64 
TRANSFORMA TIONS OF INSECTS. 
page 21. The term ‘ nymph,’ which is sometimes employed, is 
applicable only to those species in which the limbs remain free, 
but are folded up, as in the pupae of the butterfly and moth, 
and are not covered with a hard uniform case, as in many Coleo- 
pterous and most Hymenopterous 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.’ ( See page n.) 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 tirticcz, 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 chrysalis 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 
