NO. 2 DRAGONFLY LARVA — SNODGRASS 15 



engendered by a muscular diaphragm in the anterior part of the ab- 

 domen effects the opening of the labial hooks. For some reason 

 Amans did not observe the presence of abductor muscles of the palpi. 

 However, it is quite possible, and now seems probable, that his theory 

 of hydraulic action by blood pressure may account for the forward 

 thrust of the entire labium. The abdominal diaphragm that Amans 

 described in the larva of Aeschna grandis has been shown by other 

 writers to be present in other anisopterous species. The diaphragm 

 of the Anax larva (fig. 9 B, Dph) is a strongly developed sheet of 

 transverse muscle fibers closely surrounding the posterior end of the 

 mesenteron (Ment) and the ventral tracheal trunks (vTra) in the 

 fifth abdominal segment just before the Malpighian tubules. Its ven- 

 tral margin is stretched between the lower anterior angles of the fifth 

 tergum ; the lateral margins extend anteriorly and dorsally along the 

 sides of the fourth segment just below the oblique lateral muscles ; 

 the free dorsal edge allows the passage of the dorsal tracheal trunks 

 (dTra) and the dorsal blood vessel above it. The diaphragm thus 

 divides the abdominal cavity (fig. 7) into an anterior compartment 

 continuous with the thoracic and head cavities, and a posterior com- 

 partment containing the respiratory chamber of the intestine (rspC). 

 Since the diaphragm is concave anteriorly, a contraction of its muscles 

 should compress the anterior abdominal chamber. Stretched between 

 the lower anterior angles of the sixth abdominal tergum is a strong 

 transverse muscle (fig. 9 B, hncl) that goes beneath the ileal sac of 

 the intestine behind the Malpighian tubules. 



A live Anax larva persistently refuses to give any response other 

 than a very slight movement of the labium to pressure on the anterior 

 part of the abdomen or on the thorax. An anesthetized specimen, 

 however, while still limp, reacts at once to compression of the abdo- 

 men and thorax by a lifelike forward swing of the postmentum and 

 an extension of the prementum, but neither the palpi nor their hooks 

 are affected by this treatment. The protraction of the labium is accom- 

 panied by a strong inflation of the neck membrane at the base of the 

 postmentum (fig. 5 C). It seems evident, therefore, that the protrac- 

 tion of the labium results from blood pressure engendered in the 

 fore part of the body. Munscheid (1933) observes that, when the 

 labium is suddenly protracted, a Ruck (quick contraction?) always 

 goes through the body. Perhaps the diaphragm is not the only, or 

 even the chief, source of the pressure, since compression of the thorax 

 has a stronger action on the labium than compression of the abdomen, 

 but the diaphragm at least counteracts the backward extension of the 

 pressure, however the latter may be generated. According to Whedon 



