Ann. Rev. EntomoL 1982. 2Z'I19-47Copydght - PDF document

Ann. Rev. EntomoL 1982. 2Z'I19-47Copydght © 1982 by Annual Reviews lnc. .411 rights reservedBIOLOGY OF MAYFLIESJohn E. BrittainZoological Museum, University of Oslo, Oslo 5, NorwayPERSPECTIVES AND OVERVIEWThe insect order Ephemeroptera, or mayflies as they are usually called, haveattracted man's attention for centuries. As early as 1675, SwammerdamEphemeri vita (212), which contains an amazingly detailed study the biology and anatomy of the mayfly Mayflies date fromCarboniferous and Permian times and represent the oldest of the existingwinged insects. They are unique among the insects in having two wingedadult stages, the subimago and imago. Adult mayflies do not feed; they relyon reserves built up during their nymphal life. They live from 1-2 hours toa few days and even up to 14 days in some ovoviviparous species. Thusmayflies spend most of their life in the aquatic environment, either as eggsor as nymphs, and the major part of this review concerns itself with theiraquatic life. The nymphal life span in mayflies varies from 3-4 weeks toabout 21/2 years. The length of egg development varies from ovoviviparity,in which the eggs hatch immediately after oviposition, to a period of up to10-11 months in some arctic/alpine species.Because of their winged adult stage and a propensity for drift as nymphs,mayflies are often among the first macroinvertebrates to colonize virginhabitats (89, 128, 241). However, over longer distances their dispersalcapacity is limited, owing to the fragile nature and short life of the adults.Mayfly faunas on oceanic islands and isolated mountain areas are poor inspecies andusually restricted to the Baetidae and/or Caenidae (62). Theirconservative dispersal makes them useful subjects for biogeographical anal-ysis (62).The mayflies are a small insect order containing somewhat over 2000valid species, which are grouped into approximately 200 genera and 19families (102, 152). Despite their poor fossil record, the conservative dis-persal, together with the wide range of morphological, anatomical and 120 BRITTAINbehavioral data on their eggs, nymphs, and adults, has permitted consider-able phylogenetic analysis during the last decade (130, 152, 188).Mayflies are found in almost all types of freshwater habitats throughoutthe world, with the exception of Antarctica, the high Arctic, and manysmall oceanic islands. Some species also occur in brackish waters. A SouthAmerican baetid is apparently semiterrestrial [Peters in (188)].In the Arctic and in mountain areas above the tree line, there are fewmayfly species (22, 46, 127). Their greatest diversity is i~ lotic habitats temperate regions, where they are an important link in the food chain, fromprimary production to secondary consumers such as fish. More recentlytheir potential as indicators of pollution has attracted increasing attention.The literature on mayflies is extensive, especially in North America andEurope. An insight into the breadth of modern studies on the mayflies isprovided by the proceedings of the three international mayfly conferences(78, 175, 180). The last attempt to review the data on mayfly biology on worldwide basis was by Illies in 1968 (113). Therefore, in the present reviewI have concentrated on the data published during the last decade.The adult mayfly has two main functions, mating and oviposition. Thisproduces a general uniformity in adult structure. The prominent turbinateeyes of males, especially well-developed in the Baetidae and some Leptoph-lebiidae, provide both high acuity and good sensitivity (100). This enablesthem to detect and capture single females in a swarm at low light intensities.The forelegs of most mayflies also show sexual differences; those of themale are unusually long for grasping and holding the female during mating.In the Polymitarcyidae the middle and hind legs of the male and all the legsof the female are reduced, and in (Behningiidae) all the legs of bothsexes are reduced. Another interesting aspect of the biology of several members of the Polymitarcyidae and Palingeniidae is that thefemales remain in the subimaginal stage. (181). The reason for two adultstages has provoked much discussion. It has been suggested that this primi-tive trait is maintained because there has not been the selective pressure onthe short-lived stages to produce just a single molt (195). Another explana-tion is that two molts are necessary to complete the elongation of the caudalfilaments and forelegs of the adults because such a drastic increase in theirlength from those of the nymphs is not possible in a single molt (146).Most mayflies have two pairs of wings, but in the Caenidae, Tricory-thidae, Baetidae, and some Leptophlebiidae the hind wings are reduced oreven absent (66). BIOLOGY OF MAYFLIES121Spermatogenesis and oogenesis are completed in the final nymphal instar,and the eggs and sperm are physiologically mature in, the subimago (19,200). Several workers have obtained viable offspring by artificial fertiliza-tion. Although most species have fecundities in the range 500-3000 (43),values range from less than 100 in' to 12,000 in 18.1 ). There is a general trend for the females of the larger species to producemore eggs, and the fecundity values recorded for Palingenia, Hexagenia,and Epeorus are greater than in most other insect groups.except the socialHymenoptera (43). Most workers have found a positive correlation betweenfecundity and female size within a particular population. In species with along emergence period or with a bivoltine life cycle giving two summeremergence periods, early emerging females are larger and therefore morefecund than those emerging later (9, 214, 216). In some. species of with long emergence periods, female size and fecundity increase again in theautumn concomitant with failing temperatures (9, 115). Two explanationshave been offered: either a temperature effect on the number of ovarioles(214) or a combination of high water temperatures and low water flowproduce premature emergence (115).Mating and SwarmingThe swarming and mating behavior of mayflies has been the subject of anumber of reviews (19, 31, 90, 194). Swarming is a male activity, apart fromthe Caenidae and Tricorythidae where both males and females may partici-pate. The females fly into these swarms and mating occurs almost immedi-atdy and usually in flight. Swarming may take place over the water itself,over the shore area, or even remote from the water. For instance, theswarms of l~aetis, Paraleptophlebia, have been observedup to several kilometers from the nymphal habitat (65, 66). Most swarmsare orientated according to terrain markers such as areas of vegetation, theshoreline, and trees (194). The time of swarming varies considerably, al-though dusk is the most common time of day in temperate areas. Lightintensity and temperature are major factors in determining the timing ofThe majority of mayflies, including most Ephemeridae, Heptageniidae, andLeptophlebiidae, oviposit by descending to the water and releasing a feweggs at a time by dipping their abdomen into the water. Species of erella, Siphlonurus, and Centroptilum, however, release all their-eggs in asingle batch that separates immediately on contact with water. In and some Heptageniidae (71, 184) the female, resting on stone above the water, dips her abdomen into the water to lay the eggs. Thisis taken a stage further in several species of (9, 66, 69) in which thefemale actually goes underwater and lays its eggs on suitable stones. Suchbehavior may permit assessment of water quality before oviposition (207).Parthenogenesis has been reported in about 50 mayfly species, although itis not obligatory in most cases. In nonobligatory parthenogenesis, eggsdevelop more slowly than fertilized eggs and fewer of them hatch (59, 107,178). Because of the low level of hatching success, this type of partheno-genesis is unlikely to be of importance in population dynamics.Parthenogenesis is apparently obligatory in all or certain populations offive species: Ameletus ludens, Baetis hageni, B. macdunnoughi, from North America and in Caenis cuniana from Brazil (11,Caenis cuniana was reared through several parthenogenetic genera-tions in the laboratory, and only females were obtained (86). In the speciesin which parthenogenesis appears to be obligatory, hatching success isusually high and similar to the fertilized eggs of nonparthenogenetic species.Gibbs (88) studied the emergence of C. triangulifer and found the emer-gence period to be unusually long (June-November). She suggested that thiswas due to the removal of the necessity for synchronous emergence of thesexes; it has been suggested that parthenogenesis may also lead to decay ofthe diel flight activity pattern (221).Mayfly eggs vary from ovoid to nearly rectangular. Their length generallyranges between 150 and 200/.tm, although the eggs of some larger speciesare 250-300 #m wide, and even up to 1 mm long in theBehningiidae (59, 122, 124, 125). As most mayfly eggs are laid freely on thewater surface, they have a variety of attachment structures that enable themto adhere to submerged objects or to the substratum.Differences in egg morphology have enabled the construction of identifi-cation keys, purely on the basis of eggs. This has provided a useful comple-ment, not only to studies of phylogeny (125) but also to taxonomy, identification of female adults according to external characters is oftenditi~cult. In addition, the eggs are already developed in the mature nymphs,providing the possibility of associating nymphal and adult stages whenrearing is not possible. Recent comparative morphological studies of mayflyspermatozoa (200) indicate similar possibilities for males. BIOLOGY OF MAYFLIESUntil the last decade there had been no detailed work on egg development.Even today, we have very limited information, almost exclusively fromtemperate areas in Europe and North America (71). Most eggs hatch withinthe range 3-21°C. Among the European exceptions are Baetis rhodaniwhose hatching success is high even at 25°C; Rhithrogena loyolea, with eggsthat hatch in the narrow range 2-10°C; and R. semicolorata which has alower hatching limit of 5°C (71). In Hexagenia rigida, a North Americanspecies, the eggs hatch successfully between 12 and 32°C and even at 36°Cif incubation is started at lower temperatures (85). Eggs of Tortopus incertusdo not complete development at 14°C, whereas at 19°C hatching occurs(224). In Tricorythodes minutus eggs hatch between 7.5 and 23°C, butmortality is least at 23°C (171).Hatching success is variable, ranging from over 90% in Baetis rhodaniHexagenia rigida to less than 50% in the Heptageniidae studied. Thistype of difference clearly has repercussions for population dynamics. Ex-cluding the few ovoviviparous species, the total length of the egg develop-ment period varies from a week in Hexagenia rigida to almost a year inParameletus colurnbiae (61, 85). Temperature is the major factor determin-ing the length of the period of egg development in mayflies. There is noindication that photoperiod influences egg development time. In most spe-cies the temperature relationship can be well described by the power lawor as a hyperbola (71).Egg diapause has only been studied in detail in Ephemerella ignita Baetis vernus (15, 16), although there is much indirect evidence from fielddata and from limited laboratory studies. However, one should be cautiousin putting forward egg diapause as an explanation for the absence of smallnymphs in field collections. For example, on the basis of field data (129),it was suggested that Ecdyonurus dispar E. insignis spend the winterin egg diapause; but Humpesch (106, 108) has shown that the eggs hatchdirectly without diapause. The small nymphs may be deep down in thesubstratum or are perhaps too small to be detected by normal samplingmethods. Nevertheless, Clifford (42) makes the conservative estimate frompublished field data that 15 species of mayfly display summer egg diapause.This aspect of mayfly biology dearly warrants further study. Assessmentfrom field data whether winter egg diapause occurs is more ditficult owingto the irregularity and even absence of winter samples. However, manyspecies undoubtedly spend the winter months either in the egg stage or asvery small nymphs, particularly in arctic/alpine areas (23, 61).The actual period over which hatching takes place may often be short. 124 BRITTAINFor example, at temperatures above 5°C most eggs of Baetis rhodani in less than ten days (10, 69). The occurrence of small nymphs of this speciesin field collections over several months is often due to slow growth in partof the population rather than to a long hatching period. This is not alwaysthe case, however, as shown in British Ephemerella ignita (70). In theLeptophlebia cupids, extended hatching has been demonstrated inthe laboratory (44).Ovoviviparity is rare in the mayflies and is restricted to the Baetidae. InCloeon dipterum is the only species known to be ovoviviparous(58). The female imago of C. oviposits 10--14 days after mating,and as soon as the eggs come into contact with water they hatch. InAmerica a number of species in the genus are ovoviviparous(66). Here again the female imagos are especially long-lived, which, coupledwith the comparatively short life of the males, leads to abnormal sex ratiosin field collections.In contrast to the adults, mayfly nymphs show considerable diversity inhabit and appearance. Differences do not always follow taxonomic lines,and convergent and parallel evolution appears to be common.Growth and DevelopmentMayflies have a large number of postembryonic molts. Estimates of thenumber of nymphal instars vary between 10 and 50; most are in the range15-25 (77). However, because of size overlap between instars, which neces-sitates the rearing of individual nymphs from hatching to emergence, theexact number of instars has only been determined in a few species (24, 44,57). Many workers have therefore distinguished developmental stages onthe basis of morphological characters (77). Although these may encompassseveral instars, they have proved useful in analyzing complex life cyclepatterns (153, 210), in elaborating the effect of environmental factors growth and survival (36), and in comparing field and laboratory data (36,178). The number of instars does not appear to be constant for a particularspecies but probably varies within certain limits. Instar number can varyeven when nymphs are reared under the same conditions (24, 44, 57).Environmental conditions, such as food quality and temperature, also affectinstar number. Because of its simplicity, by far the most common measureof development and growth in mayflies has been body length, although headwidth and other body dimensions have also been used. However, growth ofthe various body parts is not always isometric (40). A number of authorshave also used body weight, and the length-weight relationship is usually BIOLOGY OF MAYFLIES125wall expressed by a power function where the value of b is close to 3 (108,As one might expect from the variability and flexibility of mayfly lifecycles, nymphal growth rates are influenced by several environmental fac-tors. However, in most species that have been studied in detail, temperature,in terms of mean values, the scale of diurnal fluctuations, or total daydegrees, is the major growth regulator (24, 44, 52, 75, 105, 108, 155, 176,210, 214, 215, 233). Other factors, such as food and current velocity, mayexert a modifying influence on growth rates (36, 105, 108, 126). In a fewspecies growth rates appear to be independent of water temperatures, partic-ularly in those that continue growing at the same rate during the wintermonths (150, 157, 236). No true diapausing nymphal stage has been re-ported in the Ephemeroptera. However, growth rates are often very lowduring the winter because of low temperatures. Judging from field datanymphal growth in some species may be inhibited by high summer tempera-tures, although extended hatching could create a similar picture.The gills of mayflies are very diverse in form, ranging from a single plateto fibrillar tufts in (188). Respiratory tufts aresometimes developed on other parts of the body besides the abdomen, suchas those at the base of the coxa in (66, 188).In several families the second abdominal gill has developed into an opercu-late gill cover for the remaining gills and in certain Heptageniidae the gillsare markedly expanded so that they together form an adhesion disc. Inmany of the Siphlonuridae the gills are used as swimming paddles, whichhas been put forward as their original function (188). In respiration the gillsmay either function as respiratory organs or as ventilatory organs for theother respiratory exchange surfaces.Mayflies have solved the problem of respiratory regulation in two ways.Some species, mostly from lentic habitats, are respiratory regulators (74,167). These species may be unable to regulate oxygen consumption onnonoptimal substrata or in the absence of substratum (73), a fact which particularly important when interpreting the results of laboratory respira-tion studies. Other species, primarily associated with running waters, areunable to physiologically regulate oxygen consumption over concentrationgradients. The mayflies that have immovable gills are usually restricted toenvironments with high current velocities, and their oxygen consumptioncan often be directly related to current speed (109). This gives them theopportunity of regulating their oxygen consumption by positioning them-selves with-respect to the current (240). Factors affecting respiration ratesinclude temperature, light intensity, and growth stage (37, 54, 97, 226, 229). 126 BRITTAINHigh rates of oxygen consumption are often reported in association withemergence and gonad maturation (37, 54, 229). At that time water tempera-tures are also usually high, which means that low oxygen concentrationscan be critical (170, 184).Many burrowing Ephemeridae and pond-dwelling Baetidae are able tosurvive moderately low oxygen concentrations, especially for short periods(74, 87, 170). However, so far only one species, Cloeon dipterum, has beenshown to survive long-term anoxia and respire anaerobically (168, 169).This adaptation is part of the overwintering strategy of C. dipterum in smallponds that experience low oxygen concentrations during winter ice cover.In addition to an ability to survive anoxia at low temperatures, the nymphsdisplay special behavioral adaptations under anoxic conditions, which en-able them to move into microhabitats more likely to contain Oxygen. Al-though unable to move into microhabitats more likely to contain oxygen.Although unable to survive long-term anoxia, the mayfly also displays similar behavioral adaptions under conditions oflow oxygen (30).Population MovementsAll mayfly populations undertake movements at some time during their life;they may be random or directional, daily or seasonal. Due to frequentnonrandom oviposition, a redistribution of small nymphs takes place inmany mayfly populations (213, 241). During the final stages of nymphal lifethere is also a movement to and a concentration in the shallower areas oflakes and rivers. In running waters, springtime mass movements of mayflynymphs along the banks of the main river and into slower flowing tributarystreams or into areas flooded by spring snow-melt have been reported (98,173). In running water, mayfly nymphs may move down into the sub-stratum in response to spates (185) or as part of a daily rhythm (35).Generally, however, mayflies do not extend far down into the substratum(110, 161).Mayflies, especially are a major component of invertebrate driftin running waters. Their drift shows a strong diurnal periodicity, with apeak during the hours of darkness (2, 45, 67). At high latitudes the driftactivity of certain Ephemeroptera becomes desynchronized in continuousdaylight (162). Drift rates are not constant for a particular species, and thelarger size classes are usually more in evidence. Behavioral drift may serve,as in mass movements along the substratum, to relocate the population inareas suitable for that particular stage (93, 136, 227). In addition photopedod and stage of development, several other factors have beenshown to influence mayfly drift including changes in current velocity anddischarge (34, 39, 176), increased sediment loading (38, 189), temperature BIOLOGY OF MAYFLIES127changes (119), oxygen conditions (240), nymphal morphology (38), phal density (91), and food availability (17, 119). Within the mayflies thereis a gradation in the tendency to leave the substratum and enter the watercolumn, and this appears to be related more to behavioral patterns than todensity (51). Recently the presence of predatory stoneflies has been shownto increase drift rates (53, 177).The potential reduction of upstream populations of both eggs andnymphs by drift has initiated a search for compensatory mechanisms. Driftmay well be simply a method of reducing competition which is related tobenthic population densities (91). However, there is clear evidence of otherexplanations in some cases. Upstream movement of nymphs has been dem-onstrated in several mayfly species (68, 109) and may at least partiallycompensate for drift. Another compensatory mechanism is the upstreamflight of imagos before oviposition. Upstream flight has been demonstratedin a wide range of habitats, from small streams to large rivers, both in• treeless mountain areas and in lowland forests (144, 163, 191,217). How-ever, although common, the phenomenon of upstream flight is not by anymeans universal (67, 91, 176, 181).Emergence, the transition from the aquatic nymph to the terrestrialsubimago, is a critical period for mayflies. Their movement up to the watersurface, often during daylight, makes them especially vulnerable to aquaticand aerial predators. Shedding of the nymphal skin usually occurs at thewater surface on some object such as a stone or maerophyte stem or inmid-water. The latter is more typical of the burrowing species which inhabitdeeper waters and of a number of river species. Genera such as crawl completely out of the water before they molt(66, 178). The mechanism of molting has been well described (113).Diel PatternsIn temperate regions the crepuscular emergence of mayflies is well known.However, dusk is not the only time of day that mayflies emerge, althoughmost species exhibit clear diel patterns of emergence which are, with fewexceptions (20, 84), characteristic for a given species, genus, or even a wholefamily. For example, the emergence of the short-lived Caenidae invariablytakes place either at dawn or dusk and appears to be controlled by lightintensity (164). Several baetid and leptophlebiid genera emerge aroundmidday (e.g. 13, 84, 99, 104, 164, 218). In temperate areas the higherdaytime air temperatures are less restrictive for flight activity, although theadults are probably more susceptible to predation (65). In the arctic sum-mer, with perpetual daylight, Baetis purnilis, B. macani, 128 BRITTAINstill maintain a synchronized daily rhythm of emergence with adistinct afternoon peak (218). This suggests either an endogenous circadianrhythm (162), also postulated for (181), or a response to the limiteddiurnal fluctuations in temperature and light intensity. In Baetis alpinus,which normally emerges in the afternoon under constant temperature, therhythm of emergence was disturbed in permanent light and suppressed inpermanent darkness (104). Nymphs of appear to require both lightand temperature cues for successful emergence (181).In the tropics and warm temperate regions, nighttime air temperaturesare less restrictive, and in order to escape from daytime predators it seemsthat most longer-lived forms emerge during the first two hours of darkness(65, 72). The shorter-lived genera, such as are subject to fewerrestraints on their emergence and there are few constant differences betweentropical and temperate species (65).The daily emergence of males and females is usually synchronous, espe-cially in the short-lived forms~ although there may be an excess of malesat the start of the day's emergence (13, 84). In the Behningiidae, femalesoviposit as subimagos and therefore the males, which molt to imago, emergewell before the females (181).Seasonal PatternsAs well as diel patterns, mayflies have distinct and finite emergence periods,especially in temperate and arctic areas. In cold temperate and arctic areas,mayfly emergence is more or less restricted to the summer months, owingto the physical barrier of ice cover and the low air temperatures during therest of the year (13, 26, 228). Probably only a few species, such as are able to emerge at water temperatures below 7°C (13, 23). one approaches the tropics, and also in more oceanic climates, there arefewer restrictions and emergence may occur throughout much of the year,although most emergence still occurs during the warmer months (42, 216).In the tropics emergence is often nonseasonal (220, 221), although somespecies have clear emergence patterns. The lunar rhythm of emergence ofthe Africian species, Povilla adusta, is well known from a number of lakes(50, 96). In other African lakes, however, emergence Qf/~. is lesssynchronized (50, 182). The burrowing mayflies of the Ephemeridae,Polymitarcyidae, and Oligoneuriidae are well known for their sporadicmass emergence (20, 66). The mass emergence of from theMississippi River, USA, has been well documented (82), mechanism producing such synchronicity is unknown (219).Latitudinal and .altitudinal differences result in differences in the timingof emergence. For example in both American and European emergence occurs progressively later as one moves northwards (21, 22, 44,120). In a similar way, the onset of emergence is delayed with increasing BIOLOGY OF MAYFLIES129altitude. In habitats with several mayfly species, peak emergence of themajor species may be separated in time, especially in congeneric species (e.g.13, 26-28, 109, 216). Such serial emer.gence appears to be a constant featurefrom year to year; changing temperature conditions only affect the absolutedates of the emergence peaks rather than the serial pattern itself (26, 27).The length and pattern of emergence varies in seasonal species, and it hasbeen suggested that emergence falls into two main categories: synchronizedand dispersed (95). It is thought (142) that such differences represent approaches for reducing adult mortality: synchronous emergence attempt-ing to saturate a potential predator, and dispersed emergence seeking tolower the possibility of predator-prey encounters. However, emergencepattern can vary with abundance, locality, and from year to year within thesame species (27, 112, 131,216). The degree of developmental heterogen-eity in the preemergenee nymphal population also has a major influence onemergence patterns (27, 28, 95, 116), and emergence should be viewed an intergral part of the species' overall life cycle strategy (28). In specieswith well defined emergence periods, males and females usually emergesynchronously, although there is often a tendency for the cumulative emer-gence curve of males to lie somewhat ahead of the females' (27, 44, 95, 216).Water temperature thresholds, often in conjunction with rising tempera-tures, are important for both seasonal and daily emergence of many mayflies(20, 25, 82, 104, 114, 131, 141, 181,218). It has been shown in the labora-tory that emergence can be hastened or delayed by changing the experimen-tal water temperatures (25, 214). Also, field data often show earlierephemeropteran emergence in warmer years (27, 114). However, it has beenargued that other factors such as stable flow conditions and food are ofequal importance in inducing emergence in the laboratory (131). Neverthe-: less, the later emergence of the same species at higher altitude lends supportto, the effect of temperature, either through threshold temperatures orthrot~gh nymphal growth and development (233).Photopedod has also been suggested as a potential factor regulatingseasonal :emergence in mayflies (131, 157), but little concrete data are• available and. successful emergence has been shown in nymphs reared incomplete darkness (24). Other abiotic factors, such as wind, humidity,precipitation, turbidity and irradiance, may also affect daily emergencetotals (20, 27, 131, 181, 218, 220).LIFE CYCLESThere is an extensive literature on mayfly:life cycles, mostly from temperateareas in Europe and North America (42). Care should be taken wheninterpreting mayfly life cycles, especially when based solely on field observa- 130 BRITTAINtions. Hynes (109) has rightly emphasized that a combination of field andlaboratory work yields much more information than either does alone.Particular care is necessary in interpreting the length of egg developmentfrom field data, as shown by the recent studies of Humpesch (106) The useof inadequate sampling methods for the smaller instars can also be a sourceof error (208).Several authors have classified ephemeropteran life cycles; most use acombination of voltinism, duration of egg development, and nymphalgrowth rates as criteria (42, 129, 140, 204). Multivoltine species usuallyhave two or three generations in temperate regions, often a slow growingwinter generation and one or two rapidly growing summer generations.Limited data from the tropics, where many species are nonseasonal, indi-cates that some species go through about four and possibly up to six genera-tions during the course of a year (12, 50). However, analysis of field datais diificult, and there is certainly need for complementary laboratory stud-ies. In temperate areas the univoltine life cycle is the most widespread.Several authors have distinguished two main types of univoltive cycles: inthe first, overwintering occurs during the nymphal stage after a relativelyshort egg development period, and in the second, hatching occurs in thespring after a long period of egg development. Semivoltinism appears to berelatively uncommon in mayflies. The maximum generation time so farrecorded is 3 yr, and even in such populations there are usually both 2 and3 yr cohorts.Mayfly life cycles show a distinct trend from the tropics to the arctic (42).In the tropics, nonseasonal multivoltine cycles predominate; seasonalitybecomes more distinct in mountainous and continental areas. In oceanicclimates, such as in New Zealand, synchronization may be poorly devel-oped (242). As one approaches the Arctic, univoltine dominate evenmore (22, 42, 227).Some mayflies, for example the widespread Palearctic have a univoltine winter cycle over a wide range of latitudes and climates(21, 22, 120, 129, 204). A similar constancy is found in the Nearcti¢ tophlebia cupida (44).However, a number of successful display a considerable degree oflife cycle flexibility throughout their distributional range. This is perhapsbest exemplified by many of the Baetidae. The European species, is a typical univoltine winter species in northern and mountainareas, while in much of Europe it has both a winter generation and asummer generation (140). In more southerly locations there are two sum-mer generations as well as the winter generation. The real flexibility of BIOLOGY OF MAYFLIES131Baetis rhodani's life cycle has been shown by Humpesch (105), who showedvariation from year to year depending on temperature conditions. Forexample, in an Austrian stream he was able to distinguish 10 cohorts overa 30 month period, and the duration of these cohorts varied from 2.5 to 8Several Ephemeridae are also known to exhibit a degree of life cycleflexibility, and there has been much confusion concerning life cycle lengthin certain species. For example, the European species, Ephemera danica,may have a one, two, or three year cycle, and two of these cycles are oftenpresent in the same habitat (e.g. 210, 216, 237). A similar situation existsfor the North American Temporal separation of life cycles is one of the most common mecha-nisms permitting coexistence among closely related mayflies. For example,the European spp. have essentially similar life cycles. How-ever, when they occ/ur together their cycles are out of step, L. marginataalways being larger and emerging earlier than L. vespertina (21, 22, 26).Preliminary data (28) suggest that L. marginata has a lower temperaturethreshold for growth. Such differences have also been observed in Northspecies (48, 52). Differences in life cycle, al-though often the most obvious, are usually not the only differences betweencoexisting species. For example, in a Norwegian mountain lake differencesin nutrition, fecundity, predation pressure, and size at maturity were foundamong the mayfly species (28).The majority of mayfly nymphs are herbivores, feeding on detritus andperiphyton. This explains the relative uniformity of mouthpart constructionwithin the order (206). The modifications that are present are a result different food gathering mechanisms rather than differences in diet (201,206). The herbivorous mayflies fall into two main categories: collectors andscrapers (55, 64). Among the collectors, several genera are filter-feeders,with setae on the mouthparts or forelegs acting as filters (230). Within theOligoneuridae, Leptophlebiidae, Siphlonuridae, and the Heptageniidae,there are several genera that are probably filter-feeders (230). By using theirgills to produce a current of water through their burrows, several of theEphemeridae and Polymitarcyidae may, at least for part of their foodsupply, be regarded as filter-feeders. The higher caloric and organic mattercontent of gut contents compared to the sediments they inhabitlends support to this hypothesis (248). To supplement their diet, nymphs, especially the larger ones, leave their burrows at night and graze BIOLOGY OF MAYFLIES133The carnivorous feeding more intermittently but on a higherenergy diet, has consumption indices similar to those of In all these studies there arises the question of what fraction of the foodingested is actually digested and absorbed. Two detritivores, Habrophlebia lauta, have been observed to eat their own fecesdirectly from the anus (184)---probably a mechanism for increasing theet~ciency of digestion. Brown (32) has shown that several algal species areel~ciently digested by Cloeon diptemm, although narrow filamentous formsand small cells remain viable after excretion. He suggested that such algaepass between the mouthparts without being damaged and that in the ab-sence of a cellulase they were unable to be digested. Subsequent studies haveshown little or no cellulase activity in mayflies (158). Nevertheless, organiccompounds leaked or secreted by the algae may be of nutritional importance(55). In contrast to cellulase activity the proteolytic activity of trypsin- andpepsin-like enzymes is very high in the Ephemeroptera (56). Bacteria andfungi are another potential food source for mayfly nymphs. However, noevidence for the digestion of bacteria was found in (5), and a hypomycete mycelium alone was insut~cient to support thegrowth of Mayflies are susceptible to predation throughout their life cycle. We knowleast about egg predation, although both true predators and grazing andcollecting herbivores undoubtedly take their toll (139). Mayfly nymphs areeaten by a wide range of aquatic invertebrate predators, including stoneflies,caddisflies, alderflies, dragonflies, water beetles, leeches, triclads, andcrayfish. Mayflies are also important fish food organisms (see 109). In manycases the degree of predation is closely related to the mayflies' size andabundance and therefore varies with habitat and with season (29, 147).Birds and winged insects, such as Odonata, also prey on mayflies (e.g.151, 154). Birds may take both the aquatic nymphs and the aerial adults.Several other animal groups, including spiders, amphibians, marsupials,and insectivorous mammals such as bats, shrews, and mice, have beenreported to take mayflies. Many parasites also utilize these food chain linksDespite the numerous records of predation on mayflies, its effect on theirpopulation dynamics is poorly known. In an English pond, Macan (138)found little change in populations after a change in troutdensities, although the mayflies were one of the trout's main food items.However, on the stony shores of nearby productive lakes the lack of aquaticinsects, including mayflies, was attributed to predation by groups such as 134 BRITTAINtriclads and molluscs (139). In a Norwegian mountain lake, trout consumed30-40% of the total annual mayfly production (26, 28, 29).Symbiosis, Phoresy, and ParasitismThere is a wide range of organisms that live on or in mayflies. They includethe normal spectrum of protozoan, nematode, and trematode parasites aswell as phoretic and commensal relationships with other organisms (4, 47,179). Unfortunately, much of the literature is restricted to occurrencerecords. The more detailed studies have been on phoretic Diptera (60, 202,209, 211, 239). Svensson's investigation of the relationship between theEpoicocladius ravens and the mayfly Ephemera danica southern Sweden is particularly noteworthy (209, 211). Chironomids in theare ectoparasites and may cause sterility (202), butdoes not appear to be detrimental to its host. In fact thecleaning effect, especially of the gills, may facilitate oxygen uptake in themayfly. The semivoltinism of E. (210) compared to the univoltinismor bivoltinism of considerably lessens the problem of lifecycle synchronization for the chironomid. In contrast, the apparently bivol-tine ectoparasite Symbiocladius equitans from North America has hostswith much shorter generation times, and in one area the winter generationof S. nymphs while the summer generationlives on Blackflies are also phoretic on mayflies, although far more is known aboutthe parasite than the host because of their medical importance. Althoughblackflies and mayflies occur together throughout the world, phoretic rela-tionships are restricted to upland areas in Africa and Central Asia.Mayflies can also be commensal, and a baetid, Symbiocloeon heardi, Thailand lives between the gills of a freshwater mussel (166). The mayflyhas a number of morphological adaptations to life inside the mollusc, suchas a strongly hooked apex to the tarsal claws in order to hold on to themussel's gills. The mayfly profits from the food filtered by the mussel, andthe relationship may be obligatory for the mayfly.DISTRIBUTION AND ABUNDANCEThe distribution and abundance of mayflies has received considerable atten-tion. Within the basic zoogeographical limitations, abiotic factors, notablytemperature, substratum, water quality, and in running water currentspeed, appear to be the most important. Other factors such as ice, floods,drought, food, and competition may also influence abundance and distribu-tion. These factors have been treated in detail by Hynes (109), with manyexamples drawn from mayflies. In addition to subsequent papers dealing BIOLOGY OF MAYFLIES135specifically with mayfly zonation (14, 22, 127, 187, 196, 203, 231,243, 245),numerous studies have been made on macroinvertebrate zonation in thetemperate regions of the world. Generally, the number of mayfly speciesincreases with decreasing altitude. As shown by several authors (22, 112,118) increasing temperature, by creating more emergence slots and enablingadditional species to grow and complete their life cycles, is important.Differences in the trophic nature of the community can change the natureof the mayfly community. For example, a change from a predominantlyautotrophic alpine to a heterotrophic subalpine stream community wouldresult in a shift from grazers to collectors.Many lotic mayflies are either dorsoventrally flattened or streamlined asan adaptation to life in swift current (109). Although this is generally true,such a body form does not necessarily indicate a preference for currenthabitats (143). The importance of respiration in microhabitat selectionshould not be forgotten, both with respect to substratum and in terms ofrespiratory regulation (73, 126, 240). The physical substratum also trapsdifferent amounts of detritus and silt and this is a major factor influencingmicrodistribution (e.g. 6, 26, 196, 197). The richest mayfly community often found in association with aquatic vegetation (6, 41, 132, 145, 246),which, as well as providing shelter, functions as a detrital trap and as asubstratum for periphyton. Removal of aquatic macrophytes may thereforelead to an impoverished mayfly community (49). For burrowing mayflies,the presence of the correct substratum is obviously a major determinant ofboth macro- and microdistdbution (183, 193, 213). In lentic habitats andin slow-flowing waters, oxygen concentrations may become critical in cer-tain areas (20, 183, 213). In lakes the highest mayfly diversity occurs in theshallow littoral areas. At deeper levels the mayfly fauna, although oftenreaching high densities, is usually poor in species. Mayflies are generallyabsent from the profundal of lakes.Many mayflies can tolerate a wide range of salinities, and a few specieswithin the Baetidae, Caenidae, and Leptophlebiidae occur in brackishwater habitats (81, 137). The chloride cells in the integument of mayflynymphs participate in osmoregulation, and their density appears to berelated to the osmotic strength of the medium (238).Clifford (41) has reviewed numerical abundance values in the Holarcticwere the most reported generaBaetis, Ephernerella, accounted for a major part of thehigher values. The genus provides the most data and has the highestaverage annual abundance value. Its species richness and life cycle plasticityundoubtedly contribute to its success.Mayflies constitute a major part of the macroinvertebrate biomass andproduction in freshwater habitats. Seasonal variation in density and 136BRITTAINbiomass and annual production are strongly influenced by life cycle parame-ters, indicating the importance of correct life cycle information in produc-tion studies (42, 234, 235). Most mayfly production values are in the range0.1-10.0 g dry wt/m2/yr (108, 234). The errors that may arise due to driftand to exuvia loss in estimating mayfly production have been demonstrated(93, 155). Emergence biomass has also been used as a measure of productionin mayfly communities (e.g. 26, 114).HUMAN INFLUENCEMan is having an increasing effect on the distribution and abundance ofmayflies and, by virtue of their widespread occurrence and importance inaquatic food webs and particularly in fish production, mayflies have beenwidely used as indicators of water quality (149). Mayflies play a central rolein the saprobic system that is especially well-developed in Eastern Europe(18, 192, 205). Mayflies often occur in habitats of a particular trophic status(139, ! 48), and increased eutrophication caused by man's activities can leadto the reduction or even extinction of certain species (160, 244). Even underslight pollution the mayfly community may be changed, initially oftenresulting in higher densities and production (225, 246). However, it is notlong before the mayfly fauna becomes impoverished or even totally exter-species are often among the most tolerant of mayflies toIn North America the use of mayflies as indicators of water quality hasnot escaped attention (82, 103, 135). Until recently, however, this work hasbeen hampered by problems of species identification. The mass emergenceof burrowing mayflies from Lake Erie and the Mississippi River has pro-vided a useful barometer of water quality. Organic and nutrient enrichmentof Lake Erie in the 1940s and 1950s led to an increase in the intensity andfrequency of mass emergence of until in 1953 prolonged periodsof oxygen depletion in the hypolimnion caused the population to crash tovirtual extinction (244). In the Mississippi River, mass emergence stilloccurs, but it is largely restricted to less polluted, upper reaches (82).Notably in North America, numerous laboratory bioassays have been madeusing mayflies, particularly The lethal concentrations of a num-ber of pollutants, such as heavy metals and detergents as well as naturallyoccurring compounds such as hydrogen sulphide, have been determined(see 149). DDT and other pesticides also affect non-target organisms suchas mayflies (111, 199). For example, Canadian studies in connection withblackfly control have demonstrated catastrophic drift and reduced biomassin mayfly populations over large distances in rivers treated with methoxy-chlor (79). BIOLOGY OF MAYFLIES137Contamination by petroleum products is a new threat to aquatic ecosys-tems in certain areas (7). Although most mayflies are adversely affected, few species may show small increases owing to the extensive algal growththat often occurs on oiled substrata (190). Acidification of freshwaters also a threat, especially in some areas of Europe and North America. Manymayflies are affected adversely by low pH and emergence is a particularlycritical period (8). In the northeastern USA, was theonly mayfly to survive in near normal numbers after experimental acidifica-tion of a stream. However, its growth rate decreased and recruitment to thenext generation was severely reduced (76). In Scandinavia the mayfly faunaof affected areas is poor in mayfly species, and in lotic habitats notable by their absence and are often replaced by (94; Bdttain unpublished data). Although it was suggested that theabsence of from acid streams was due to the lack of periphyton (207),more recent evidence implicates direct chemical action on the mayfliesthemselves (92, 156).The impoundment and diversion of watercourses for water supply andpower is commonplace and can have profound effects on the mayfly com-munity, especially when there is a hypolimnion drain (121, 232). An in-crease in winter temperatures and a fall in summer temperatures mayremove obligatory life cycle thresholds and produce changes in energybudgeting, leading to extinction (133, 214, 232). Heated efltuent usually hasdetrimental effects on diversity and production (172), although in somecases the observed effects may be small (131). The seasonality of the climate(continental or oceanic) of the actual area may also determine the degreeof disturbance. In reservoirs themselves, lentic conditions and increasedwater level fluctuations usually produce a depauperate mayfly fauna, espe-cially for species typical of stony substrata (117), although there may be increase in the abundance of burrowing and silt-dwelling species. The flood-ing of new areas can create new habitats for mayflies, and in many of thelarge African reservoirs the mayfly Povilla adusta has developed largepopulations, which burrow into the submerged trees and play an importantrole in tree breakdown (183).All biological studies must have a firm basis in taxonomy, and speciesidentification has been, and still is in large areas of the world, a majorobstacle to progress in mayfly biology. This is especially true of the nymphs.Our ideas of mayfly biology are largely based on data from the temperateareas of Europe and North America. In order to obtain a more balanced 138 BRITTAINpicture we need more information from the tropics, the Arctic, and frommuch of the southern hemisphere.In the same way as taxonomy presents an obstacle to further progress inbiology, so the lack of accurate life history information hinders progress inexplaining community processes and in monitoring and assessing man'simpact on aquatic ecosystems (235). While field data on mayfly life cyclesis extensive, there is often considerable doubt as to whether the species inquestion is in the egg stage or is present as early instar nymphs. In suchcases laboratory studies can provide the answer, and much can be gainedfrom a combined laboratory and field approach. The culture techniquesbeing developed for the eggs and nymphs of mayflies (83, 85, 106, 237) arean important aid to progress in this field, in addition to their value inbioassay work. We also need to know the ecological requirements of eachlife cycle stage in order to assess the effect of manmade perturbations (133,By virtue of their high and easily measured fecundity, mayflies are inter-esting objects for population dynamics, Data on indicate thatpopulation regulation occurs in the nymphal stage (101). However, al-though total life cycle mortality has been estimated in several species (9, 28,43), we know little about the causes of mortality and how mortality varieswith population density.We already know of several cases where the mayfly fauna has either beendrastically changed or exterminated (63). This is especially true of largerivers because of their restricted number and their vulnerability to pollution(63, 135). For example, the genus studied by Swammerdam inthe 17th century, which I mentioned at the beginning of this review, is nowextinct in the Netherlands and western Europe (186). Nevertheless, re-search activity during the last decade or so has set us on the right coursetowards solving both the academic problems that concern us as biologistsand the more applied problems of resource management in freshwatersystems. To fulfill both these goals we must continue to increase our knowl-edge of mayfly biology.I am grateful to the following persons for commenting on aspects of thepaper: J. J. Ciborowski, H. F. Clifford, L. D. Corkum, T. J. Fink, M. D.Hubbard, U. H. Humpesch, A. Lillehammer, B. Nagell, M. L. Pescador,J. Peters, W. L. Peters, B. S. Svensson, and J. V. Ward. The assistance ofJ. F. Flannagan and K.E. Marshall in literature search is acknowledged.Financial support was given by the Norwegian Research Council forScience and the Humanities. Literature CitedA~new, J. D. 1962. The distribution ofCentroptiloides bifasciata (E.P) (Ba-&idae: Ephem.) in Southern Africa,with ecological observations on the2.Allan, J. D. 1978. Trout predation andsize composition of stream drift. nol. Oceanogr. 3.Anderson, N. H., Cummins, K. W.1979. Influences of diet on the life histo-ries of aquatic insects. Z Fish. Res.Board Can. 4.Arvy, L., Peters, W. L. 1973. 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