156 



Jewett: Plecoptera 



parts and by the fact that stoneflies always have 

 three tarsal segments and two or three ocelli. In 

 size stoneflies vary in body length from 4-5 mm. 

 (small Capnia and Nemoura) to 40-50 mm. (large 

 Pteronarcys). Most stoneflies are normally winged, 

 but a number are known to be brachypterous; one 

 little-known western species, recorded only from 

 New Mexico, is apparently wingless in the male {Cap- 

 nia fibula Claassen); this is true also of another 

 capniid found in the Midwest, Allocapnia vivipara 

 (Claassen). Filipalpia are primarily diurnal; many 

 Setipalpia are crepuscular or nocturnal and are at- 

 tracted at night to artificial lights. 



Egg. — The eggs of stoneflies (fig. 6:2) are most 

 frequently deposited in flight over water, but some 

 species, notably among the Filipalpia, crawl to the 

 water's edge for egg deposition. Egg laying may 

 occur only once or several times. The total number of 

 eggs deposited is known to exceed a thousand in 

 some species studied. Egg shape usually differs in 

 the two suborders: those of the Filipalpia are spheri- 

 cal in general shape and have a sticky coating when 

 moistened, an adaptation which enables them to 

 adhere to the substrate; those of the Setipalpia are 

 usually longer than wide but variously shaped and 

 sculptured, without an adhesive coating but with an 

 anchor plate. 



Nymph. — The nymphs of European stoneflies and 

 those found in the midwestern United States (fig. 6:3) 

 are rather well known, but those of a great many 

 North American species have not yet been described; 

 this is particularly true of our western species. Many 

 stonefly nymphs occur in waters with a gravel bottom, 

 but some species occur where the substrate is mostly 

 detritus. Most species of Filipalpia feed primarily 

 on plant tissue, but some are omnivorous; most Seti- 

 palpia are carnivorous. Generally, the Filipalpia are 

 found most abundantly in cooler waters, and the 

 Setipalpia, most commonly in warmer waters. Many 

 exceptions occur, and in some tropical regions only 

 Setipalpia are found. For the stoneflies which have 

 been reared from eggs, the number of instars has 

 varied from twenty-two to thirty-three (Claassen, 1931, 

 pp. 7-8). Development of the nymph may occur grad- 

 ually, but for the majority of species there is ap- 

 parently a period shortly after hatching when growth 

 virtually ceases (Brinck, 1949, pp. 131-140). The 

 nymphal stage lasts about a year in most species, 

 but two or three years are required in some. Nymphs 

 may or may not have external gills located on the 

 mentum, submentum, neck, thoracic segments, the 

 first few abdominal segments, or extruded from the 

 anus. Remnants of nymphal gills are present on some 

 adults and are important in classifying the order. 



PRESERVING AND COLLECTING 



Stonefly nymphs and adults should be preserved in 

 70-75 per cent ethyl or isopropyl alcohol and pref- 

 erably placed in this liquid as they are collected or 

 very soon afterward. Stoneflies are soft-bodied in- 

 sects and particularly during warm weather will 



soon dry and shrivel if left exposed to the air after 

 death. A convenient sized vial for temporary preser- 

 vation is one of three-dram capacity measuring 65 

 by 17 mm. Cork or rubber stoppers may be used but 

 the latter give better protection from evaporation. 

 Care should be exercised not to place so many speci- 

 mens in a vial they they will be improperly preserved. 

 Specimens in vials of somewhat smaller diameter 

 with cotton plugs are best stored permanently in 

 large jars filled with 70 per cent alcohol. 



Adult stoneflies may be collected in several ways. 

 From late autumn to early spring, when most Capniids 

 and many Nemourids emerge, concrete bridges over 

 streams are excellent sources of specimens which 

 can be easily collected with a pair of forceps. During 

 the rest of the year the most productive method of 

 collecting is by sweeping vegetation along streams 

 with an insect net. The foliage of conifers is a favor- 

 ite resting place for stoneflies where such trees 

 border streams. Nocturnal species may be taken at 

 artificial lights, from under large stones bordering 

 streams, and particularly from under loose bark of 

 logs which extend into the water. Cast nymphal 

 skins found on rocks, tree trunks, and so on near the 

 water should be preserved. 



Nymphs are easily collected by overturning rocks 

 and stirring gravel in stream beds upstream from the 



antenna 



cpicrama 



epicranial 



stem 



-narqinal 



qroove 

 marctina' 



(lanqe 



thoracic qi' 



anal q 



tarsus 



Fig. 6:3. Dorsal view of a constructed stonefly 

 nymph (Frison, 1935). 



