DIEL EMERGENCE PATTERNS OF

TRICORYTHODES STYGIATUS

(Ephemeroptera: Leptohyphidae)

ON THE LITTLE LEHIGH CREEK

NEAR ALLENTOWN, PENNSYLVANIA1

Fran Gough, Bruce L. Haase2

ABSTRACT: Tricorythodes stygiatus subimagos and adults show a diel periodicity in drift rates. Most of their activity is centered around the hours of sunrise and sunset. There is a differential emergence of male and female subimagos, males emerge after sunset and females emerge after sunrise. Light periodicity is the primary influence in the timing of T. stygiatus emergence, molting, and oviposition. This species is univoltine at this study site.

      

      Tricorythodes stygiatus McDunnough (Ephemeroptera: Leptohyphidae) is a common species in the limestone streams of southeastern Pennsylvania. The species emerges throughout the summer. Tricorythodes species have been described as univoltine (Newell and Minshall 1978), bivoltine (Hall 1975), or multivoltine (Newell and Minshall 1978) depending on geographic location and water temperatures.

      Tricorythodes species are atypical of most mayflies in that the subimago emerges from the larval shuck underwater and swims/floats/crawls to the surface; with males emerging after dark and females emerging early the next morning (Hall 1975; Edmunds and McCafferty 1988).

      In Hall’s (1975) study of T. allectus Needham, the surface drifting of male subimagos took place from sunset until the early hours of the morning, while female subimagos showed a peak in surface drift rate around sunrise. The emergence of male subimagos of T. allectus was correlated with low light intensity in the evening, while female subimago emergence is correlated with increasing light intensity in the morning (Hall 1975). However, in a study by Newell and Minshall (1978) T. minutus always emerged at the water surface, usually in the afternoon. T. minutus seems to be little influenced by photoperiod (Newell and Minshall 1978).

      T. allectus male subimagos molt to adults before dawn, so the length of the male subimago stage is 5-7 hours. The female’s subimago stage is very short. Some female subimagos molt immediately after emergence, others may wait up to 2 hours (Hall 1975). The male adult life of T. allectus lasts about 9-10 hours, beginning after dusk until the sun is well above the horizon. The female's adult stage spans a few short hours after dawn (Hall, Berner and Cook 1975). In T. minutus, the subimago stage usually lasts less than 30 minutes and the adult forms live less than 6 hours (Newell and Minshall 1978).

      The purpose of this study is to determine if there is a differential emergence of male and female subimagos of T. stygiatus in the Little Lehigh Creek in Allentown, PA. Then to determine if this differential emergence can be correlated with water temperature or time of day.

METHODS

      This study took place on the Little Lehigh Creek, a 4th order stream in Salisbury Township, Lehigh County, Pennsylvania. The study site was at the end of a riffle in the main channel of the stream. It is a limestone spring creek with a typical riffle-pool complex, with substrate composed mainly of marl and silt.

      Sampling was done using a Munro style drift net with a 250 micron mesh size and a net opening measuring 39cm x 39cm and a length of approximately one meter. A removable (250 cm) catch basket, composed of 3 inch PVC pipe and mesh screen was attached to the end of the drift net. The frame, made of 0.5 inch PVC pipe, was slid over steel reinforcing rods that were hammered into the stream bottom. When in place, the Munro drift net extended from the stream bottom through the water's surface (anywhere from 5 cm to 15 cm above the surface depending on water levels).

      The net was placed in the selected riffle of the study area for 24 hours at a time. Samples were removed and the net replaced every hour during the study period. At this time, the stream temperature was also taken and recorded. These 24 hour study periods were conducted three times, about one month apart, on July 13, August 11, and September 8 of 1995.

      The samples were rinsed from the net into a pan, concentrated with an aquarium net and preserved in 70% ethanol. Using a 20x Swift dissecting microscope, these insects were counted, aged (as subimagos or adults) and the subimagos sexed. To determine the life stage of the adult T. stygiatus, the methods described by Edmunds and McCafferty (1988) were used. Subimagos had translucent wings with cilia along the hind edge. Adults had transparent wings without cilia on the hind edge of the wing. Subimagos and adults were sexed as males if claspers were present at the end of the abdomen, and as females if claspers were absent. Voucher specimens are deposited at the Department of Biological Sciences, East Stroudsburg University, East Stroudsburg, Pa..

RESULTS

      The largest numbers of adult T. stygiatus were caught in the July 1995 sample with 891 organisms collected, including 73 subimagos and 818 adults. During the July 13th sample period, the water temperatures ranged from 16° C. in the early morning hours to 21° C. in the late afternoon. Sunrise was at 0542 and sunset was 2033 so the day length was 14 hours 51 minutes. The peak drift time for male subimagos occurred between 2200 and 2400, for female subimagos this peak occurred between the hours of 0600 and 0900 (Table 1, Figure 1). Adults showed a peak drift rate between the hours of 0800 and 1100 (Table 1, Figure 2).

      Fifty- four adult T. stygiatus were collected in the August drift samples, including 8 subimagos and 46 adults. During the August 11th sample period, the water temperatures ranged from 15° C. in the early morning hours to 19° C. in the late afternoon. Sunrise was at 0608 and sunset was 2006, so the day length was 13 hours and 58 minutes. The peak drift time for male subimagos occurred between 2100 and 2300, for female subimagos this peak occurred between 0900 and 1000 (Table 1, Figure 1). Adults showed a peak drift rate between the hours of 0900 and 1000 (Table 1, Figure 2).

      Fifty- five adult T. stygiatus were collected in the September drift samples, including 4 subimagos and 51 adults. During the September 8th sample period, the water temperatures ranged between 15° C. and 16° C. Sunrise was at 0636 and sunset was 1921, so the day length was 12 hours 45 minutes. The peaks in drift time for male subimagos were at 1800 and 2100, for female subimagos these peaks were at 0700 to 0900 (Table 1, Figure 1). Adults showed a peak in drift rate from 0700 to 1100 (Table 1, Figure 2).

DISCUSSION

      Adult forms of T. stygiatus adults on the Little Lehigh show a diel periodicity in their drift rates. Water temperature did not appear to trigger the differential emergence of T. stygiatus in this study. The temperature of the stream did not fluctuate much (± 2° C.) on a 24 hour basis, or on a seasonal basis (15°-21° C.) throughout the study period. Instead, the peak for subimago and adult drift revolves around the hours of sunrise and sunset. This can be seen in the data as the season progresses and the day length shortens. It is apparent that light periodicity is the primary influence in the timing of the T. stygiatus adult life activities of emergence, molting, mating and oviposition. This agrees with the findings of Hall (1975) and Friesen, Flannagan and Laufersweiler (1980) (T. allectus.

      There was differential emergence of male and female subimagos of T. stygiatus in the Little Lehigh. This is similar to what Hall (1975), Friesen, Flannagan and Laufersweiler (1980) and Edmunds and McCafferty (1988) have found in other parts of North America for other species of Tricorythodes. In this study 83% (30 of 36) of the male subimagos emerged in the first 3 hours after sunset, and 93% (43 of 49) of the female subimagos emerged in the first 3 hours after sunrise. About 98% (894 of 915) of the spent adults were collected from 1 to 5 hours after sunrise, so mating and oviposition had occurred prior to this event. Mating and oviposition occur for about 3-4 hours after sunrise based on the first/last appearance of spent wing adults in the drift samples. This is a somewhat longer period than was observed by Hall, Berner and Cook (1975) in T. allectus. These findings were similar to R. J. Hall's (1975) study of T. allectus except that male subimago emergence did not occur steadily through the night, while the peak emergence of female subimagos occurred after sunrise.

      The short life span of Tricorythodes species has been well documented (Hall 1975; Hall, Berner and Cook 1975; Edmunds and McCafferty 1988). On the Little Lehigh, adult males live 8-14 hours and adult females live 2-5 hours on average, based on the time from first/last appearance of subimagos and the first/last appearance of adults.

      It appears that the T. stygiatus on the Little Lehigh Creek are univoltine based on this study. This would agree with Newell and Minshall's (1978) study on T. minutus. The drift rate of adult T. stygiatus was many times greater in July than in August or September. In this study, 89% (891 of 996) of the total drift is accounted for in the July sample alone.

      In a number of the T. stygiatus larvae, subimagos and adults, a distinct red coloration on the thorax was noted. In the 0600 hour sample on July 13, 1995, 70% of the female subimagos had a red thorax. The cause of this red coloration is another area for further investigation. A number of hypotheses have been offered from 1) something in the water chemistry causing it (Gary Borger personal communication 1995), 2) parasitic mites (Greg Hoover personal communication 1996, Brittain 1982), to 3) a type of polymorphism (Peters & Peters 1977).

ACKNOWLEDGMENTS

      We would like to acknowledge the following people who helped make this project a success: for much needed assistance from Rod Rohrbach, Rich Heiserman, Steve Hoovler, Joe Kohler, Gary Pyle and other members of the Little Lehigh Fly Fishers and Little Lehigh Trout Unlimited; to Andy Shiels of the PA Fish and Boat Commission and Don Marushak of the Allentown Parks Dept., to Jim Munro for his drift nets; to Mark Gerber for his word processing skills; and the primary author gives special thanks to his wife, Maura, for her endless patience and understanding. The identification of this species as T. stygiatus was verified by Don Baylor of Aquatic Resource Consulting of Saylorsburg, PA and by Greg Hoover, Penn State University Department of Entomology. The authors would like to thank R.D. Waltz and W.P. McCafferty for their suggestions in reviewing this article.

 

LITERATURE CITED

Brittain, J. E. 1982. Biology of mayflies. Ann. Rev. Entomol. 27:119-147.

Edmunds, G. F and W. P. McCafferty 1988. The mayfly subimago. Ann. Rev. Entomol. 33:509-529.

Friesen, M. K., J. F. Flannagan and P. M. Laufersweiler. 1980. Diel emergence patterns of some mayflies (Ephemeroptera) of the Roseau River (Manitoba, Canada). Advances in Ephemeropteran Biology. Plenum Publishing Co. p. 287-296.

Hall, R. J. 1975. Life history, drift and production rate of the stream mayfly, Tricorythodes atratus McDunnough in the headwaters of the Mississippi River. Ph.D. Thesis, Univ. of Minnesota. 288 pp. (T. allectus as T. atratus)

Hall, R. J., L. Berner and E. F. Cook. 1975. Observations on the biology of Tricorythodes atratus McDunnough. Proc. Entomol .Soc. Wash. 77 (1): 34-49.

Hall, R. J., T. F. Waters and E. F. Cook E. F. 1980. The role of drift dispersal in production ecology of a stream mayfly. Ecology 61 (1): 37-43.

Newell, R. L., and G. W. Minshall. 1978. Life history of a multivoltine mayfly, Tricorythodes minutus: an example of the effect of temperature on the life cycle. Ann. Entomol. Soc. Am. 71: 876-881.

Peters, W. L., and J. G. Peters. 1977. Adult life and emergence of Dolania americana in northwestern Florida. Int. Revue ges Hydrobiol. 62(3): 409-438.

 

FOOTNOTES

1. Received June 9, 1997. Accepted

1. Department of Biological Sciences, East Stroudsburg University, East Stroudsburg, Pa. 18301.