NO. 2 DRAGONFLY LARVA — SNODGRASS 1 9 



Junius the "primary extensors" (tentorial muscles) are retained in 

 the adult, but the pair of muscles he identifies as such are probably 

 the apodemal muscles ("primary flexors"), since he says they arise 

 from a semicircular sclerite near the base of the labium. The apod- 

 emal muscles (^o), because of the shortening or suppression of the 

 apodeme in the adult, as Munscheid shows, come to arise in the imago 

 on the base of the hypopharynx beneath the salivary orifice, which is 

 their normal position in the cockroach. The elbow muscles of the 

 larval labium appear to be retained in the adult. 



The labium of the adult dragonfly (fig. 6E, F), though it has a 

 more generalized structure than that of the larva, is nevertheless spe- 

 cialized in its own way for the purposes of the adult. In particular 

 the large, flat, lobelike palpi, together with the ligula, are evidently 

 adapted for closing in the other mouth parts (G, H) to prevent the 

 escape of captured prey from the mandibles and maxillae. 



III. THE ALIMENTARY CANAL 



From the mouth at the inner end of the cibarial chamber in the 

 larval head (fig. 7, Cb) the slender oesophagus (Oc) extends through 

 the head and the thorax, enlarging posteriorly to form a crop (Cr), 

 which opens by a narrow neck into the proventriculus (Prvent) lying 

 in the first abdominal segment. From the proventriculus the simple, 

 cylindrical ventriculus (Vent) goes back to the fifth segment after 

 penetrating the diaphragm (Dph), and tapers to its connection with 

 the ileum (Ilm). At the junction arise the numerous, threadlike 

 Malpighian tubules (Mai, not fully shown in the figure). The ileum 

 turns upward in the fifth segment, close in front of the transverse 

 muscle (tmcl) of the abdomen, and expands into an oval sac. The 

 intestine then continues as the huge respiratory chamber (rspC), 

 which arises by a narrow extremity from the upper end of the ileal 

 sac and reaches to the end of the eighth abdominal segment. The en- 

 trance into the respiratory chamber from the ileum is guarded by 

 three high, convergent folds of the ileal wall. The respiratory cham- 

 ber is commonly regarded as an enlarged anterior part of the rectum, 

 but the narrow, cylindrical following part of the intestine (Red) has 

 in itself the typical features of the rectum of other insects. The walls 

 of this end chamber alone contain the six characteristic longitudinal 

 thickenings known as rectal pads, and on the intervals between them 

 are attached six rows of dilator muscle fibers arising on the body wall. 

 It is true that the pads are continuous with the six double rows of gills 

 in the respiratory chamber, but there is no reason for regarding the 

 gills as rectal pads. It seems probable, therefore, that the respiratory 



