INSECT METAMORPHOSIS * 
Il, 
MAY naturalists of eminence have insisted so strongly upon 
the connection of the growth of wings and metamorphosis, 
that I shall now proceed to examine into this part of the subject. 
These beautiful organs of flight, so elegant in their outlines, so 
exquisite in the artistic blending of their colours, so marvellous 
in their minute construction, are popularly associated with the 
perfection of insect life. A suspicion, of their existence arises 
when the curious swathings of a pupa are examined ; but it re- 
quires the patience of a Landois to trace these future glories of a 
butterfly within the chest of the caterpillar but lately escaped 
from the egg. 
But in considering the relation of growth to metamorpho- 
sis, it must be remembered that some insects have no wings, 
and yet undergo. metamorphosis, and that others possess organs 
of flight, and yet only submit to skin-shedding. 
In describing the general form of the body of the larva, it was 
noticed that the openings for the passage inwards of the air 
tubes were visible on either side of each segment. The open- 
ings, or stigmata as they are called, of the second and fifth seg- 
ments of the larva, whose structures have been already described 
‘in part, are very distinct, and they lead to large air-tubes which 
branch off in all directions, and especially send a twig back- 
wards and forwards along the inside of the third and fourth 
segments respectively. 
The openings, or stigmata, of the third and fourth segments, 
on the contrary, are blind ones, and do not lead to tracheze or 
air-tubes ; but the delicate offshoots of the second and fifth 
masses of air tubes pass inside close to them, and it is on these 
that the wings are developed as new organs, as new structures 
fashioned out of the protoplasm of the blood. The wings are 
Fic, 7.—Wing-Cells 
acquired, and are added to the bulk of the belongings of the 
larva. They continue to grow and to be perfected during the 
whole of the life of the insect, until their function is called into 
action. They originate after the escape from the egg ; but the 
structures, upon the consideration of which so much time has 
been spent this evening, originated during the embryonic or egg 
condition, and clearly do not advance through long stages of 
inutility and imperfection to one of use and perfect adaptation. 
The first indication of the wing is observable in a caterpillar 
four millimetres in length, and one day after birth from the egg. 
The whole of the air-tubes are at that time as they are at 
all others covered by a very delicate layer of cells, which sepa- 
rates them, in fact, from the other tissues or blood, as the case 
may be, and with which they are in contact. Some flat and 
very small five-sided projections of delicate tissue are seen upon 
the fine air tubes running along the inside of the third and fourth 
segments, 
There are four of these, two on either side, and the hinder are 
smaller, but are close to the front pair. In this stage they are 
composed of simple cells placed side by side to form the expan- 
sion of the tissue, and they rest upon and cannot be separated 
artificially or microscopically from the fine layer of cells which 
intervenes between their bases and the air-tubes. The tissues 
and cells of the air tubes remain intact, but these additional 
structures are fixed upon them, and are destined for a very dif- 
ferent series of developments. 
When the caterpillar has changed its skin for the first time, 
the expansions have increased in size and in complexity of struc- 
ture. Each expansion is found to consist of a structureless, flat, 
* A Lecture delivered before the British Association, 1872, by Prof. 
Duncan, F.R.8., continued from p. 34. 
pentagonal bag, which is very thin, and to contain a well-marked 
layer of globular cells of nearly equal sizes. Moreover, at the 
base of the expansion, where it rests on the cellular layer of the 
air-tube, a crowded group of elongated cells is observed resting — 
on this layer, and situated amongst the’globular cells and within 
the structureless expansion. eS 
These elongated club-shaped cells are sometimes fusiform, and 
contain a structureless liquid, and attached within their equally 
structureless walls isa nucleus and its contents. They did not 
exist before the skin-shedding, but"are readily observed subse- 
quently to it. The expansions of this tissue consist of the three’ 
histological elements just noticed, and out of them the futu 
wings are gradually developed. ne 
After the second skin-shedding of the caterpillar, the expan- 
sions are found to have increased slightly in size ; and a care 
microscopical examination detects an excessively delicate and 
twisted cylindrical tube within each of the long cells which are 
situated at the base of the expansion, and which would bein con- 
tact with the air-tube, were it not for its investing cellular layer. 
The nucleus of the elongated cell has been absorbed, and ii 
walls look thinner, and their tissues appear to have been observed 
to contribute to the twisted-looking thread which floats in the — 
liquid contents. Itis evident that this thread-like tube is con- 
nected through the cellular layer with the interior of the air-tube, — 
but it is at present a simple tubular expansion of plain structure- 
less membrane, and does not contain air. 'e 
Alterations progress in the developing wings during the inter 
val between the third and fourth skin-sheddings, and they be- 
Fic. 8—Advanced Wing Development. 
come sufficiently large as to be seen with the naked eye, for they 
have attained nearly one-tenth of the length of the whole cater- 
pillar. The globular cells within the structureless membrane are 
found to have increased in size, and the delicate tubes of the 
elongated cells to have increased in length and numbers. The 
cell wall and nucleus are lost to sight in some instances, and the 
twisted tubular connection of the undermining air-tube may be — 
observed to have passed here and there amongst the globular — 
cells. At this time these tubes take on the appearance of air. 
tubes, and the delicate circular fibre which is seen in the other 
air tubes of the caterpillar is to be recognised. fen 
The next stage appears to bring about an increase in the num- 
ber, length, and size of these coils of air-tubes within the bag- 
like wing, and in the dimensions of the bag, but not of its 
contained globular cells. ae q 
By the time that the caterpillar leaves off taking food and 
begins to attach itself by its tail end, preparatory to skin- 
shedding for the last time, considerable progress has been made 
in the development of the wings. The cord-like air-tubes have 
grown sufficiently to reach nearly to the top of the wing, and 
they branch off in several directions. They contain air, and are 
surrounded by the layer of globular cells, but they are closer to 
the under side of the wing bag than to the upper. Moreover, 
the wing bag, so structureless hitherto, has acquired on its out- 
side a glistening surface of extremely delicate cells, which is 
called epidermis. Beneath this coating is the main thickness of 
