Data collected from an ecological study of the mayfly Potamanthus rnyops (Walsh) in Michigan showed intraspecific variability in taxonomic characteristics that have been employed by previous investigators for species separation. Nymphal dorsal maculation patterns varied considerably within a single population. Also, the ratio of mandibular tusk length to head length increased with successive nymphal instars. Certain adult taxonomic characteristics, particularly relative male imago eye size and distance of separation, were either too poorly defined or too variable to be conclusive in species identification

Lord, Robert J

Meier, Peter G

English

Valparaiso University

The Great Lakes Entomologist Volume 10 Number 2 - Summer 1977 Number 2 - Summer 1977 Article 3 June 1977 Intraspecific Variation in Taxonomic Characteristics of the Mayfly Potamanthus Myops (Walsh) Robert J. Lord Department of Natural Resources Peter G. Meier University of Michigan Follow this and additional works at: https://scholar.valpo.edu/tgle  Part of the Entomology Commons Recommended Citation Lord, Robert J. and Meier, Peter G. 1977. "Intraspecific Variation in Taxonomic Characteristics of the Mayfly Potamanthus Myops (Walsh)," The Great Lakes Entomologist, vol 10 (2) Available at: https://scholar.valpo.edu/tgle/vol10/iss2/3 This Peer-Review Article is brought to you for free and open access by the Department of Biology at ValpoScholar. It has been accepted for inclusion in The Great Lakes Entomologist by an authorized administrator of ValpoScholar. For more information, please contact a ValpoScholar staff member at scholar@valpo.edu. 1977 THE GREAT LAKES ENTOMOLOGIST 5 1 INTRASPECIFIC VARIATION I N  TAXONOMIC CHARACTERISTICS OF THE MAYFLY POTAMANTHUS MYOPS (WALSH) Robert J.   or dl and Peter G. ~ e i e r '  ABSTRACT Data collected from an ecological study of the mayfly Potamanthus rnyops (Walsh) in Michigan showed intraspecific variability in taxonomic characteristics that have been employed by previous investigators for species separation. Nymphal dorsal maculation patterns varied considerably within a single population. Also, the ratio of mandibular tusk length to head length increased with successive nymphal instars. Certain adult taxonomic characteristics, particularly relative male imago eye size and distance of separation, were either too poorly defined or too variable to be conclusive in species identification. Mayflies of the genus Potamanthus occur locally in scattered but concentrated populations in several larger rivers of southeastern Michigan. As a part of a larger study on the ecology, taxonomy, and life history of this group (Lord, 1975), the feasibility of developing a key to species based on nymphal characteristics was examined. There is very little information available concerning the taxonomic characteristics of Potamanthus nymphs, except for the recent publication by McCafferty (1975). Morgan (1913) was one of the first authors to mention that size and relative length of mandibular tusks may be a species distinguishing characteristic, while Ide (1935) attempted to differentiate nymphs on the basis of dorsal head maculation (Figure 1). Current Potamanthzis taxonomy, however, is based exclusively on adult characteristics. According to  Walsh (1863), male imagos could be classified to  species by employing eye size and the respective distance between them measured in terms of eye diameters. On this basis he described Potamanthus (originally Ephemera) myops and flaveola. Subse- quently, this characteristic has been widely used by many authors in descriptions of new specles (i.e. Argo, 1927; Ide, 1935; Needham, et  al., 1935). However, examination of these groupings by eye size and eye separation distance shows many discrepancies, in particular within the species Potal?mnthus verticis. This species was originally described by Say (1839), as possessing large eyes separated by a distance less than or equal to  one eye diameter (male imago). McDunnough (1926) suggested that verticis and flaveola were synonyms because they shared this characteristic of eye sue. However, Argo (1927) put verticis into the Potamnthus group having small eyes separated by a distance of two or more eye diameters and retained flaveola in the large eyed group. Needham, et al. (1935) returned verticis to the large eyed group, but later, in their description of the new species P. neglectus, a species having small, widely separated eyes, they stated that neglectus may be a synonym of verticis! Burks (1953) is the only author to illustrate reIative eye sues (Figure 2) and includes P. distinctus in the "intermediate" eye size group. Male imago wing Iength, darkening of cross veins in both sexes, an& the presence of pale lateral abdominal markings were other adult taxonomic characteristics utilized for species separation by past researchers. Both Needham, et aL (1935) and Edmunds and Allen (1957) recognize eight species of Potamnthus in North America, n-hile Burks (1953) describes only four. A summary of adult taxonomic characteristics for these eight species by Edmunds and Allen (1957) is presented in Table 1. 'Department of Natural Resources, Brunswick, Georgia. 2~epa r tmen t  of Environmental and Industrial Health, The University of Michigan, Ann Arbor. 3Iichigan 481 09. 1Lord and Meier: Intraspecific Variation in Taxonomic Characteristics of the MayflP blished by ValpoScholar, 19775 2 THE GREAT LAKES ENTOMOLOGIST Vol. 10, No. 2 Fig. 1. Dorsal maculation of Potamanthus nymphs (Ide, 1935). A)  rufous; B) walkeri; C) flaveola; and D)-earIy instar o f  flaveola. Fig. 2. Eye sizes of Potamanthus male imagos (Burks, 1953). A) large eyes-verticis; B) smalI eyes-myops; C) medium eyes-distinctus. METHODS The area of study was Michigan within a 50 mile radius of Detroit. Live material was collected and maintained from the Huron and Black Rivers. Nymphs were collected from May through'July, 1975, and adults from June through August, 1975. Some nymphs from each sampling area were reared in aquaria for association of nymphs to adult. Preserved material collected in 1970 by the author was examined from the Saline and Raisin Rivers. 2The Great Lakes Entomologist, Vol. 10, No. 2 [1977], Art. 3https://scholar.valpo.edu/tgle/vol10/iss2/31977 THE GREAT LAKES ENTOMOLOGIST 5 3 Table 1. Adult Potamanthus species characteristics (after Needham, Traver, Hsu, 1935) wing length male eye size male cross-veins male female Species abdominal markings none 1. stripes distribution diaphanus distinctus lg med inequalis 17zyops (=m edius) neglectus rufous verticis (=flaveola) none midwest MD NY PA NY none* 1. spots 1. spots 1. spots midwest & northeast Ontario walkeri none *very pale spots in freshly killed specimens lg = large; med = medium; sm = small; hy = hyaline; dk = darkened; 1. = lateral RESULTS Although only one species of Potamanthus was collected in the course of the study, considerable variation of taxonomic characteristics in both nymphs and adults was observed. In the nymph, mandibular tusk length was found to increase in proportion to head length with later instars. This ratio, of head length to tusk length, changed from 1 to .63 in an early instar with 6 mm body length (excluding cerci), to 1 to 1.4 in the last instar with a 15 mm body length. Since the nymphs of several instars are present at the same time, this progressive development of tusk length with age, as illustrated in Figure 3, precludes the use of this characteristic as suggested by Morgan (1913) and others, in differentiating Potamanthus species at the immature level. However, McCafferty (1975) feels that the ground color of the naiad and the shape of the tusks may suffice in the separation of these species. Dorsal head maculation was also found to be variable within a single population of nymphs of approximately the same size. Figure 4 presents some examples of head maculation differences, while Figure 5 illustrates the general maculation pattern in the last instar of Potamanthus myops.  Several of the generally accepted adult taxonomic characteristics also showed a degree of variation. In this Potamanthus species wing cross veins were found to be, without exception, hyaline in both sexes. Faint lateral abdominal markings were observed in some individuals of a given population and not in others. However, when present, these lateral abdominal markings were most prominent in the subimago stage and virtually disappeared in the adult form. Preservation of subimagos in 7@% ethanol removed all traces of lateral abdominal markings as indicated by Burks. Since identification is carried out mostly from preserved specimens, the usefulness of lateral abdominal markings for differentiating Potamanthus imagos is questionable. Wing lengths of male imagos showed a range of 9.46 mm to 12.54 mm with the average being 11.15 mm. This variability of wing length within one population of this local species overlaps nearly all other species presented in Table 1. Probably the most consistent taxonomic characteristic found for these populations of Potamanthus myops was that of eye size and respective distance of separation. This morphological feature (Figure 6), which compares most closely with Burk's illustration of distinctus (Figure 21, 3Lord and Meier: Intraspecific Variation in Taxonomic Characteristics of the MayflPublished by ValpoScholar, 1977THE GREAT LAKES ENTOMOLOGIST Vol. 10. No. 2 Fig. 3. Progressive increase in tusk length relative t o  head length of  P. myops. Body lengths (excluding cerci), A )  6.0 m m ;  B) 7.5 nini; C) 9.0 mm: D) 11.0 m m ;  E) 13.0 mm:  F) 1 5  mm. 4The Great Lakes Entomologist, Vol. 10, No. 2 [1977], Art. 3https://scholar.valpo.edu/tgle/vol10/iss2/3THE GREAT LAKES ENTOMOLOGIST Fig. 4.  Variation in dorsal head maculation of P. ntyops nymphs of a single population (scale 1 :40). would appear to place this species in the intermediate eye size group. However, measurements show that the eyes are separated b y  slightly more than two eye diameters fitting the definition of the small eyed Pofatnanfhus group. Thus, even here some confusion exists as to the exact definition and usefulness of eye size and separation distance in differentiating Potamanthus species. The populations of Potamanthus collected locally consisted of a single species, myops. This identification was based upon those characteristics described in Table 1 and the midwestern distribution of this species. CONCLUSlONS The nymphal characteristics of dorsal head maculation and mandibular tusk length as employed by Ide (1935) and Morgan (1913) for species separation were found to be too variable even within a single population of Potamanthus myops to be considered of taxonomic value. Male imago wing lengths displayed a range of variation over-lapping that 5Lord and Meier: Intraspecific Variation in Taxonomic Characteristics of the MayflPublished by ValpoScholar, 1977THE GREAT LAKES ENTOMOLOGIST Vol. 10. No. 2 Fig. 5 .  P. myops, dorsal view of last nymphal stage (scale 1 : l O ) .  6The Great Lakes Entomologist, Vol. 10, No. 2 [1977], Art. 3https://scholar.valpo.edu/tgle/vol10/iss2/3THE GREAT LAKES ENTOMOLOGIST Fig. 6. P, myops, Dorsal view of imago heads (scale 1:40). A) and B) male fore and middle tarsi. C )  and D) female fore and middle tarsi (scale 1:20). E) male genitalia, ventral aspect (scale 1: 50). 7Lord and Meier: Intraspecific Variation in Taxonomic Characteristics of the MayflPublished by ValpoScholar, 19775 8 THE GREAT LAKES ENTOMOLOGIST Vol. 10, No. 2 of other species. Other characteristics of taxonomic value such as lateral abdominal markings occurred too infrequently and disappeared upon preservation in 70% ethanol. The primary taxonomic characteristics of eye size and distance of separation was determined to be poorly defined in past literature and probably was ~nisused by previous authors. Thus the mayfly genus Potamanthus may be in need of re-examination and revision due to  wide variability and inconsistant use of species distinguishing taxonomic characteristics by past authors. LITERATURE CITED Argo, V. M. 1927. The North American species of the genus Potamanthus with a description of a new species. J. N.Y. Entomol. Soc. 35:319-325. Burks, B. D. 1953. The mayflies (or Ephemeroptera) of Illinois. Bull. Ill. Nat. Hist. Surv. 26. Edmunds, E. F. and R. K. Allen 1957. A checklist of the Ephemeroptera of North America, north of Mexico. Ann. Entomol. Soc. Amer. 50:317-24. Ide, F. P. 1935. Life history notes on Ephoron, Potamanthus, Leptophlebia, and B ~ Q S ~ U ~ U S  with descriptions. Can. EntomoI. 67. Lord, R. J. 1975. Potamanthus: ecology, taxonomy and life history. M.S. Thesis, Water Res. Prog. Univ. of Mich. McCafferty, W. P. 1975. The burrowing mayflies (Ephemeroptera: Ephemeroidea) of the United States. Trans. Amer. Entomol. Soc. 101:447-504. McDunnough, J. 1926. Notes on North American Ephemeroptera with description of a new species. Can. Entomol. 58. Morgan, A. H. 1913. A contribution to the biology of mayflies. Ann. Entomol. Soc. Amer. 6:371-413. Needham, J. G., J. R. Traver and Y. Hsu 1935. The biology of mayflies. Comstock Pub., N.Y. Say, T. 1839. Descriptions of new North American neuropterous insects, and observations on some already described. Acad. Nat. Sci. Phil. 8:9-46. Walsh, B. D. 1863. Observations on certain N.A. Neuroptera, by H. Hagen, M.D. of Koeningsberg, Prussia; translated from the original French M.S., and published by permission of the author, with notes and descriptions of about 20 new N.A. species of Pseudoneuroptera. Proc. Entomol. Soc. Phila. 2: 167-272. 8The Great Lakes Entomologist, Vol. 10, No. 2 [1977], Art. 3https://scholar.valpo.edu/tgle/vol10/iss2/3

Intraspecific Variation in Taxonomic Characteristics of the Mayfly \u3ci\u3ePotamanthus Myops\u3c/i\u3e (Walsh)

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Intraspecific Variation in Taxonomic Characteristics of the Mayfly \u3ci\u3ePotamanthus Myops\u3c/i\u3e (Walsh)

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