182 REPORT UNITED STATES ENTOMOLOGICAL COMMISSION. 
•lowly but gradually increased, until a few seconds before it attempted to expand its 
■wings and to elevate itself upon tbem, when the acts of respiration became exceedingly 
rapid, and amounted to at least 120 per minute. These were most rapidly performed, 
and were then suddenly arrested at the instant before it attempted to unfold the 
wings. During this increased respiration the abdomen of the insect was distinctly 
enlarged, and it was quite evident that this enlargement and the expansion of its 
wings wore being effected by forced inspirations, and maintained by the expansion 
of the air-sacs over the whole body, and the communication of these with the tracheal 
vessels in the wings themselves. As, howover, the wings had become stiffened and 
dried through many hours, it did not completely succeed in its attempts to escape, 
but only partially raised itself upon them. The results were nevertheless sufficiently 
•atisfactory to prove to me that the respiratory organs became distended previous to 
the act of flight, as the entire body was distinctly enlarged; the effect of which en- 
largement, together with an increased evolution of heat in the body, as the result of 
increased respiration, must, of consequence, bo to diminish the specific gravity of the 
insect, and thus, by lessening the degree of muscular force required to raise it on its 
wings, considerably augment its powers of locomotion, which seems to be the chief 
use for whicli the vesicles are developed. 
Origin of the air-sacs. — When we carefully examine the tracheae and 
observe that in the locusts many are dilated and so expanded that at 
first one is inclined to regard them as simply long vesicles, and then 
observe the form of some of the vesicles and their relations to the tracheae 
from which they originate, one is inclined to accept the following theory 
as to their origin given by the distinguished English anatomist whom 
we have just quoted. 
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 approaching that they begin 
to be enlarged even in those in which vesicles are afterwards the most numerous. The 
enlargement, as I have elsewhero shown, commences in Lepidopterous insects at about 
the time when the larva ceases to feed. It is perceptible first in the longitudinal 
trachea? of the thoracic segments 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 completed, the trachea? from the second to the fifth spiracles are distinctly enlarged. 
In the diurnal species, which do not enter the earth but undergo their changes in the 
open air, the dilatation of the trachea? commences while the insects are spinning their 
silken threads. When this labor 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 trachea in the fifth and 
sixth segments are dilated into vesicles, which continue to be enlarged during the first 
few days after the change. The trachea? 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 formd 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 trachea? from these minute trunks, 
and very closely resembles in appearance the external abdominal branchia? of the 
aquatic larva? of Neuroptera. It is these folds which become the most important or- 
gans in the perfect state of the insect, its wings. When the old skin of the larva is 
fissured and the thoracic segments become shortened, as the skin is thrown off, pre- 
vious to their forming one region, the thorax, the trachea? in these folds are rapidly en- 
larged and elongated, and mainly assist in inducing a rush of blood into these struct- 
ures, which are thus expanded on the sides of the new pupa as the rudimentary wings. 
This elongation of the small trunks at the sides of the longitudinal trachea? in the 
thorax relieves them of a portion of that tension which results from the powerful 
