64 TRANSFORMATIONS OF INSECTS. 



page 21. The term 'nymph,' which is sometimes employed, is 

 apphcable only to those species in which the limbs remain free, 

 but are folded up, as in the pupse of the butterfly and moth, 

 and are not covered with a hard uniform case, as in many Cole- 

 opterous and most Hymcnopterous insects." At present the term 

 nymph is usually restricted to active pupae. " When the pupa is 

 enclosed in a smooth uniform case, but no signs of the limbs and 

 other parts of the body are visible, as in the Diptera, it is called 

 ' coarctate.' {^Scc page ii.) In these insects the skin of the larva 

 is not cast off at the period of changing, but becomes the covering 

 or cocoon of the included pupa, which is also enclosed in its own 

 proper skin within it. 



" In all insects which undergo a complete metamorphosis 

 this is the period of quiescence and entire abstinence. Many 

 species remain in this state during the greatest part of their exist- 

 ence, particularly the true pupae of moths and sphinxes, which 

 often continue in it for nearly nine months of the whole year. 

 But in most of those insects which assume the particular con- 

 dition in which the body remains soft and delicate — as the hor- 

 nets, ants, and bees — this pupa state is the shortest period of their 

 existence, being often scarcely more than a week or ten days. In 

 every species the length of this period is much affected by the 

 influence of external circumstances. Thus if the larva of the 

 common nettle butterfly, Vanessa iirticcs, changes to a chrysalis in 

 the hottest part of the summer, it will often, as we have found, be 

 developed into the perfect insect in eight or nine days ; whilst if 

 its change into the chrysalis takes place at the beginning of 

 summer, it is fourteen days before the perfect insect appears ; and 

 if it enters the chiysalis state at the end of summer, it remains in 

 that condition through the winter, until the following spring. On 

 the other hand, it was proved by Reaumur, if the chrysalis be 

 placed in an ice-house, its development into the perfect insect 

 may be retarded for two or three years. Again, if the chrysalis be 

 taken in the midst of winter into a hothouse, it is developed into 

 the perfect insect in from ten to fourteen days. This period of 

 quiescence is absolutely necessary in all those species which 

 undergo an entire change of form and habits for the completion of 

 those structural metamorphoses by which the creature is not only 



