440 



that when nymphs are brouglit into the warm laboratory they molt 

 within a few days. Balfour-Browne found surprising differences in 

 the time between molts in nymphs kept at constant temperatures, so 

 that it would seem probable that other factors enter into the problem 

 besides temperature. He was able, however, to reduce the length of 

 the stages by raising the temperature, and found that in some cases 

 these lasted, in low temperatures, for 150 days, while in others they 

 lasted only five days at higher temperatures. The number of molts 

 varies from ten to fifteen in the Coenagrionidae, and the length of the 

 nymphal life may range from 229-624 days (Balfour-Browne, '09). 



Habitat. — In nature, the nymphs are most often found hiding 

 among the weeds and rubbish along the margins of lakes, ponds, 

 and streams. A few have been taken under rocks in swift currents, 

 among them Argia piitrida- (Needham, '03) and Argia tibialis. The 

 Agrionidae frequent the swifter currents, and seem to prefer these 

 situations to any others. They are never found in stagnant ponds. 

 Nymphs of Lestes, on the other hand, do not occur except in stagnant 

 woodland pools, and are never taken along the banks of streams unless 

 a stagnant condition is present. They prefer the shade, and hide 

 among the broad-leaved types of small water-weeds, being rarely found 

 among the narrow-leaved rushes and saw-grass. Riley ('12) says 

 that the nymphs of Zygoptera react negatively to light from a pro- 

 jection lantern but that such a reaction is often inhibited by the habit 

 of clinging to objects. He was unable, however, to obtain similar 

 reactions to moderatelv strong daylight. Reactions to heat have not 

 been studied, but the nymphs are able to withstand temperatures near 

 the freezing-point and may be collected during the winter from be- 

 neath the ice. They readily succumb when the temperature of the 

 water rises much above /0°F., but flourish well at 66.2°F. or I9''C. 

 (Balfour-Browne, '09). Lestes is particularly sensitive to high tem- 

 peratures, and when in captivity considerable care must be taken to 

 keep the temperature low enough for them. 



Food. — The food of the nymphs consists almost entirely of Crus- 

 tacea, the larvae of nematocerous Diptera, such as mosquitoes and 

 chironomids, and ephemerids. Very young nymphs have been known 

 to thrive on Paramecium and other Protozoa. Of a large number of 

 Lestes which were dissected, nearly all contained Daphnia and Cyclops, 

 while the coenagrionines dissected contained many heads of chirono- 

 mids and only occasionally small Crustacea. However, a single small 

 Ischnura verticalis nymph contained eight specimens of Daphnia, and 

 it seems highly probable that other insects are also taken when the 

 normal food supply is scarce. Diatoms and other minute organisms 



