378 Royal Institution :— 
of the back of its own head, and fluttered away. The whole process 
occupied less than ten seconds. 
The speaker in this case wished particularly to impress on his 
hearers, first, the gradual nature of the changes, and, secondly, 
that some of them have no reference to the fourm of the perfect insect, 
but are entirely of an adaptational character. Thus the young larva 
is born without branchiz, and with two caudal appendages. It gradu- 
ally acquires a thin tail and seven pairs of branchie ; but the perfect 
insect has only two tails and no branchize. Thus, then, the changes 
which an insect undergoes are of two kinds, developmental and adap- 
tational. 
External forces act upon the larveeas much as on the perfect insects. 
And we can thus understand the remarkable fact that some animals, 
which differ much when young, are very similar at maturity. 
The speaker then entered into some theoretical considerations as 
to the nature and causes of metamorphoses, dividing the subject into 
three questions. 
Ist. How these changes of form might have originated. 
2ndly. Why they are, in insects, so abrupt in their character ; and 
ordly. Why the pupa condition, a period of approximate immobility, 
should intervene between the active larva and the still more active 
imago. 
1. The changes of form depend on the early condition at which 
some insects quit the egg. There is reason to believe that all insects 
pass through the stage of fat, fleshy grubs, and subsequently acquire 
legs*. Some, however, are hatched in the first state, while’ others 
remain in the egg until they attain the second. In the former case 
additional changes are produced by the fact that external forces do 
not affect the larva in the same manner as the perfect insect; and 
thus there is a tendency to still greater differentiation. 
2. The abruptness of the change is more apparent than real. The 
actual change itself is merely the withdrawal of the curtain, the 
casting of the old skin, by which the alterations which have perhaps 
been in preparation for days, or even weeks, are rendered visible. In 
fact there can be no great change in insects without a moult. Insects 
have no bones, and the muscles are attached to the skin, which there- 
fore is necessarily hardened to afford them a solid and sufficient 
fulcrum. But it follows from this that no change of form can take 
place without a change of skin. 
In Chloéon we have seen that each moult is accompanied by a 
slight change. 
In caterpillars, on the contrary, there is little alteration during 
growth, and the changes are concentrated, so to say, on the last two 
moults. The advantage of this is obvious; the mouth, digestive, 
and other organs of the larva are very different from those of the 
perfect insect ; and if the change from the one type to the other were 
gradual and slow, the insect would be liable to perish of starvation in 
the midst of plenty. 
* See, for instance, Professor Huxley’s admirable memoir on Aphis, in 
the ‘ Linnean Transactions.’ 
