Mayfly Nymphs Research Articles

COPPER, CADMIUM, AND ZINC CONCENTRATIONS IN JUVENILE CHINOOK SALMON AND SELECTED FISH-FORAGE ORGANISMS (AQUATIC INSECTS) IN THE UPPER SACRAMENTO RIVER, CALIFORNIA

This study assessed the downstream extent andseverity of copper (Cu), cadmium (Cd), and zinc (Zn)contamination from acid mine drainage on juvenile chinook salmon(Oncorhynchus tshawytscha) and aquatic insects over aroughly 270-km reach of the Sacramento River below KeswickReservoir. During April–May 1998, salmon were collected fromfour sites in the river and from a fish hatchery that receiveswater from Battle Creek. Salmon from river sites were examinedfor gut contents to document their consumption of variousinvertebrate taxa, whereas salmon from river sites and thehatchery were used for metal determinations. Midge(Chironomidae) and caddisfly (Trichoptera) larvae and mayfly(Ephemeroptera) nymphs were collected for metal determinationsduring April–June from river sites and from Battle and Buttecreeks. The fish hatchery and Battle and Butte creeks served asreference sites because they had no history of receiving minedrainage. Salmon consumed mostly midge larvae and pupae (44.0%,damp-dry biomass), caddisfly larvae (18.9%), Cladocera (5.8%),and mayfly nymphs (5.7%). These results demonstrated thatinsects selected for metal determinations were important as fishforage. Dry-weight concentrations of Cu, Cd, and Zn weregenerally far higher in salmon and insects from the river thanfrom reference sites. Within the river, high metalconcentrations persisted as far downstream as South Meridian (thelowermost sampling site). Maximum concentrations of Cd (30.7 μg g-1) and Zn (1230 μg g-1),but not Cu (87.4 μg g-1), in insects exceeded amounts that other investigators reported as toxic when fed for prolonged periods to juvenile salmonids.

Changing Abundance ofHexagenia Mayfly Nymphs in Western Lake Erie of the Laurentian Great Lakes: Impediments to Assessment of Lake Recovery?

After an absence of 40 years, mayfly nymphs of the genus Hexagenia were found in sediments of western Lake Erie of the Laurentian Great Lakes in 1993 and, by 1997, were abundant enough to meet a mayfly-density management goal (ca. 350 nymphs m -2 ) based on pollution-abatement programs. We sampled nymphs in western Lake Erie and Lake St. Clair, located upstream of western Lake Erie, to determine the importance of seasonal abundance and life-history characteristics of nymphs (e.g., emergence and recruitment) on density estimates relative to the mayfly-density management goal. Two types of density patterns were observed: (1) densities were relatively high in spring and gradually decreased through late summer (observed in Lake Erie and Lake St. Clair in 1997 and Lake St. Clair in 1999) and (2) densities were relatively high in spring, gradually decreased to mid summer, abruptly decreased in mid summer, and then increased between summer and late fall (Lake Erie and Lake St. Clair in 1998 and Lake Erie in 1999). Length-frequency distributions of nymphs and observations of adults indicate that the primary cause for the two density patterns was attributed to failed (first pattern) and successful (second pattern) reproduction and emergence of nymphs into adults in mid summer. Gradual declines in densities were attributed to mortality of nymphs. Our results indicate that caution should be used when evaluating progress of pollution-abatement programs based on mayfly densities because recruitment success is variable both between and within years. Additionally, the interpretation of progress toward management goals, relative to the restoration of Hexagenia populations in the Great Lakes and possibly other water bodies throughout the world, is influenced by the number of years in which consequtive collections are made.

EFFECTS OF TRIOPS NEWBERRYI (NOTOSTRACA: TRIOPSIDAE) ON AQUATIC INSECT COMMUNITIES IN PONDS IN THE COLORADO DESERT OF SOUTHERN CALIFORNIA

The association between a size-selective floodwater predator, the tadpole shrimp Triops newberryi, and aspects of the aquatic insect community (successional pattern, size structure, and composition) were studied in replicate ponds in the Coachella Valley of southern California. The abundance of early colonists, particularly the mosquito Culex tarsalis, chironomid larvae, and mayfly nymphs, and the size structure of the aquatic insect communities in ponds containing tadpole shrimps differed significantly from ponds without shrimps. Based on headwidths of co-occurring aquatic insects, ponds containing tadpole shrimps at densities >5 individuals m–2 had fewer small instars of aquatic insects relative to ponds without Triops. The composition of aquatic insect communities in ponds containing tadpole shrimps differed from ponds without Triops, particularly during the first 3 weeks after inundation. These results suggest that effects of tadpole shrimps, such as predation on early life history stages of co-occurr...

Annual Production of Burrowing Mayfly Nymphs ( Hexagenia spp.) in U.S. Waters of Lake St. Clair

Burrowing mayfly nymphs ( Hexagenia spp.) were sampled monthly, September through October 1995 and April through August 1996, with a standard Ponar grab (538 cm 2 jaw opening) at 16 stations in U.S. waters of Lake St. Clair. Annual production (production, P) was 0 to 477 mg dry weight/m 2 at three stations where pollution and sediment grain-size distribution limited the population, and was 738 to 5,255 mg dry weight/m 2 at the other 13 stations. The highest production value measured for Hexagenia in Lake St. Clair was about three times higher than the highest value reported for other areas in the northern United States and Canada (39° to 53° North latitude). The production-mean annual biomass (biomass, B) ratio (P/B) for Hexagenia in Lake St. Clair in 1995–96 was described by the straight line P = 2.4 B (R 2 = 0.94). Adding published P/B data for other North American populations changed the relation only slightly to P = 2.5B (R 2 = 0.96). A P/B ratio of 2.5 is consistent with the expected value for an aquatic insect with a 2-year life cycle and overlapping cohorts, and these data suggest this relation has general applicability for estimating production of Hexagenia in the northern United States and Canada. Size-class and seasonal partitioning of Hexagenia biomass and production were evident in the data. Both biomass and production were highest among nymphs 16.0 mm and larger, and biomass was highest in October and again in June, immediately before the annual emergence of subimagos. The large size of the mature nymphs and the concentration of biomass and production among the larger nymphs in the population is consistent with their importance in the diets of many fishes in the northern United States and Canada.

Changing Abundance of Hexagenia Mayfly Nymphs in Western Lake Erie of the Laurentian Great Lakes: Impediments to Assessment of Lake Recovery?

Changing Abundance of Hexagenia Mayfly Nymphs in Western Lake Erie of the Laurentian Great Lakes: Impediments to Assessment of Lake Recovery?

Combined effects of predatory fish and sublethal pesticide contamination on the behavior and mortality of mayfly nymphs.

We evaluated the potential interaction of pesticide effect and predatory fish on behavior and mortality of a stream mayfly. Experiments in laboratory stream microcosms compared the activity, drift, and mortality of Baetis mayfly nymphs in the absence of fish with that in the presence of Cape galaxias (Galaxias zebratus), both species inhabiting the same stream environments in the Western Cape of South Africa. These two predator treatments were combined with exposure either to no pesticide or to 0.2 microg/L of the organophosphate insecticide azinphos-methyl (AZP) or 0.2 microg/L of the pyrethroid insecticide fenvalerate (FV). Such pesticide levels are known from transient spraydrift- or runoff-related pesticide input into running waters. Each of the six combinations was replicated six times as 30-min trials during the day and effects were analyzed using 2 x 2 factorial analysis of variance (ANOVA). A single exposure to either fish or pesticide significantly increased the absolute activity of mayflies, measured as number of animals visible on top of stones, and the absolute mayfly drift in the fish treatment and in the FV treatment but did not increase the mortality above 0.8%. The combination of predator presence and sublethal pesticide exposure resulted in a significant increase in the mortality rate, to about 9% in the AZP x fish and 25% in the FV x fish treatment, although the activity and drift rates were not increased compared with the single-stressor treatments. Two-by-two factorial ANOVA and the comparison of expected and measured responses indicated that the mortality resulted from a synergistic interaction of the two stressors. The observed mortality was without exception caused by predation of the fish on drifting mayflies. The relative drift rate in the FV x fish treatment was decreased, again due to a synergistic interaction, which suggests an active drift avoidance reaction of the mayflies exposed to the combined pesticide x fish treatment, in contrast with those exposed just to FV. We conclude that the drift response of mayflies to transient sublethal pesticide exposure results in a synergistically increased adverse effect in the presence of predatory fish.

Nocturnal drift of mayfly nymphs as a post‐contact antipredator mechanism

1. The predominantly nocturnal constrained drift of stream invertebrates is commonly regarded as a behaviour that avoids encounters with visually foraging fish in the water column. The alternative explanation, that drift peaks are caused by bottom‐feeding, nocturnal predators, has rarely been tested. 2. We examined these hypotheses by collecting invertebrate drift in five streams in northern Finland: one with brown trout (Salmotrutta, a drift‐feeding fish), one with alpine bullhead (Cottuspoecilopus, a benthic fish), one with both species, and two fishless streams. 3. Drift by Baetis mayflies was aperiodic or slightly diurnal in both fishless streams on all sampling occasions. In contrast, drift was nocturnal in streams with trout and, to a lesser extent, in the stream with bullhead. Non‐dipteran prey drifted mainly nocturnally in all streams with fish, whereas Diptera larvae were less responsive to the presence of fish. 4. In laboratory experiments, bullheads were night‐active, causing a much higher frequency of drift by touching Baetis at night than during the day. Thus, increased nocturnal drift may serve to avoid both visual predators (a pre‐contact response) and benthic fish (a post‐contact response). In streams with bottom‐feeding fish, nocturnal drift should be caused by increased drift by night rather than by reduced drift by day.

Riverine endosulfan concentrations in the Namoi River, Australia: Link to cotton field runoff and macroinvertebrate population densities

AbstractPopulation densities of six dominant macroinvertebrate taxa (mayfly nymphs Jappa kutera, Atalophlebia sp., Tasma‐nocoenis sp., Baetis sp. and the caddisfly larvae Cheumatopsyche sp. and Ecnomus sp.) were negatively correlated to total endosulfan concentrations in the Namoi River (New South Wales, Australia) in the 1995/96 and 1997/98 cotton growing seasons (November‐March). Total endosulfan concentrations measured in solvent‐filled polyethylene bags (passive samplers) at the exposed sites correlated with rainfall, suggesting that endosulfan entered the riverine environment in runoff from land. At the start of both surveys, in November 1995 and November 1997, there was no significant difference (p < 0.05) between the reference and exposed sites for both total endosulfan concentrations and mean population densities of the combined study taxa. This indicates distance downstream, which was negatively correlated with exposure, was not an important variable in explaining the significantly higher population densities in other months at the reference sites compared with those at sites subsequently exposed to increased (10–25‐fold) total endosulfan concentrations. The multivariate analysis of the 1997/98 data with the principal response curves (PRC) method indicated that endosulfan explained a significant proportion (25%) of the variation in the total macroinvertebrate community. Principal components analysis (PCA) indicated other covariables were involved, including river discharge. River discharge was positively correlated to increased densities of the mayfly taxa at the reference sites in the 1995/96 survey, but these correlations were reduced to near zero, except for Baetis sp., in the 1997/98 survey.

Particle Mixing by Freshwater Infaunal Bioirrigators: Midges (Chironomidae: Diptera) and Mayflies (Ephemeridae: Ephemeroptera)

Mechanisms and rates of sediment mixing by the infaunal chironomids Coelotanypus sp. and Chironomus sp., and the mayfly nymph, Hexagenia spp., were quantitatively investigated in the laboratory using a multiple 137Cs tracer layer microcosm technique. These insect larvae construct and irrigate burrows up to 10 cm beneath the sediment-water interface. Obliteration of tracer layers by these larvae was attributed to mixing by both diffusive (random smearing)—and feeding (directed particle motion)—style processes. Biodiffusion coefficients were obtained from the rate of widening of the tracer layers and feeding rates were obtained from the rate of decrease in activity of the tracer layers. The biodiffusion coefficients and feeding rates normalized by experimental population densities were used to compare the results of these experiments with those obtained in similar experiments using other organisms. The results suggest that of the organisms studied here, only Chironomus sp. mix sediments by biodiffusion at rates comparable to conveyor-belt deposit feeding oligochaete worms on a per individual basis. Individual rates of non-local mixing by feeding for the chironomid and mayfly larvae in these experiments are similar to or less than those reported for oligochaetes in similar experiments. These results indicate that although chironomid larvae and mayfly nymphs are burrow irrigators, they may also deposit-feed or at least rework sediments in a non-local fashion. Some of the differences in reported individual biodiffusion coefficients and feeding rates may be related to organism size, because larger organisms exhibited greater biodiffusion coefficients and feeding rates per individual. The values for the biodiffusion coefficients on a per individual basis were multiplied by approximate in situ population densities to evaluate the relative importance of each of these species at natural densities. The results indicate that the sparse populations of bivalves, some chironomids ( Coelotanypus sp.), and mayflies cause a relatively small amount of mixing compared to the more abundant amphipods, tubificid oligochaetes, and Chironomus sp. Surprisingly, the bioirrigator Chironomus sp. appears to be as important in mixing sediment as depositfeeding conveyor-belt tubificid oligochaetes; however, whereas sediment mixing by tubificid oligochaetes is primarily the result of feeding, sediment mixing by Chironomous sp. appears to be largely the result of burrow construction.

The macroinvertebrate fauna and hydrogeology of springs in central Sweden.

The macroinvertebrate fauna of 23 springs in central Sweden was sampled. A total of 79 taxa was recorded, including three crenobiont, nine crenophilic, 33 lotic, 11 lentic and 21 ubiquitous taxa. Only eight were non-insect taxa. Geological and hydrogeological maps were used to categorize each spring, resulting in eight glaciofluvial, 11 moraine and four limestone springs. Faunal composition and environmental factors showed an overall similarity among spring types, except for geographic position, the almost exclusive occurrence of crenobionts in glaciofluvial springs, and factors associated with high concentrations of calcium carbonate in limestone springs. Canonical correspondence analysis showed that the crenobiont caddisflies Parachiona picicornis(Pictet), Crunoecia irrorataCurtis were associated with high discharge and high summer temperature. Lotic taxa, including the stonefly nymph Isoperla grammatica(Poda), the caddis larvae Halesussp. and the mayfly nymph Baetis bundyaeLehmkuhl, were associated with high discharge and low summer temperature, while many lentic and ubiquitous taxa, e.g. dytiscids, were characteristic of small springs. It is suggested that the dominance of insect taxa, and the restricted occurrence of crenophilic and crenobiont taxa, are consequences of the glacial history and/or the hydrogeological setting of springs. Also, the possible influence of `diffusive rarity' of C. irrorataand P. picicornis, is discussed.

Diel vertical movements by lotic mayfly nymphs under variable predation risk

Summary1. The diel foraging periodicities of two grazing mayfly (Ephemeroptera) nymphs, Heptagenia dalecarlica and Baetis rhodani, under variable fish (European minnow) predation risk were examined in a series of laboratory experiments.2. Heptagenia dalecarlica were almost exclusively nocturnal in their use of feeding areas on stone tops. There was a sharp increase in the proportion of nymphs out of refuge at nightfall, both in the control and fish‐odour treatments. In a treatment with freely moving fish, H. dalecarlica decreased their activity during both the day and night. In feeding trials with three freely foraging minnows, nymphs were completely safe when they had access to refuges beneath stones, whereas almost half the nymphs were consumed when no refuges were available.3. Baetis rhodani nymphs reduced their use of stone tops when exposed to four caged minnows or a freely moving fish, but this occurred both day and night. In feeding trials, B. rhodani were captured only while in the water column, and their mortality risk was independent of refuge availability.4. It is suggested that because H. dalecarlica lack efficient post‐encounter defences, they must rely on pre‐encounter mechanisms to reduce the threat of fish predation. It appears that in order to dwell sympatrically with fish, H. dalecarlica have evolved a coexistence by hiding strategy. In contrast, Baetis are vulnerable to fish attacks only if they enter drift in daylight, and are thus able to remain on stone tops both during the day and at night.

Occurrence of the mayfly family Teloganodidae in northern New South Wales

Teloganodid mayfly nymphs, previously known in Australia only from south‐eastern Queensland, have now been recorded from numerous localities in the coastal drainages of northern New South Wales (NSW) from the Barrington Tops district to the Richmond River system. The nymphs seem to be restricted to riffles in forest streams and occur over a wide altitudinal range with records up to 940 m. They appear identical to those of Austremerella picta Riek, but rearing to the adult is needed to be certain that they represent the same species. The apparent restriction of Australian Teloganodidae to southern Queensland and northern NSW poses a biogeographical puzzle.

Small‐scale movements of lotic macroinvertebrates with variations in flow

Summary1. The small‐scale movements and distribution patterns of invertebrates were observed in an attempt to identify the various mechanisms by which organisms may use flow refugia during flow disturbances. The microdistribution of lotic macroinvertebrates was examined in two replicate, non‐circulating laboratory flumes with variations in flow among microhabitat patches (≈ 0.015–0.035 m2). The discharge in one experimental flume was manipulated to mimic spates and alter near‐bed flow patterns; the other flume acted as a control. After an initial settling period, the position and behaviour of animals within the flumes was recorded before, during and after a simulated spate. Three species with contrasting flow microhabitat preferences and movement behaviour were examined.2. At low discharge, the microdistribution of all three study species in flumes was broadly consistent with field observations. In the field, the optimum current speed was lowest for adults of the dytiscid beetle, Oreodytes sanmarkii, and highest for mayfly nymphs, Ephemerella ignita, with nymphs of the stonefly, Leuctra inermis, most abundant at intermediate velocities. In the flumes, O. sanmarkii occurred only in very low velocity areas, L. inermis occurred widely throughout the flumes with highest density in low velocity areas and E. ignita also occurred throughout the flumes, but maximum density was in moderately high velocity areas.3. Increased discharge did not reduce the total number of individuals in experimental versus control flumes for any of the three species studied, although total numbers did decrease over the observation period in both treatments. Simulated spates resulted in a change in the microdistribution of O. sanmarkii and E. ignita, but not L. inermis, such that numbers were reduced in very high velocity microhabitats and animals accumulated in lower flow areas, analogous to flow refugia. These distributional shifts were attributed to movements of individuals among microhabitats.4. Both active and passive modes of movement contributed to the accumulation of E. ignita and O. sanmarkii in low flow microhabitats (i.e. flow refugia). Some nymphs of E. ignita actively crawled from high to low flow microhabitats. Both species drifted between microhabitats. Drift entry could be active or passive, whereas regaining the substratum was active: O. sanmarkii swam down and E. ignita altered its body posture to promote sinking.

Two New Harpellales Living in Ephemeroptera Nymphs in Colorado Rocky Mountain Streams

Two new species of harpellid gut fungi (Zygomycota: Trichomycetes) are described from the hindguts of mayfly nymphs inhabiting high altitude Rocky Mountain streams: the new genus and species Legeriosimilis tricaudata living in Ameletus sp. (Siphlonuridae), and the new species Glotzia coloradense from Baetis tricaudatus (Baetidae). Legeriomyces aenigmaticus is reported from a new site and a new ephemeropteran host, Ephemerella sp. (Ephemerellidae), and previously unknown zygospores are described.

Two new Harpellales living in Ephemeroptera nymphs in Colorado Rocky Mountain streams

Two new species of harpellid gut fungi (Zygomycota: Trichomycetes) are described from the hindguts of mayfly nymphs inhabiting high altitude Rocky Mountain streams: the new genus and species Legeriosimilis tricaudata living in Ameletus sp. (Siphlonuridae), and the new species Glotzia coloradense from Baetis tricaudatus (Baetidae). Legeriomyces aenigmaticus is reported from a new site and a new ephemeropteran host, Ephemerella sp. (Ephemerellidae), and previously unknown zygospores are described.

Rapid size-specific changes in the drift of Baetis bicaudatus (Ephemeroptera) caused by alterations in fish odour concentration.

Chemical cues from fish can alter the behaviour of stream invertebrates in experimental tanks but their effect in natural streams has received little attention. By adding brook trout (Salvelinus fontinalis) odour to a trout stream in the Rocky Mountains of Colorado, USA, we tested whether changes in the concentration of chemical cues from visually feeding predatory fish would alter the drift of mayfly nymphs (Ephemeroptera). Stream water was piped from stream-side tanks with (odour) and without (control) three brook trout to two locations in the stream 3.5 m upstream of drift nets at six replicate sites. Five-minute drift samples were collected downstream from odour and control pipes before, during and after the release of water from the tanks into the stream during both the day and night. Almost all drift occurred at night and consisted predominantly of Baetis bicaudatus nymphs. The odour manipulation had no measurable effect on Baetis drift during the day but statistical power was low. During the night, however, the drift of large (>0.65 mm head capsule width, HCW) Baetis nymphs decreased significantly during the odour addition compared to control drift. In contrast, the drift of small nymphs (≤0.65 mm HCW) increased both during and after the odour addition in comparison to control drift. Since the stream contains brook trout (0.04-0.18 m-2), and water from the stream (presumably containing fish odour) altered the behaviour of fishless-stream Baetis nymphs in another experiment, we conclude that the changes in Baetis drift density were a response to an increase in the concentration of fish odour in the stream. Furthermore, we were able to detect the effect within 5 min. of odour addition, indicating that mayfly behavioural response to trout odour was rapid. These results suggest that mayflies can distinguish different concentrations of trout odour in natural streams and that the response is size-specific, according to the relative risk of predation of large and small Baetis.

Effects of drift- and benthic-foraging fish on the drift dispersal of three species of mayfly nymphs in a Japanese stream

Differences in the effects of two common predatory fishes, rainbow trout (Oncorhynchus mykiss) and freshwater sculpin (Cottus nozawae), on drift dispersal of the nymphs of three mayfly species, Baetis thermicus, Cinygmula sp., and Paraleptophlebia chocorata, were examined by manipulative field experiments using stream enclosures in a northern Japanese stream. The diel periodicity of prey consumption rate differed considerably between the two fishes; the prey consumption of the drift-foraging trout was greater at both dawn and dusk, whereas the benthic-foraging sculpin showed a higher consumption rate at night. The drift propensity at night was much greater than during the day for all the mayfly species in each of the trout, sculpin and fishless treatments, as well as under natural conditions with both fishes present. The nighttime drift propensities of the epibenthic grazers, B. thermicus and Cinygmula sp., were greater in the sculpin treatment than in trout and fishless treatments, and they did not differ between the trout and fishless treatments. By contrast, the nighttime drift propensity of P. chocorata, an infaunal collector-gatherer, did not differ among the treatments. The daytime drift propensity did not differ among the treatments for any of the mayfly species. Thus differences in drift response to the predatory fishes could be related to the characteristic microhabitats of the mayfly species. Moreover, the experiments demonstrated that benthic-foraging sculpin had greater overall effects on prey drift than drift-foraging trout.

Flat-Tipped Sensillum in Baetidae (Ephemeroptera): A Microcharacter for Taxonomic and Phylogenetic Considerations

In some mayfly nymphs, family Baetidae, flat-tipped sensilla occur on antennal segments and on other body parts (tergal borders and cerci). These fine, flat-tipped setae have an apical pore, a basal tubular body, and a branched dendrite along the shaft, an organization consistent with a chemo-mechanosensory function. Phylogenetic considerations are presented on the basis of a survey on the presence or absence of flat-tipped sensilla in some members of the Baetidae. The results of this investigation, and those of other authors, support the hypothesis that this sensillum is an apomorphic character. Additional key words: mayflies, sensilla, phylogeny, taxonomy Insect sensilla have mainly been studied from two different perspectives: first, from the morphological and electrophysiological point of view, to examine their specific function (Crnjar & Prokopy 1982; Kapoor & Zachariah 1983; Keil & Steinbrecht 1984; Mclver 1985; Zacharuk 1985; Solinas et al. 1987; Stadler et al. 1994; Isidoro et al. 1996), and second, for the taxonomic relevance assigned to these structures, including setae, scales, and spines. In the Ephemeroptera, cuticular armatures represent a useful set of characteristics that taxonomists have considered in comparing different species. Scanning electron microscopy (SEM) has allowed new microcharacters to be highlighted. In this regard, Morihara & McCafferty (1979) used SEM in the taxonomic revision of the North American genus Baetis to examine and analyze the setae, scales, and spines of the nymphs. Among the three main types of setae studied by these authors, hairlike or bristlelike fine setae that were distinctly clubbed apically were considered to be phylogenetically important traits. This kind of fine sea Author for correspondence. E-mail: gaino@unipg.it tae has been described in the antennae of Baetis rhodani (Baetidae) and of Rhithrogena loyolaea and Epeorus sylvicola (Heptageniidae) (Gaino & Rebora 1996). The location of this setal type and the presence of an apical pore have supported the hypothesis that it could be a sensillum, defined as a flat-tipped sensillum (Gaino & Rebora 1996). Ultrastructural serial sections showed the presence of both a tubular body and the dendrites extending along the shaft, further supporting the hypothesis of a chemo-mechanosensory function (Gaino & Rebora 1998). Considering the possible systematic relevance of this sensillum to systematics, we have documented its presence (or absence) and its distribution in species representing almost all the European genera of the family Baetidae: Acentrella, Alainites, Baetis, Centroptilum, Cloeon, Labiobaetis, Nigrobaetis, Procloeon, and Pseudocentroptilum. For comparison, we examined the antennae of the New Zealand mayfly Siphlaenigma janae, belonging to Siphlaenigmatidae, generally regarded as the sister group of Baetidae. In this paper we report that flat-tipped sensilla are a Figs. 1-4. Flat-tipped sensilla in Baetis rhodani. Fig. 1. The sensillum connects two consecutive antennal articles. Note the pore (P). Proximal article (Px); distal article (Ds). Fig. 2. Basal region of a flat-tipped sensillum. Note the tubular body (TB) in longitudinal section. Inset: distal portion of the sensillum in cross section, with dendrites in the middle (arrow). Fig. 3. Numerous sensilla (arrowheads) located on a cercus. Fig. 4. Sensilla on a urotergite (arrows). Figs. 5-9. Distal border of antennal articles in several species that lack flat-tipped sensilla but which may present other kinds of fine setae (arrows). Fig. 5. Siphlaenigma janae. Fig. 6. Centroptilum luteolum. Fig. 7. Cloeon dipterum. Fig. 8. Procloeon bifidum. Fig. 9. Pseudocentroptilum pennulatum. This content downloaded from 157.55.39.247 on Wed, 27 Apr 2016 06:14:50 UTC All use subject to http://about.jstor.org/terms Flat-tipped sensillum as microcharacter

Flightlessness in mayflies and its relevance to hypotheses on the origin of insect flight

Until now, only fully winged mayflies have been known. It has been proposed recently that brachyptery could be a missing link in the development of insect flight, via sailing or skimming aquatic insects. To our knowledge, we report here the first documented case of brachyptery in mayflies. The flightless genus Cheirogenesia is endemic to Madagascar, and the adults skim the water surface. This loss of the flight function has induced important physiological changes, such as a shift from lipids to carbohydrates in the energy reserves used during their adult life. Comparison of wing area of living mayflies with fossil species indicates that brachyptery could have already occurred in early flying insects (in the Permian). We argue that flight loss in Cheirogenesia has been made possible by the lack of fish predation in its natural habitats.

Influences of natural acidity and introduced fish on faunal assemblages in California alpine lakes

In an alpine area of the Sierra Nevada of California, naturally acidic waters and introduced fishes both strongly affect the distributions of native amphibians, zooplankton, and macroinvertebrates. The study area in Kings Canyon National Park contains 104 lakes with pH values between 5.0 and 9.3, including 10 lakes with pH < 6.0 (defined here as acidic lakes) and 18 lakes with introduced trout. We surveyed 33 of these lakes (8 acidic, 7 non-acidic with trout, 18 non-acidic without trout) for water chemistry and faunal assemblages. Yellow-legged frog tadpoles (Rana muscosa), common microcrustaceans (Daphnia, Hesperodiaptomus, Diaptomus), and larvae of a caddisfly (Hesperophylax) were rare or absent in acidic lakes but common in non-acidic lakes, and microcrustacean and macroinvertebrate species richness decreased with decreasing pH. Large and (or) mobile, conspicuous taxa, including tadpoles, large-bodied microcrustaceans (Hesperodiaptomus, Daphnia middendorffiana), and many epibenthic or limnetic macroinvertebrates (baetid and siphlonurid mayfly nymphs, notonectids, corixids, limnephilid caddis larvae, and dytiscid beetles), were rare or absent in trout lakes but were relatively common in lakes lacking trout, and the taxon richness of macroinvertebrates was reduced by trout.