AIR-SACS OF THE LOCUST. 
179 
in purely creeping and running insects, but only in those which leap 
and always in those which fly. In the two-winged flies (Diptera) the 
vesicles are both large and numerous, as any one can see by opening the 
body of the common house fly. 
According to Marcel ds Serrcs, the Asilidce have an immense number of small 
elongated vesicles on eacli side. In one species they amount to so many as sixty. 
Burmeister remarks that in the Lepi ioptera vesicles in the Sphingidw and moths are 
chiefly found in the males, which agrees with our own observations in Hymenoptera. 
The air-sacs in the family of locusts (Acrydii) are as numerous and there 
are as many large ones as in any group of insects, the females being as 
well provided with them as the males. To the general account given 
in our First Eeport (pp. 267-270) we may add a few facts, and present 
in Plate I some enlarged views of tiie upper side of the head and the face, 
showing better than in the wood-cuts in the First Report their mode of 
distribution. Without repeating the distribution of the air-sacs in the 
head the reader is referred to page 269 of the First Eeport and to Figs. 
1 and 2 of Plate I of the present report. 
Our Fig. 3 (Plate I) represents the course of the main ventral tra- 
cheae or air-tubes extending along the floor of the hind-body or abdomen 
under the digestive canal. They are indicated in Fig. 16 V of the First 
Report. In that figure, however, the relation of this ventral system of 
air tubes to the air-sacs is not shown. By reference to our figure on 
Plate I it will be seen that from these tubes arise small tracheal twigs 
in the thorax, which give off numerous minute globular or oval air-sacs; 
these are also very abundant at the base of the abdomen, especially in 
the front and hinder part of the first or basal segment of the abdomen, 
while a few line the walls behind, and others are seen near the middle 
of the abdomen in the fourth segment. Again they become noticeable 
in the end of the abdomen near the base of the ovipositor, where the 
smaller twigs from the ends of the two main tracheae each end in numer- 
ous small sacs. 
Fig. 4 of the same plate represents the distribution of the air-tubes in 
the hind legs of a common locust (CEdipoda sordida). In the fore and 
middle as well as hinder pair of legs, the thigh joint (femur) is provided 
with two large dilated air- tubes, but there are only four minute, slender 
air-sacs at the base (not represented in the drawing). At the end of 
the femur these two tubes dilate or expand considerably. A large di- 
lated air-tube (t. tr), with numerous small branches, passes along through 
the shin-joint or tibia, lying between the muscles, and this trachea ex- 
tends through the five toe-joints, ending in several small branches in the 
fifth joint at the insertion of the claws. 
Use of the air-sacs in flight. — We described in our First Report (pp. 
269, 270) the mode of inhalation of the air, or of breathing in the locust, 
and briefly pointed out the way in which the air-sacs are filled by the 
air drawn in through the spiracle or breathing holes. It thus appears 
