I90 BUI.I.ETIN Olf run BUREAU OP FISHERIES. 



Nymph-Skin Counts, Ponds i to 4 D, Fairport, Iowa, 1916. 



Nvmoh ki obtai ed 



July 3. 



July 16. 



July 31. 



Aug. 15. 





No. 



P. ct. 



No. 



P. c/. 



No. 

 30 



?2 



P. cf. 



No. 

 20 



P. ct. 



4 









10 

 I 













6 





2 



15 



10 



3 

 2 













4 







2 













40 



4 



8-5 









3 

 I 





4 

 2 















16 





8 







S 

 2 





3 

 I 







2-5 













10 



2 



3 



12 



2 



25 

 26 



5-5 





3 





2 















6 





SI 

 246 



20 

 i6c 

 8 



8 



20 

 137 



4 



29-5 





442 



89 



310 

 6 



66' 



40 









2 







24 



3 



26 



4 



112 



24 





S 





114 



I 





















Certain additional facts were noted during the gathering of the nymph skins. 



I. Choice of Locality. — Upon reaching maturity the nymph does not crawl out 

 of the water blindly wherever he may happen to be, but shows a definite preference for 

 certain localities. Most of the nymphs here recorded transform in the early morning, at 

 which time the west and north margins of the ponds receive the early sunshine, while 

 the east and south margins are in the shade. It has been stated that the counts were 

 made on the north shores ; this was because those shores were found by actual trial to 

 yield many more nymph skins than the east or south shores, and considerably more 

 than the west shore. Another important reason is that the north shores border the 

 shallower water of the ponds, and are thus naturally frequented by the nymphs when 

 nearly ready for transformation. 



A third factor which may influence the nymph in its choice of a locality for transfor- 

 mation is the kind of support obtainable. Some nymphs, such as those of Libellula- 

 luctuosa, Anax, and Epicordulia crawl up on anything that may be convenient, including 

 wire screening, old boards, fence posts, and the like. Others show a decided preference 

 for certain kinds of support and will even choose between different water plants. 

 Erythemis simplicicollis, for example, selects the arrowleaf, Sagittaria latifolia, in 

 preference to the cat-tail, Trypha latifolia, when the two are equally available. And 

 this same species was the only one found in any abundance upon the stems of the crex 

 grass, Carex stricta. 



Erythemis transforms later in the day than many of the other species, and its 

 exuviae were found in large numbers along the eastern shores of the ponds. In five of 

 the ponds these shores contain both cat-tails and arrowleaf in addition to the crex grass, 

 but almost without exception the Erythemis nymphs had chosen the latter. 



On the other hand, Anax usually emerges during the night, and its exmdae were 

 found upon the western and northern shores, and more of them upon the cat-tails than 

 upon all other kinds of support combined. The large, sprawling nymphs of Tramea 

 lacerata also take very kindly to the cat-tails, but shun the crex grass altogether. 



Among the damselflies the Enallagmas are always found upon some convenient stem, 

 a short distance above the water. Often there will be several exuviae upon the same 

 stem, and in one instance the number reached 21, as recorded upon page 230. 



On the other hand, the nymphs of Ischnura very frequently crawl out upon the 

 top of the lily pads and perch for transformation on the margin of the leaf, where it has 



