METAMORPHOSIS OF INSECTS. 333 
Most caterpillars moult four or five times; at each 
moult the outer layer of the skin is cast off, the new 
skin arising from the hypodermis, or inner layer of the in- 
tegument. The skin opens on the back behind the head, 
the caterpillar drawing itself out of the rent. In the 
change from the caterpillar to the chrysalis, there are re- 
markable transformations in the muscles, the nervous, 
digestive, and circulatory system, inducing a change of 
form, external and internal, characterizing the different 
stages in the metamorphosis. 
While the changes in form are 
comparatively sudden in flies and 
butterflies, the steps that lead to 
them are gradual. How gradual 
they are may be seen by a study of 
the metamorphosis of a bee. In 
the nest of the humble or honey 
bee, the young may be found in all 
stages, from the egg to the pupa 
gayly colored and ready to emerge 
from its celi. It is difficult to 
indicate where the chrysalis stage 
begins and the larva stage ends, 
yet the metamorphosis is more 
complete—namely, the adult bee 
is more unlike the larva, than in 
any other insect. 
Besides the normal mode of de- 
velopment, certain insects, as the _ Fig. 298.—Embryo of the Louse. 
. am, serous membrane; db, amnion; 
plant-louse (Aphis), the bark-louse as, antenne ; vk forehead.—After 
(Coccus), the honey-bee, the Po- rican 
listes wasp, the currant saw-fly (Nematus), the gall-flies, 
and a few others, produce young from unfertilized eggs. 
Certain moths, as the silk-worm moth (Bombyx mori) and 
others, have been known to lay unfertilized eggs from which 
caterpillars have hatched. This anomalous mode of repro- 
duction is called parthenogenesis, and fundamentally is only 
a modification of the mode of producing young by budding 
which is universal in plants, and is not unusual, as we have 
