192 



BULIvETiN OF^ THE BUREAU OF FISHERIES. 



in some similar body of water. The Anax imagos are present in' considerable numbers 

 around the ponds by the first of August, and may be seen mating and ovipositing. The 

 iEschna imagos delay much longer, and none have thus far been seen depositing eggs 

 before the second week in September. In Massachusetts the same species, constricta, 

 may be seen depositing its eggs as late as October. 



Sympetrum corruptum appeared late in June and lasted for about three weeks and 

 then entirely disappeared, its place being taken by S. rubicundulum, which remained 

 the rest of the season. 



Perithemis domitia was never present in sufficient numbers to really enter into the 

 reckoning. 



Plathemis lydia and Libellula pulchella were much more abundant in 191 7 than 

 in 1 91 6, and both took an active part in the odonate life of the ponds. Previously 

 they had remained quite constantly along the ditches beside the railroad tracks, but 

 finally deserted them and assumed their appropriate places around the ponds. 



LIFE HISTORY OF AN ODONATE. 



In order to properly appreciate the relations between fish and dragonflies and 

 damselflies it is well to consider briefly the life history of these insects. 



Eggs. — ^The eggs are laid in the water and hatch into larvae called nymphs. The 

 period of incubation varies greatly in different species; perhaps the average for dragon- 

 flies is from 5 to 10 days, and for damselflies about 20 days. Eggs laid by a Pachydiplax 

 female and kept in the laboratory hatched in 5 days; Warren (191 5, p. 8) also found 

 the period of incubation in Pantala flavescens to be 5 days for two females and one 

 male and 7 days for another male. The dragonfly's egg is ellipsoidal, narrowed a little 

 at either end, and surrounded by a gelatinous envelope (fig. i). There is a small pro- 

 jection or knob at the anterior end of the egg, which is known as the pedicel. It is 

 formed of a thickening of the egg shell or chorion and furnishes the means by which 

 the egg is attached to the egg string inside the ovary of the female. 



The eggs of damselflies and of Anax, ^Eschna, and their relatives among the 

 dragonflies are considerably elongated and assum.e a cylindrical form (fig. 58). The 

 anterior end is pointed, with a short and wide pedicel, while the posterior end is bluntly 

 rounded. 



Nymph. — ^At the end of the third day the larval pronymph could be seen inside 

 the Pachydiplax egg and appeared as shown in figures 2 and 3, the long concentric 

 lines being the folded legs. On emerging from the shell the pronymph is closely covered 

 by a chitin sheath which holds the legs tightly to the body. It quickly molted out of 

 this sheath and took the form shown in figure 5, but one was pulled out of the sheath 

 before it had time to molt, and this one looked like figure 4. The body was elongate 

 and fully segmented and the legs were more or less twisted from their previous folding. 

 The pronymphal stage lasts usually but a very short time, less than a minute, sometimes 

 only two or three seconds. The nymph, on the other hand, continues until it is ready 

 to be transformed into the imago or perfect insect. Most nymphs require a year in 

 which to fully mature; a few, like Gomphus, require more than a year, while others, 

 like the damselflies Enallagma and Ischnura, may produce more than one brood in a 

 season. 



