NO. 2 DRAGONFLY LARVA — SNODGRASS 'Z.y 



broad bands of intersegmental dorsal and ventral muscles, and thick 

 tergosternal lateral muscles. The dorsal muscles in each segment are 

 paired layers of external and internal fibers (fig. 9 C, de, di) slightly 

 oblique in opposite directions. The ventral muscles are thicker bands 

 of fibers not so definitely arranged in two layers {F,v). The lateral 

 muscles include in each segment external, intrasegmental, vertical 

 tergosternal muscles (C, F, /^), and a long oblique intersegmental 

 muscle (lo) from the side of each tergum in front to the laterosternite 

 of the segment behind. In most of the segments the fibers of the ex- 

 ternal lateral muscles are grouped on each side in two thick bundles 

 attached on the side of the tergum and inserted on the inner margin 

 of the laterosternite, but in the anterior segments the fibers are less 

 concentrated and some of them attach ventrally on the median sternal 

 plates. Besides these principal abdominal muscles there are also, as 

 shown by Whedon (1918), short outer intersegmental muscles lying 

 close against the tergal and sternal walls (fig. 9D, E). Finally, in 

 addition to the segmentally repeated muscles, are the muscular dia- 

 phragm of the fourth segment {B,Dph), the transverse muscle of 

 the sixth segment (tmcl), and in the tenth segment special muscles 

 of the apical lobes. 



The breathing movements of the larva are most pronounced in the 

 wider posterior half of the abdomen. The expiratory movement in- 

 cludes a slight depression of the tergum accompanied by an expansion 

 of its lower margins, and a much more pronounced retraction of the 

 sternum (fig. 9G) during which the laterosternites turn upward on 

 the tergal margins. The expiratory muscles, therefore, are the lateral 

 muscles attached on the inner margins of the laterosternites. If the 

 sternum is more strongly retracted (H), the laterosternites turn up 

 at a sharper angle and the tergum is transversely compressed. Dur- 

 ing inspiration the lateral muscles relax, the tergal arch is restored, 

 the sternum descends and resumes its ventral convexity (F). The ac- 

 tion of the muscles is easily seen in a live larva through the semitrans- 

 parent lateral parts of the abdominal segments. 



The inspiratory movements appear to be due in all cases to the elas- 

 ticity of the tergal plates. Wallengren (1914), however, contended 

 that when the sternum of a segment has reached its maximum inflec- 

 tion, the tergum would be incapable of restoring it to the depressed 

 position. He, therefore, accepted and elaborated the idea of Matula 

 (1911) that the inspiratory movements are caused mostly by contrac- 

 tions of the diaphragm and the transverse muscle. On cutting out a 

 narrow strip of the dorsal integument of the abdomen of a live larva, 

 he observed that, when the insect resumed breathing, the contraction 



