420 — Mr. Newport on the Formation and Use of the Air-sacs 
considerably enlarged throughout the greater part of their course, their ex- 
treme ramifications only retaining their original setiform structure and distri- 
bution. In the truly apterous insects the trachee are invariably arborescent, 
and diminish in size from their origin to their extremest point in their perfect 
as in their larva condition; and they are invariably smaller in diameter, and 
have fewer ramifications, in the most inactive species. 
The respiratory organs are always simply tracheal in the larva state of all 
insects, and it is not until the period of change to the pupa is fast approach- 
ing that they begin to be enlarged, even in those in which vesicles afterwards 
are most numerous. The enlargement, as I have elsewhere shown *, com- 
mences in Lepidopterous insects at about the time when the larva ceases to 
feed. It is perceptible first in the longitudinal tracheze of the thoracic seg- 
ments of the Sphinx, immediately before the insect enters the earth ; and by 
the time that the cell in which it is to undergo its transformation is com- 
pleted, the tracheæ from the second to the fifth spiracles are distinctly en- 
larged. In the Diurnal species, which do not enter the earth, but undergo 
their changes in the open air, the dilatation of these tracheze commences while 
the insects are spinning their silken threads. When this labour is finished, 
. and they have remained for a few hours at rest, the skin is fissured along the 
dorsal surface of the thoracic segments and thrown off, the change to the pupa 
is effected, and the longitudinal tracheze in the fifth and sixth. segments are 
dilated into vesicles, which continue to be enlarged during the first few days 
after the change. The tracheze of the third and fourth segments each give off 
a small trunk on their external surface, which is divided into two branches, 
and is involved in a fold of the new tegument that is formed beneath the old - 
skin of the larva some days before its change. The fold of tegument on each 
side of the third and fourth segments is supplied with ramifications of trachez `- 
from these minute trunks, and very closely resembles in appearance the ex- 
ternal abdominal branchie of the aquatic larvze of Neuroptera. It is these 
folds which become the most important organs in the perfect state of the in; 
sect, its wings. When the old skin of the larva is fissured, and the thoracic 
segments become shortened, as the skin is thrown off, previous to their form- 
mg one region, the thorax, the trachez in these folds are rapidly enlarged 
* Phil. Trans. 1836, part ii. p. 535. 
