188 BULLETIN OF THE BUREAU OF FISHERIES. 
PLANT DISTRIBUTION IN PONDS OF SERIES D, Farrport, Iowa, 1916. - 
[a=abundant, c=common, s=scarce.] 
Pond numbers. 
Species. 
I 2 3 
IN THE WATER. 
Potamogeton illinolensis: Illinois pondweed ...... ..........06.2000.)o0000- a a 
Potamogeton pusillus: Small pondweed........... 00. .0.0cecceeee eee sfeeeeee a |ieeens[acesen 
Potamogeton pectinatus: Fennel-leaved pondweed..................[...000feeeees a 
Trypha latifolia: Cattail. 0.0.0.0. 00.00 ees Sale a a 
Ceratophyllum demersum: Hornwort..... s c c 
Ranunculus aquatilis: Common water crow Same] Odeo ¢ 
Oedogonium, sp.: Filamentous alga....... CET Te Reeser c 
Elodea canadensis: Water weed.......... wha a a 
Vallisneria spiralis: Eelgrass............ BARA Soop Aaah Ser Plael mrapa ea MRtome: c 
Naias flexilis:\Slendét fatas.. FAST ls. eo aetna dele s s s 
Castalia odorata’ White pond lily. oc oo Ve wot en phys sien dd Bea itlacaaed -asses re 
Blankatalgaty i iii\aOs..\. 28 RE Ae AA AW Ee a a c 
ALONG THE MARGINS. 
GoirexiStictatiCrextprasst/P7-k6 . ai. 2. NE eRe te raat» a a a 
Rinnexicraspiyss Citled Sorrel cnc saecn. op stein ascin seins cg ae pas ie OR all eae ere en 
Eleocharis palustris: Creeping spike rush + iain (Doc dobae ae 
Sagittaria latifolia: Broad-leaved arrowh 
Homalocenchrus oryzoides: Rice cut-gras: 
Juncus effusus: Common rush.......... 
Echinocloa crussgalli: Cockspur grass... 
ON THE EMBANEMENTS. 
Convolvulus sepium: Bindweed...... 20.0.6 0 ccc cece cece cence eee 
Lactuca scariloa: Prickly lettuce........ 
Equisetum fluviatile: Swamp horsetail. 
Trifolium pratense: Red clover......... 
Plantago rugelii: Common plantain...........5.. 05.000 e cece eee eee 
Ppnaarnn 
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. PERIopICITy.—The emergence of the imago is periodic in occurrence; most of 
the Libellulide 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 A!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 Gomphide, 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 Gomphide 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 
