INTERNAL STRUCTURE. 69 



especially equipped for this purpose with a large number of 

 air sacks. Such organs are found in largest numbers in those 

 that sail for many hours in the air, hence they are more 

 numerous in the migratory locusts than in other members 

 of the orthoptera or in bees and flies. By possessing such 

 air sacks the locust can buoyitself upin theair, and as it con- 

 stantly fills and refills these balloons without any great 

 muscular exertion, it is easily borne along by favorable 

 winds. The air sacks are intimately connected with the com- 

 plicated s\^stem of air-tubes, or tracheae, w^hich ramify 

 throughout the body ; theair enters these tubes through a 

 row of spiracles or breathing-holes [stigmata] located in 

 the sides of the body. In locusts are found two pairs of 

 thoracic and eight pairs of abdominal spiracles. The system 

 of thoracic air-tubes, which is entirely independent of that of 

 the abdomen, is not easily made out. The tubes arising 

 from the two thoracic stigmata send out two well-defined 

 tracheae into the head, w^hich subdivide and form the ocular 

 air-sacks and some smaller ones in the front of the head. The 

 five pairs of large abdominal air-sacks are derived directly 

 from the spiracles ; the}- can be readily seen b}' removing the 

 integument from the back of the locust. There is also a 

 large pair of air-sacks in the mesothorax, and two enormous 

 ones in the prothorax. A number of smaller air-sacks are 

 buried among the muscles, while others, spindle-shaped ones, 

 occur in the end of the abdomen. As the process of respira- 

 tion is much more easily performed in clear and sunny 

 weather, we find locusts flying only at such times; in 

 cloudy or damp weather, or after the sun sets, they are less 

 active. 



The Digestive Organs.— These are quite dark in color, 

 and vary slightly in structure according to the genus. The 

 following description is true only of the genus Schistocerca. 

 By removing the corpus adiposum,the wing-muscles and the 

 heart, the oesophagus (Fig. 41, oes) is seen to be a tough, 

 dark-brown, cylindrical tube, which runs up from the mouth, 



