NO. 2 DRAGONFLY LARVA — SNODGRASS 5 



of thorax serves equally well for the larva, and therefore has not been 

 essentially modified in the larval evolution. 



Nothing is known of the early odonate larvae, but when they took 

 to the water their first concern must have been with developing some 

 means of aquatic respiration. Inasmuch as a few modern zygopterous 

 larvae have lateral gills along the sides of the abdomen, and in this 

 respect resemble the larvae of mayflies, we may reasonably suppose 

 that the primary odonate larvae were first thus equipped for breathing 

 in the water. This assumption is all the more reasonable if we wish 

 to believe that such gills represented more primitive abdominal styli, 

 such as those still preserved in terrestrial Thysanura. Speculation on 

 the origin and evolution of an animal is quite safe where there is com- 

 plete ignorance of historical facts. 



I. THE HEAD 



The larval head is clearly adapted in its shape to the use of the 

 labium as a grasping organ. The head is prognathous (fig. 5 C), the 

 mandibles and maxillae being directed horizontally forward beneath 

 the labrum, where they are in a position to receive the food brought 

 back to them by the prehensile labium. In an aeschnid larva particu- 

 larly, the head is much flattened (fig. 5 C). The postoccipital margin 

 on which the neck is attached is strongly oblique from above down- 

 ward and forward to the base of the labium, which latter thus becomes 

 suspended below the center of the cranium to give it a longer forward 

 reach when protracted. The large compound eyes project on the sides 

 of the head (fig. 2 A), but they do not meet on the vertex as they 

 will in the adult (D). The antennae have a forward position just be- 

 hind the clypeus (A). The broad clypeus (Clp) gives articulation to 

 the mandibles on its basal angles, and supports the large labrum 

 (Lm). The cranial walls are braced internally by a well-developed 

 tentorium (B,Tnt), consisting of a posterior bridge arising from 

 pits ipt) in the anterior ends of the postoccipital sulcus (pos), and 

 of posteriorly convergent anterior arms arising from slits between the 

 compound eyes and the bases of the mandibles (A, at). Short dorsal 

 branches of the anterior arms are attached on the cranial wall by 

 brushes of apparently nonmuscular fibers. The ventral margins of 

 the cranium (B) surround the neck foramen posteriorly, but are 

 widely divergent anteriorly along the sides of the maxillae. The man- 

 dibles and the maxillae are articulated on the cranial margin (a,a') ; 

 the labium (fig. ^ A, Lb) is suspended from the membranous ventral 

 head wall behind the hypopharynx and has no direct support on the 

 cranium. 



