NO. 2 DRAGONFLY LARVA — SNODGRASS 25 



numerous fine circular fibers, but the sac has no dilator muscles. The 

 branchial chamber, therefore, can contract but it cannot of itself ex- 

 pand. The narrow terminal rectum (fig. 7, Red), however, is provided 

 with six rows of dilator fibers arising on the body wall, as well as with 

 longitudinal and circular fibers in its own walls. This end section of 

 the intestine, therefore, is capable of active expansion and contraction, 

 and, according to Tonner (1936), functions for "swallowing" water 

 and driving it into the respiratory chamber for respiration {Schluckat- 

 niung). Since the narrow anterior end of the branchial chamber can 

 be closed by the ileal valve, the water may be discharged by the ac- 

 tion of the chamber itself. Again, the water is sometimes held in the 

 chamber and agitated or churned for better aeration of the gills 

 {Kauatmung of Tonner), The three small circumanal lobes between 

 the bases of the apical processes of the larva (fig. 11 G) apparently 

 act as an anal valve ; in a living larva they may be seen actively open- 

 ing and closing. 



The usual breathing movements of the branchial chamber as seen 

 through the body wall are exactly synchronous with dorsoventral and 

 transverse expansions and contractions of the abdominal wall itself, 

 and these skeletal movements appear to be the cause of the ordinary 

 breathing movements of the chamber. On the other hand, when the 

 water is forcibly ejected from the anus, and the larva suddenly darts 

 forward with its legs laid back against the body, it is to be seen that 

 with each spurt the abdomen contracts lengthwise from behind. 



The mechanism of the respiratory and locomotory movements may 

 be readily understood by a study of the abdominal skeleton and mus- 

 culature. The tergal plates of the abdomen are simple, strongly con- 

 vex arches with slightly projecting lateral margins (fig. 9F). The 

 sterna are convex, and on the first nine segments each sternum is di- 

 vided into three parts (A, I), a broad median plate (mst), and two 

 small lateral plates (1st), which latter are movably hinged on the 

 edges of the tergum and on the median sternal plate. The lateral plates 

 contain the spiracles on the first eight segments, and some writers, 

 therefore, call these plates "pleurites." The term pleuron, however, 

 has no very definite meaning as applied to the abdomen, so we may as 

 well distinguish the two sets of ventral plates as median sternites and 

 later St emit es. The median sternites are almost of uniform width 

 throughout the length of the abdomen (A), but the laterosternites are 

 very small on the anterior segments (J) and largest on the middle 

 region of the abdomen. 



The abdominal musculature is strongly developed in the dragonfly 

 larva, and has been fully described by Whedon (1918). It includes 



