THE PAT BODY AND THE (ENOCYTES. 119 



them as follows: There are two dorsal sets: (1) The internal dorsal^ 

 going from the anterior edge of one tergum to the anterior edge of 

 the next following tergum, and (2) the external dorsal, going from 

 the lateral edge of one tergum to the corresponding edge of the fol- 

 lowing tergum. Both of these are expiratory, since their contrac- 

 tions bring the two segments together. On the sides are three sets: 

 (3) The external oblique, going from the anterior edge of each tergum 

 to the side of the corresponding sternum; (4) the internal oblique, 

 crossing under the last from the anterior edge of each tergum to the 

 side of the preceding sternum. These two sets are likewise expira- 

 tory, because their contractions approximate the terga and sterna. 

 The third set of lateral muscles is (5) the transverse, lying between 

 the overlapping surfaces of each tergum and its corresponding 

 sternum and being, therefore, inspiratory, because the contraction 

 separates the terga and sterna. Finally, there are two sets of ventral 

 muscles: (6) The external ventrals and the internal ventrals, forming 

 a letter M between the anterior edge of each sternum and the one 

 following, and (7) the interventral, situated between the overlap- 

 ping surfaces of consecutive sterna and causing their separation by 

 contraction. These last are therefore also inspiratory. 



It would thus seem that the abdomen is much better equipped with 

 expiratory than with inspiratory muscles. Perhaps the expansion 

 is partly due to elasticity, and perhaps, also, it is true that the abdo- 

 men contracts upon the full tracheae and air sacs, before the spiracles 

 open to allow exhalation, in order to drive the air into the farthest 

 recesses and terminal tubes of the tracheal system, which necessitates 

 an extra contractive force. 



IX. THE FAT BODY AND THE <ENOCYTES. 



The fat tissue of insects is not miscellaneously distributed through 

 the tissues, imbedded beneath the skin and packed between the 

 muscles, but is disposed in sheets and strands within the body 

 cavity, especially in the abdomen, or forms a definite mass, the 

 fat body. The fat cells are large and extensively vacuoled with 

 clear globules of fatty oils. In some insects the fat bodies have a 

 brilliant yellow, golden, or orange color. Associated with the fat 

 cells are other much larger and often gigantic cells, called wnocytes, 

 attaining the largest size of all the cells in the body except the eggs. 

 They were first discovered in segmental clusters attached to the 

 tracheae near the spiracles, but they are now known also to be scat- 

 tered through the depths of the body cavity, where they occur im- 

 bedded especially between the fat cells. The term " cenocyte " signi- 



