AIR-SACS OF THE LOCUST. 
181 
treme ramifications only retaining their original setiform structure and distribution. 
In the truly apterous insects the tracheaB are invariably arborescent, and diminish in 
size from their origin to their extremest point in their perfect as in their larva condi- 
• tion, and they are invariably smaller in diameter and have fewer ramifications in 
the most inactive species. 
In the flying locusts the air-sacs or vesicles are not less numerous 
than in the bee, and we thiuk that in proportion to the body they are 
more numerous, while it is obvious that there is a greater number of 
large sacs. When we add to these the large number of broad, expanded, 
or dilated air-tubes, some of which would easily be confounded with the 
sacs themselves, we can appreciate the wonderful powers of the Rocky 
Mountain locust as an aeronaut. With a greater expanse of wing than 
in any of its congeners, and as complex an arrangement of air-tubes and 
sacs, it is able to rise from the ground in the morning and, if the day 
proves clear, remain floating in the air for hours, until near sunset, 
borne hither and thither by gentle baffling breezes, or wafted straight on 
in its course eastward from its mountain home for miles over the plains, 
should the breeze be strong and steady. Meanwhile scarcely as much 
muscular force is spent through the day as is exerted during a few 
vigorous hops when it alights on the ground. 
In the possession and use during flight of these air-sacs, locusts and 
other insects may, as Newport suggests, be compared with birds. 
In birds the respiratory organs are not only vesicular but are more extensively dis- 
tributed over the whole body than in any other vertobrata. These, as every anato- 
mist knows, are not confined merely to the great cavities of the body, but are extended 
to every part of the skeleton, as in insects. They communicate directly with the 
interior of the bones of the wings and legs, as the trachese of the thorax are extended 
also into these parts in insects. This distribution in both is more extensive and com- 
plete in the most active species. In birds which are unaccustomed to flight, as in the 
ostrich, as remarked by Mr. Owen, 2 - 2 the communication of the respiratory organs with 
the bones is imperfect ; while in insects, although trachese exist in all, the vesicles 
are found only in those of flight. This fact extends even to the sexes of the same 
species. Thus vesicles exist in the males of the common glow-worm, which is winged, 
and designed to search out the apterous female, in which the respiratory organs are 
simply tracheal. The like conditions exist in the common winter-moth, Geometra 
brunaria. In the male of this insect I have found the vesicles large and numerous, 
but not a trace of these occurs in the female. The trachese in this sex, which has only 
the rudiments of wings, are larger relatively than in the female glow-worm, and aie 
precisely in that condition in which I have found them in the diurnal Lepidoptera 
shortly before changing to the pupa. These anatomical facts are inferential of the 
real use of the vesicles, aud are supported by an observation which I have been able 
to make on the common dung-beetle, Geotrupes stercorarius, at the moment when it is 
preparing to take flight. A specimen of this insect which had been in confinement 
for about twenty-four hours, and consequently had not expanded its wings during 
that time, when placed on a table immediately prepared to escape. After walking 
away quickly for a short distance it began to respire freely, alternately shortening 
and elongating its abdominal segments at the rate of about forty respirations per 
minute. It then ceased for an instant and slightly separated its elytra without 
elevating them, and began again to respire more rapidly. At first its respiration was 
^Cyclopaedia of Anatomy and Physiology, Art. Aves., vol. i, p. 341. 
