GRASS THRIPS. IO7 



pair, attached by a single head in the posterior part of this 

 somite, diverge as they run forward, and are attached to the 

 integumental fold between the head and prothorax. In the 

 anterior part of this somite there are, on each side, three muscles 

 which are attached at their ventral ends to the thorax, and at 

 their dorsal ends to the head. In the posterior part of the head 

 there are six muscies that are attached at their dorsal ends to 

 the thorax and at their ventral ends to the head. These two 

 sets of muscles cross one another at their middle points. The 

 muscles that have to do with the movements of the mouth parts 

 are situated in the anterior part of the head, on its ventral wall. 

 A part of these muscles run anteriorly and dorsally to be attached 

 to the front part of the head. The remainder run dorsally to be 

 attached to the dorsal wall of the head. 



Each leg is supplied with four muscles. One of these muscles 

 lies along the floor of the thorax, and is attached at one end to 

 a median ridge of the integument, and at the other end to the 

 fold between the coxa and trochanter. This muscle serves as 

 the flexor of the coxa. The other thoracic muscles of each leg 

 are really the three heads of one muscle, the extensor of the 

 coxa. These muscles are attached to the integument on the dor- 

 sal walls of the thoracic somites and at the other end to the 

 integumental fold between the coxa and trochanter. The mus- 

 cles in the next two joints of the leg (trochanter and femur) are 

 arranged in much the same way, one of them acting as a flexor 

 and the others as extensor. They are attached intersegmentally. 

 In the next joint (tibia) there are only two muscles, one flexor 

 and one extensor. Both of these muscles are continued into the 

 tarsus by tendons. The tarsus has no muscles of its own. 



In the winged forms there are, in the thorax, the muscles for 

 the movement of the wings. These consist of two series on each 

 side, one of which elevates and the other depresses the wings. 

 Each series is made up of several muscles. There are two ele- 

 vators and four depressors. Of these there is in each series, a 

 single muscle that is much larger than the others and that does 

 the greater part of the work in flight. The others serve to keep 

 the wings in their proper position. All of these muscles are 

 attached at one end to the ventral wall of the thorax, and at the 

 other to the wings. The elevators are attached inside, and the 

 depressors outside of the point which serves as the fulcrum for 

 the movements of the winsrs. 



