Pe. : 
tional supply of food. In the former case the embryo is 
‘more likely to survive ; but, on the other hand, when the 
ggs are large, they cannot be numerous, and a multi- 
plicity of germs is, in some circumstances, a great ad- 
vantage. Even in the same species the development of 
_the egg offers certain differences.* 
_ The metamorphoses of insects depend then primarily 
‘on the fact that they quit the egg in a very early condi- 
tion ; many—-as, for instance, flies and bees—before the 
thoracic segments are differentiated ; others—as locusts, 
dragon flies, &c., after the formation of the legs, but 
_ before that of the wings. 
We may now pass to the second part of the sub- 
ject, that is to say, the sudden and abrupt instance 
‘of the changes which insects undergo. The deve- 
lopment of an Orthopterous insect, indeed —say, for 
| instance, of a grasshopper—from birth to maturity is so 
gradual, that but for the influence on our nomenclature 
exercised by the most striking changes which occur in 
insects of the Heteromorphous series, they would perhaps 
never have been classed as metamorphoses. But though 
the changes from the caterpillar to the chrysalis, as from 
the chrysalis to the butterfly, are apparently sudden and 
‘abrupt, this is in reality more apparent than real ; the 
_ changes in the internal organs, though rapid, are in reality 
gradual ; and even as regards the external form, though 
the metamorphosis may take only a few moments, this is 
but the change of outer skin—the drawing away, as it 
were, of the curtain; and the new form which then ap- 
= has been in preparation for days or, perhaps, weeks 
before. 
- Swammerdam, indeed, supposed (and his view was 
adopted by Kirby and Spence) that the larva contained 
within itself “the germ of the future butterfly, enclosed 
_ in what will be the case of the pupa, which is itself in- 
cluded in three or more skins, one over the other, that 
will successively cover the larva.” This isa mistake ; but 
it is true that, if a larva is examined shortly before it 
is full grown, the future pupa may be traced within it. In 
the same minner, if we examine a pupa which is about 
_ to disclose the butterfly, we find the future insect, soft in- 
deed and imperfect, but still easily recognisable, lying 
_ more or less loosely within the pupa-skin. 
_ One important difference between an insect and a 
vertebrate animal is, that whereas in the latter, as for 
instance in ourselves, the muscles are attached to an 
_ internal bony skeleton, in insects no such skeleton exists. 
_ They have no bones, and their muscles are attached to 
_ the skin. Hence the necessity for the hard and horny 
dermal investment of insects, so different from the scft- 
ness and suppleness of our own skin. Moreover the 
result is, that without a change of skin a change 
_ of form is impossible. The chitine, or horny substance, 
forming the outside of an insect, is formed by a layer of 
cells lying beneath it, and, once secreted, cannot be altered. 
From this it follows that every change of form is neces- 
sarily accompanied bya change of skin. In some cases, 
as for instance in Ch/oéon, each change of skin is accom- 
panied by a change of form, and thus the perfect insect is 
more or less gradually evolved. In others, as for in- 
stance in caterpillars, several changes of skin take place 
without any material alteration of form, and the change, 
instead of being spread over many, is confined to the 
last two moults. 
The explanation of this difference is, I believe, to be 
found in the structure of the mouth. That of the cater- 
pillar is provided with a pair of strong jaws, fitted to eat 
leaves ; and the digestive organs are adapted for this kind 
of food. On the contrary, the mouth of the butterfly is 
suctorial ; it has a long proboscis, beautifully adapted to 
suck the nectar from flowers, but which would be quite 
useless, and indeed only an embarrassment to the larva. 
ae ee 
i ie 
—s 
_* For differences in larvz consequent on variation in the external condi- 
tions, see ante, p. 31. 
NATURE. 
SSS —_ ee ee 
1 oo 
The digestive organs also are adapted for the assimila- 
tion, not of leaves, but of honey. Now it is evident that 
if the mouth-parts of the larva were slowly metamor- 
phosed into those of the perfect insect, through a number 
of small changes, the insect would in the meantime be 
unable to feed, and liable to perish of starvation in the 
midst of plenty. On the contrary, in the Orthoptera,and 
as a general rule, among those insects in which the 
changes are gradual, the mouth of the so-called larva 
resembles that of the perfect insect, and the principal 
difference is in the presence of wings. 
Similar considerations throw much light on the nature 
of the chrysalis or pupa state—that remarkable period of 
death-like quiescence which is one of the most striking 
characteristics of insect metamorphosis. The compara- 
tive quiescence of the pupa is mainly owing to the rapi- 
dity of the changes going on init. In the chrysalis of a 
butterfly, for instance, not only (as has been already men- 
tioned) are the mouth and digestive organs undergoing 
change, but the same is the case with the muscles. The 
powerful ones which move the wings are in process of 
formation ; and even if they were in a condition favour- 
able to motion, still the nervous system, by which the 
movements are set on foot and regulated, is also in a 
state of such rapid change that it could scarcely act. 
It must not be forgotten that all insects, indeed all 
articulate animals, are inactive for a longer or shorter 
space of time after each moult. 
The slighter the change the shorter the period of in- 
action. Thus, after the ordinary moult of a caterpillar, 
the insect only requires rest until the new skin is hard- 
ened. When, however, the change is great and gradual, 
the period of inaction is correspondingly prolonged. The 
inactivity of the pupa is therefore not a new condition 
peculiar to this stage, but a prolongation of the inaction 
which accompanies every change of skin. Most pupze 
indeed have some slight powers of motion; those which 
assume the chrysalis state in wood or under ground 
usually come to the surface when about to assume the 
perfect state, and the aquatic pupz of certain Diptera, 
swim about with much activity. Among the Neuroptera 
certain families have pupz as quiescent as those of the 
Lepidoptera ; others, as, for instance, Raphidia, are quies- 
cent at first, but at length acquire sufficient strength to 
walk, though enclosed within the pupa skin, a power de- 
pendent partly on the fact that this skin is very thin. 
Others again, as, for instance, dragon-flies, are quiescent 
on assuming the pup3 state, only in the same manner and 
for a similar time as at other changes of skin. 
JoHN LUBBOCK 
(To be continued.) 
NOTES FROM THE “CHALLENGER” 
Ill. 
THE MILLER-CASELLA THERMOMETER 
T8A.M., on March 26, we sounded, lat. 19° 41’ N. 
long. 65° 7’ W., in 3,875 fathoms. The sounding 
was perfectly satisfactory, and left no doubt that the 
depth was estimated within a very small error. The 
“Hydra” sounding instrument was used weighted to 
3 cwt. A slip water-bottle, and two Miller-Casella ther- 
mometers (Nos. 39 and 42) were sent down along with 
it as usual, The tube of the “ Hydra” came up filled 
with a reddish clay containing a considerable quan- 
tity of carbonate of lime. The two thermometers 
were broken, and as the mode in which the fracture 
occurred is in itself curious, and has an important bear- 
ing upon the use of these instruments at extreme depths, 
I will briefly describe the condition of the thermometers 
when they came to the surface. 
No. 39, a valuable instrument, with a small and con- 
stant error, which we had used for some time whenever 
