INSECTS. 327 



weather and from their enemies during this stage, 

 many of them prepare in advance a silky envelope, 

 some cover themselves with clay, others wrap themselves 

 in leaves, and still others hide themselves under the 



FIG. 284. 



CATERPILLAR, CHRYSALIS, AND IMAGO OF THE GOAT-MOTH (Cossus ligniperda). 



bark of trees or in the soil. If a chrysalis be examined 

 after short intervals of time, the gradual development 

 of the organs of life may be seen through the thin ex- 

 terior envelope. At last, after an interval that depends 

 on the species and the external conditions, sometimes 

 more than a year, the fully developed perfect insect, 

 or imago, bursts its prison walls. This last state is 

 ordinarily of short duration : in most cases it does 

 not last longer than a few weeks, in many it is limited 

 to several days, while in some it is ended in an hour or 

 so. and the insect dies. The occupation of the perfect 



