AQUATIC INSECTS 



and stealing up on their prey, which they seize with the ex- 

 tended lower lip (Fig. i6o). Ordinarily this is folded back 

 under the head; in some species it covers the face like a mask. 

 When a nymph is capturing prey it thrusts its labium far 

 forward (Fig. 21), ready to clutch at anything which moves 

 and to jerk it back into its mouth and waiting jaws. 



Damselfly nymphs are slender with three leaf-shaped gills 

 extending from the hind end of the body (Fig. 167). Those 

 of dragonfiies are stout-bodied and without external gills 

 (Fig. 162). They breathe by taking water into a chamber at 

 the rear end of the alimentary canal whose walls are elaborate- 

 ly infolded and bespread with a lace work of airtubes. Water 

 is drawn in and out of this tracheal chamber by way of the 

 anus which is guarded by five pointed spines. In collecting- 

 pans nymphs will often turn over on their backs and while 

 they are thus "playing dead" with their legs bent double, 

 they will be suddenly shot forward by the forcible expulsion 

 of a stream of water from the tracheal chamber. Thus the 

 chamber proves itself not only an efficient respirator}^ organ, 

 concealed and out of the way, but a locomotor organ as well 

 (PI. IX.) 



Occurrence. — Nymphs of some species of dragonfiies or 

 damselfiies are almost certain to be abundant in mucky- 

 bottomed ponds half filled with vegetation (Fig. 21). But 

 some species of each group have settled into clear brooks 

 and slow-moving marsh streams (Fig. 22). Adult dragon- 

 flies and damselfiies begin to emerge in late April and con- 

 tinue till September, but in New England active nymphs may 

 be found in small ponds after the first of November. 



Food. — These nymphs, especially the dragonfiies, eat every 

 kind of aquatic insect, beetle larvae, midges, and small mem- 

 bers of their own kind but they prefer mayfiies, small crusta- 

 ceans, and snails. Nymphs and adults as well are great enemies 

 of mosquitoes. 



Associates. — Dragonfly nymphs and 3'oung fishes are com- 



215 



