. Bulletin. Natural history; Natural history. 302 ILLINOIS NATURAL HISTORY SURVEY BULLETIN XX:IV EMERGENCE.—It has been my observation that diurnal species of stoneflies emerge mostlj' during early morning and that nocturnal species emerge late in the evening or at night. I know of no instance in which the adults pop out of the water free of the nymphal skin as in some aquatic groups, or have another molt after leaving the water and flying as in the case of the Ephemerida. In all instances the nymphs crawl out of the water on rocks, vegetation or any object protruding out of the. Fig. 25.—Smal
. Bulletin. Natural history; Natural history. 302 ILLINOIS NATURAL HISTORY SURVEY BULLETIN XX:IV EMERGENCE.—It has been my observation that diurnal species of stoneflies emerge mostlj' during early morning and that nocturnal species emerge late in the evening or at night. I know of no instance in which the adults pop out of the water free of the nymphal skin as in some aquatic groups, or have another molt after leaving the water and flying as in the case of the Ephemerida. In all instances the nymphs crawl out of the water on rocks, vegetation or any object protruding out of the. Fig. 25.—Small river habitat of Illinois stoneflies. Various species of the winter group develop in such streams and on sunny days crawl up and congregate on bridges. water. There, after a short rest, the nymphal skin splits down the back, as has been described by the writer (1929), and the soft adult crawls out. After a short time the wings and body structures harden and the adult crawls or flies away. DEVELOPMENT.—Our method of rearing adults from nymphs was aimed at securing accurate association of adults with nymphs, with a minimum of time and efiort, and scant attention was paid to the number of developmental instars. In fact, except for the observations of Samal (1923), Schoenemund (1912) and Wu (1923), very few reliable data have been gathered concerning the number of instars in stonefly nymphs. These students indicate that there are many instars, Wu giving 22 for Neiiioura vallicularia, Schoenemund 36 for Perla cephnlotes and Samal 22 instars for Perln abdominalis. The size classes of nymphs and seasonal occurrences offer some clue, however, to the total time of the life cycle. In Illinois it appears that. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Illinois. Natural History Survey Division. Urbana, Sta
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