892 FRESH-WATER BIOLOGY 



among green stems, where they are quite inconspicuous. But 

 nearly all dragonfly nymphs get their living by waiting in hiding 

 for the approach of their prey, and comparatively few of them roam 

 freely about in the water. Most of the LibelluKdae are bottom 

 sprawlers (Fig. 1362) ; most of the Gomphines are burrowers beneath 



Fig. 1362. The sprawling nymph of Didytnops transversa. 



the bottom silt, and the nymphs of Cordulegaster are expert 

 ambuscaders, scratching a hole in the sand of the bottom and 

 gettmg into it, kicking the sand up over their backs until covered 

 excepting the tips of the eyes and of the respiratory orifice at 

 the end of the abdomen, and lying in wait until some unsuspecting 

 little animal suitable for food wanders within reach. 



The chief organ for capturing prey in the nymphs of all the 

 Odonata is the remarkably developed labium (Fig. 1389^), which 

 has become elongated, hinged in the middle and folded back under 

 the thorax. It has acquired a formidable array of grappUng hooks 

 and spines at its tip. It is often longer than the fore legs when 

 extended and possesses muscles capable of extending it with light- 

 ning-like speed. It is thrown forward and opened by a single 

 movement, and when it closes on its victim it is withdrawn again 

 instantly, dragging the struggling captive back under the jaws, 

 which then come into play. 



The problem of getting air has been solved in two ways in the 

 nymphs of the two suborders of Odonata. In the damselflies 



