies. 

 When about to be transformed, they prepare a suitable 

 place in which the nympha may await the final change. 

 Every caterpillar can spin, but all do not spin a perfect 

 cocoon in which they are enclosed as in a tomb; many 

 content themselves with attaching silken threads to suit- 

 able objects, and thus form a ruder cell. The spinnaret 

 is situated in the mouth ; the material used a species of 

 saliva. Every one knows that silk is spun by caterpil- 

 lars ; the single thread, however, like that of the spider, 

 is not a simple filament, but composed of many fine 

 strands. Many of the species, whose spinnarets do not 

 elaborate a sufficient quantity of the silky material to 

 form an envelope for the pupa, attach themselves to 

 leaves, which serves for a defense ; others creep into 

 crevices in the bark of trees, and a few remain without 

 any covering whatever. The life of a caterpillar is but 

 a succession of changes until reaching the growth neces- 

 sary for the pupa state, often throwing off its external 

 covering and assuming a new one. The nympha never 

 alters. The time required for the development of the 

 imago or perfect insect varies, according to the different 

 genera ; but the most beautiful and interesting specimen 

 of this wonderful process is to observe the butterfly at 

 the moment of its final transformation. The pupa is 

 seen to move and turn within the inclosing envelope, 

 until at length it bursts, and the perfect butterfly comes 

 forth. If entombed within a cocoon, they discharge an 

 acrid liquid, \jbich softens the silk, and allows its escape. 

 At the moment of its coming forth, the wings are small 



