34 DRAGONFLIES OF NORTH AMERICA 
at the end of the abdomen, one median and two lateral. They are 
generally thin, as befits the respiratory function, but sometimes 
(Agrioninae) they are thick and ridged lengthwise, and they are very 
variable in shape and coloration. They develop during early nymphal 
life, and if broken off before the nymph is grown they are regenerated; 
wherefore a nymph is often found bearing one or more smaller (re- 
generating) gills. 

Fig. 16. Gomphus graslinellus. X, new hatched nymph; Y, its labium; 
Z, labium of an old nymph. 
