192 BULLETIN 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 
#ischna 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 1917 than 
in 1916, 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. 
Eccs.—The eggs are laid in the water and hatch into larve 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 (1915, 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. 1). 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, A’schna, and their relatives among the 
dragonflies are considerably elongated and assume a cylindrical form (fig. 58). The 
anterior end is pointed, with a short and wide pedicel, while the posterior end is bluntly 
rounded. 
Nympu.—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. 
