30 
INSECT METAMORPHOSIS * 
7BODY, whether learned or unlearned, is aware that 
laser undergo changes in their shapes and habits. Great 
numbers of popular works on natural history have made ee 
description of these changes or metamorphoses familiar her 
public ; and Newport, Duges, Heroldt, Fabre, and those eo 
entomologists and naturalists whose names are household words 
amongst us, have informed the scientific world upon the ana- 
tomical and minute changes of structure which accompany ae 
wonderful varieties in form and in method of life. The array © 
facts is enormous, and yet, with all this vast amount of fae 
knowledge to build upon, very little progress has been made 
towards recognising the cause and meaning of metamorphosis ie 
biology—in the science of life. The facts and details of the sub- 
ject have been accumulating, but the nature of its philosophy | 
has been studied by very few naturalists, and il is only of late 
years that Lubbock and Fritz Miiller, and a few others, have | 
been stimulated by the light of the theory of evolution to ex- 
NATURE 
[Wov. 14, 1872 
ine into it. Believing that the subject is increasing in interest, 
| cal hee its peieteratien bears upon some of the most im- 
portant theories respecting life, it is proposed to devote this lec- 
ture to a description of the different kinds of metamorphoses in 
insects, and to a consideration of the biological meaning of the 
Bie ue eel to your recollection two instances of what 
may be called perfect and complete metamorphoses. When 
the tenderest cabbages are growing in the early summer, a 
number of very small caterpillars or larve may be seen upon 
the plants, devouring them in a regular and systematic =e 
Avoiding the leaf-veins as indigestible, they nibble the juicy lea’ p 
and consume daily more than their own weight. These pea 
the gardener have small heads and ends, and the body is greenis| 
and striped with yellow bands, being at the same time pen 
At first very small in size, the caterpillars do not attract muc 
attention, and especially, as after living for a few days, they pee 
up out of the light, and look shrivelled and ill. Be pre 
| time, the caterpillar in retreat bends its back violently, an 
Fic, 1,—Metamorphosis 
splits the skin of one of the rings or segments of the part nearest 
the head, then a vigorous struggle enables the legs and the head 
to be withdrawn through the crack, and the larva is noticed to 
have attained a new skin within the old one. It crawls on to its 
favourite plant and makes up for lost time, grows rapidly, and 
really may be said to live to eat. It cares not for its fellows, 
nor for any other leaves ; it is content with its own cabbage, and 
has no ambition and no desire to quarrel or to move away. 
During growth the powers of mastication and of digestion in- 
crease, but they are checked several times by the larva having to 
pass a period of quietude whilst a new skin is finished under the 
old, and whilst this is cast off. These skin sheddings have a de- 
finite relation to the increasing size of the insect, but they are 
not simple changes of skin because the old one has become too 
tight for its rapidly growing possessor. They accompany certain 
important changes within the insect, and not only is the outside 
skin shed, but the mucous membrane of the digestive organs 
* A Lecture delivered before the British Association, 1872, by Prof. 
Duncan, F.R.8. 
of Tortoise-shell Butterfly, 
and of the air tubes which enable the creature to breathe, suffer 
also. They are really important elements in the metamorphosis, 
which term includes the sum of the changes of shape, habit, and 
instinct, 
When full growth has been attained, the caterpillar crawls 
from its cabbage and wanders restlessly about, even to con- 
siderable distances, in search of a dry sheltered spot. After 
having discovered such a locality, it fills up the space between 
its hind legs with silk, and attaches this part of the body to the 
wood or stone, as the case may be. The larva then hangs head 
downwards, and forthwith begins to bend its head backwards, 
upwards, and then from side to side, until, after a little practice, 
it is enabled to touch the solid substance to which it is hanging 
on either side of its body. Then some silk is secreted, and by 
applying the mouth to the spots touched one after the other, a 
fine sling of silk thread binds the insect down and prevents it 
from being swayed to and fro by the wind. This is the last act 
of the larva which shows any evidence of will. Then it begins 
to look shrivelled, shorter than before, and broader behind the 
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