1 88 BULLETIN THE BUREAU OF FISHERIES. 



Plant Distribution in Ponds op Skries D, Fairport, Iowa, 1916. 



[a= abundant, c= common, s= scarce.] 



Species. 



Pond numbers. 



I 





3 



4 



5 



6 



7 



8 



9 



IN THE WATER. 



Potamogetonillinoiensis: Illinois pond weed 





a 



a 



a 















Potamogeton pusillus: Small pondweed 







c 











Potamogeton pectinatus: Fennel-leaved pondweed 





a 

 a 

 c 

 c 

 c 

 a 

 c 

 s 

 c 

 c 



a 





s 





a 





Trypha latifolia: Cattail 



a 



s 



a 

 c 







s 

 c 



s 

 c 



Ceratophyllum demersum: Hornwort 



s 



s 





s 

 c 









c 

 a 











c 

 a 





Elodea canadensis: Water weed 



a 



s 







c 

 s 

 s 









s 

 s 







s 



s 



s 



s 



s 









Blanket algae 



a 



c 

 s 



a 

 a 



a 

 a 



a 



a 



c 



a 

 a 



c 



a 



s 



a 

 a 



a 



a 

 c 

 s 



ALONG THE MARGINS. 





Eleocharis palustris: Creeping spike rush 







c 

 s 



c 







c 



c 















c 



s 

 s 















s 









s 

 a 



Echinocloa crussgalli: Cockspur grass 





s 





a 







ON THE EMBANKMENTS. 



Convolvulus sepium: Bindweed 













c 

 c 





a 

 c 

 c 



a 



s 



s 

 a 

 a 





c 

 a 







s 

 a 



Lactuca scariloa: Prickly lettuce 





a 









c 

 a 







Trifolium pratense: Red clover .- 



Plantagorugelii: Common plantain 



a 



c 

 c 



c 



c 



c 

 c 



s 

 s 



s 

 s 



ABUNDANCE OF DRAGONFLIES AND DAMSELFLIES. 



The actual number of both dragonflies and damselflies varies greatly from time to 

 time, due to a variety of causes. 



1. Periodicity. — Th.e emergence of the imago is periodic in occurrence; most of 

 the Libellulidae have a one-year cycle, and the great majority of any given species 

 emerge at or near the same time. Just after this period of maximum emergence their 

 numbers reach the highest point for the season, and then gradually decrease. Some 

 species of Anax, Tramea, and ^schna may have two broods during the year, in spring 

 or early summer, and again in late summer or early fall, and consequently would have 

 two periods of maximum abundance. Even the Gomphidse, whose nymphs require more 

 than a year in which to mature, show similar periods of maximum emergence, large 

 numbers being transformed within a few days and then diminishing rapidly in abundance. 

 On the other hand, the damselflies apparently have several broods during the season, 

 and their numbers rise and fall accordingly. 



2. Migration. — The imagos of many species have the habit of scattering rapidly 

 soon after their emergence, even while they are still teneral, and they may sometimes 

 entirely disappear. This is true of Epicordulia princeps, whose fresh nymph skins were 

 second in abundance during the last of June, and yet not a single imago could be seen 

 about the ponds at that time. The Gomphidse furnish numerous similar examples; the 

 most abundant of their nymph skins was that of Gomphus plagiatus, over a thousand of 

 which were secured along the bank of the Mississippi opposite the ponds, but not a single 



