AIR-SACS THAT ASSIST FLIGHT. 269 



hypodermis or inner layer of the integument, while the smaller ones are, 

 in many cases, buried among the muscles. Between the first and sec- 

 ond thoracic sacs is a pair of small ones with numerous others still 

 smaller. From the dorsal tracheae arise series of minute subcutaneous 

 air-sacs. Besides these ordinary air-sacs there is in the end of the ab- 

 domen, behind the ovaries, a plexus of six dilated air-sacs (Figs.lG and 17, 

 I, II, III), which are long, spindle-shaped, and are easily detected in 

 dissecting. Behind them, in the extreme end of the abdomen, are three 

 pairs of small rounded air-sacs, and there is a complicated system of 

 dilated tracheae among the muscles of the ovipositor. Between the six 

 gastric caeca are six dilated air-tubes. 



The system of dilated tracheae and air-sacs in the head is rather com- 

 plicated, and the following description is based on vivi- dissections of 

 Caloptenus femur-ruhnim and (Edipoda sordida. Of the air sacs there is 

 a median one (not represented in the figures) in the posterior edge of 

 the head, and buried between the muscles and resting directly on the 

 crop. Between the eyes are two large sacs (seen in Fig. 17), arising from 

 the main cephalic tracheae; these air-sacs connect with an unpaired 

 median transverse sac. Under the eyes are four sacs connected 

 together by tracheae, and in fact this whole region contains a large num- 

 ber of subcutaneous air-sacs. The main cephalic tracheae subdivide, and 

 send two branches around the eye under the cornea (Figs. 16 and 17, oc, 

 ocular tracheae), and from these ocular tracheae are sent off numerous 

 subcutaneous sacs. On the vertex between the eyes and insertion of 

 the antennae are nine sacs of unequal size, one median, one being larger 

 than the eight others. There is also a pair of sacs on the extreme ver- 

 tex and two pairs behind the eyes ; a little in front of the antennae are 

 four well-marked median round sacs, and two much smaller ones. 

 Along the front edge of the clypeus is a large transverse dilated trachea, 

 from which four tracheae, two on each side, the external ones the small- 

 est, pass down to the front edge of the labrum, and send off a number 

 of small spherical sacs, so that in the labrum alone there are about 16 

 air-sacs. We have found 53 air-sacs in all in the head of the locust. 

 The lateral region between the base of the jaws and the vertex is quite 

 free from air-sacs, as we have not detected more thaji four or five quite 

 small ones. 



In the legs (described from (E, sordida) two tracheae pass down each 

 side of the femora, sending off at quite regular intervals numerous much- 

 branching transverse twigs; .there is one large and a very small trachea 

 in the tibia, and the main one extends to the extremity of the last tarsal 

 joint. There are four slender air-sacs at the base of the femora. 



Mode of breathing. — By holding the Ked-legged locust in our hand 

 we observed the mode of breathing. During this act the portion of 

 the side of the body between the stigmata and the pleurum (Fig. 12, 

 A) contracts and expands ; the contraction of this region causes the 

 spiracles to open. The general movement is caused by the sternal 



