172 NATURAL HISTORY. 



but these, unlike those of the spider, do not proceed from the end of the 

 abdomen, but from glands opening near the mouth. The larva, in con- 

 structing its cocoon, works from the inside, and at first it presents a very 

 gauzy appearance, as is shown just to the left of the centre of the plate 

 opposite page 273. Through its walls the larva can be seen spinning more 

 threads, making the net thicker and thicker, until at last the whole forms 

 a thick, firm case formed by the winding of a single thread. Inside this 

 cocoon, after all the silk is spun, the larva sheds its skin, and emerges a 

 pupa much like those of its allies which are not thus protected. It may 

 be well to state, in passing, that the pupa of the butterflies is frequently 

 called a chrysalis. 



The pupal condition usually lasts through the winter, and then, when 

 spring comes, the dry pupal skin splits open, and out comes the perfect 

 insect, but in what a, lax and limp condition ! The legs have hardly 

 strength to support the body, while the wings appear like flabby sacs on 

 either side. Gradually the limbs harden, while the blood flowing into the 

 wings distends them to their full size, and in a short time the butterfly sails 

 away to spend its few remaining days. This period of emergence from the 

 cocoon is a critical one. Any slight obstacle will produce serious results, 

 and one can easily obtain butterflies with deformed wings by pinching these 

 organs before they become distended and hardened. 



The moths which form cocoons have an even harder task. After they 

 have cast off their pupal clothes, there is still the silken case before them. 



It is usually stated that they secrete a fluid 

 which softens and dissolves the silken threads, 

 but there is often more than this. On the 

 base of the wings of some of these moths 

 Fig. 155. — Front view of the head and there is a little hook with saw-like teeth, and 



thorax of aCecn>i>ia moth, showing at , . 



the base of the wings the hooks used in this is used to cut the threads and pull them 



escaping from the cocoon. 



apart. 

 The duration of the life of insects varies ; and as a rule exact statistics 

 are lacking. The perfect state lasts for a shorter time than the" preparatory 

 stages; indeed, there is a whole group of insects known as the Ephemeras, 

 because the adults live so short a time ; in some instances but a single 

 'lay, it is said. On the other hand, the larval and pupal stages may 

 last for an enormous length of time. The seventeen-year cicada lays its 

 jgs, and yet seventeen years must elapse before these will become perfect 

 insects; in others the period is thirteen years. There is a case of lon- 

 gevity among the beetles which exceeds even these limits. An apple-tree 

 was cut down, and from its timber a table was manufactured. " Many 

 .years afterwards the gnawing of an insect was heard in one of the leaves 



