Mayfly Species Research Articles

Mayflies (Ephemeroptera) of North Carolina and South Carolina: An Update

A total of 204 valid species representing 56 genera in 18 families of mayflies are known to occur in North Carolina and South Carolina. Twenty-six of the 182 species of mayflies that are known from North Carolina are reported for the first time. South Carolina has 170 species of mayflies recorded, of which 26 are new state records. Taxonomic changes relevant to the mayfly fauna of both states are discussed.

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.

Influence of food type on the growth of early and late instars of three mayfly (Ephemeroptera) species

Growth of Ephemerella subvaria, Epeorus sp. and Paraleptophlebia sp. were determined while on diets restricted to diatoms, microbially colonized leaves, uncolonized leaves, or cultured bacteria. Early-instar E. subvaria showed significantly higher percent weight increases (>1011 %) on diatom and colonized leaf diets than uncolonized leaves and bacteria, whereas late instars showed significant weight increases only on diatoms: both leaf diets and cultured bacteria produced little growth (<30%) in late instars. Early instar Epeorus larvae showed similar growth responses (20-60% increase) for all four diets in early instars. However. late instars of Epeorus showed significant growth (60% only on diatoms; little or no growth was shown for leaf diets and bacteria. In contrast, early-instar Paraleptophlebia lost weight on diatoms and showed significant growth (ca. 30%) for both leaf diets and bacteria. Late-instar Paraleptophlebia showed a 25 % weight increase for all four diets. Our data suggest that early instars of all three species may rely heavily on detrital food resources, whereas late instars of E. subvaria and Epeorus appear to shift to algal resources; these two species also may occupy different functional-feeding groups at different developmental stages since E. subvaria and Epeorus larvae did shred leaves in their early instars. Paraleptophlebia larvae showed little evidence of leaf-shredding activity but instead appeared to act as detritivores (feeding on fine particulates even from leaf surfaces) throughout their life cycle.

The role of parthenogenesis in the biology of two species of mayfly (Ephemeroptera)

1. From a single founder virgin female of each species, nine and seven successive parthenogenetic generations of Cloeon dipterum and Cloeon simile, respectively, were reared: all offspring were female. Eggs were removed from final instar nymphs, subimagos and virgin imagos of each generation and their development assessed. The life cycles of 487 individual C. dipterum and 315 C. simile were recorded, together with details of growth rate, egg production and adult size of these and other representatives of each generation. In order to distinguish between the effects of parthenogenesis and the effects of culture conditions, fertilized eggs of both species were reared under the same laboratory conditions.2. Fewer unfertilized than fertilized eggs hatched. There was no consistent trend in the percentage hatch in successive parthenogenetic generations. In both species the time taken for parthenogenetic individuals to complete a generation varied considerably, and no relationship was found between generation length and the number of parthenogenetic generations that had passed, the developmental stage of the donor, or the time taken for the eggs to hatch. After 50 weeks there was, in both species, an overlap of up to six generations. The number of eggs produced by both species was significantly greater in the first parthenogenetic generation than in later generations but the numbers for later generations lie within the recorded range of egg number in imagos captured in the field.3. To find if parthenogenetic individuals occur in the field, the sex ratios of last instar nymphs and subimagos of C. simile were measured in samples collected over 13 successive years from a set of small isolated pools. There was a female bias both in winter and at the start of the emergence season. It is possible that, when mature nymphs and emerging subimagos are damaged by predators, the eggs which are released from them survive and develop.4. Mature, egg‐containing nymphs of both species were present in the field throughout the year, although their numbers were small at the beginning of winter.5. Oviposition by imagos captured from mating swarms and by virgin imagos, of both species, was recorded for the first time, showing that C. dipterum is not always ovoviviparous. It is suggested that early instar nymphs occurring in mid‐winter come not only from unfertilized eggs released from injured nymphs and subimagos, but also from fertilized eggs laid in autumn.

The effect of continuous physical disturbance on mayflies of a tropical stream: an experimental approach

Field experiments to examine the effect of continuous physical disturbance on the Ephemeroptera of the Naro Moru River, Kenya, wereundertaken from June 1993 to January 1994. Continuous disturbance wasadministered on a randomly selected subsite of the sediment surface.Artificial physical disturbance within the experimental subsite involvedcontinuous local displacement, shifting and stirring of the streambedsurface substrates (about 10 cm depth) by hand every one minute for 10 or 14min. Three control samples were also taken randomly from the sedimentsurface of an undisturbed stratified area of the study riffle at the startof each disturbance occasion. All samples were collected using a Hesssampler (surface sampling area of 3.142 dm2; meshsize 80μm). Seven mayfly species were particularly abundant and these included Afronurus sp., Afroptilum sudafricanum LESTAGE, Baetis s.l., Baetis(Nigrobaetis) sp. 1, Baetis (Nigrobaetis) sp. 2, Caenis sp. and Choroterpes(Euthraulus) sp. About 83,8% of the total mayfly density and88.1% of the biomass were removed from the streambed surface withinthe first three minutes of continuous physical disturbance. A mayfly biomassof 33.7391 mg dm2 and total density of 1357.6 inddm2 were collected from the disturbed subsite during the studyduration. Further, a biomass of about 42.8335 mg dm−2and total density of 2366 ind dm−2 were collected fromthe control sites. There was a near-complete depletion of mayflyindividuals from the topmost sediment layer within 14 min of continuousdisturbance.

Overall pesticide effects on growth and emergence of two species of Ephemeroptera in a model stream carrying pesticide- polluted river water

The overall pesticide effects on mortality, growth and emergence of two dominant species of Ephemeroptera in Japanese rivers, Epeorus latifolium and Ecdyonurus yoshidae, were assessed using an outdoor channel carrying water from the Kokai River. Young larvae collected from another river were introduced into cages in the channel after their body lengths were measured. The concentrations of 17 pesticides were measured three times a week from April to August 1993. A shrimp mortality test on water samples was conducted concurrently. A relationship between the high mortality of the shrimp in the water samples and of E. latifolium in the channel was recognized. The mortality of E. yoshidae increased only when the shrimp mortality increased drastically in early June, reflecting the difference in insecticide susceptibility between the two mayfly species. Almost all the larvae which had been introduced into the channel in winter and/or early spring, when pesticides had cleared from the river, emerged in spring, although their growth rate during the winter was low. The shrimp mortality in the river water samples was caused by the overall pesticide toxicity. The increase in the mortality of the mayfly larvae in the channel might be due to the overall pesticide toxicity, although their concentrations were low and varied independently

Determining the influence of habitat and chemical factors on instream biotic integrity for a Southern Ohio watershed

The relative influences of water quality and in-streamhabitat on receiving water biology were investigatedin the Little Miami River, Ohio, a national and statescenic river located in the southwest corner of thestate. Monitored sample data for water chemistry,habitat, invertebrates and fish were obtained fromfederal and state sources and integrated via ageographical information system (GIS), using a uniquestream segmenting system. Locations and flowinformation for municipal wastewater treatment plants(WWTPs) were also integrated into the system. Therelative importance of water chemistry and habitat onbiological sample information was determined usingmultivariate forward stepwise regression. Theprincipal environmental factors that influencedbiological responses were in-stream habitat andpercent cumulative WWTP effluent at mean flow. Cumulative WWTP effluent at low flow dilution,nutrients and total toxic units from metals andammonia did not correlate with biological responses(e.g. number of mayfly species, invertebrate communityindex (ICI), number of fish species and index ofbiotic integrity (IBI)). We found the GIS-based datamethods to be highly useful for understanding theprincipal processes that guide receiving water health.

Effects of hydropower-induced flow perturbations on mayfly (Ephemeroptera) richness and abundance in north Swedish river rapids

Mayfly species richness and abundance were investigated at 52 Swedish river rapids. These were either unregulated, or regulated with or without reduced discharge. Sites impacted by regulation had lower mayfly richness and abundances than unregulated reference sites. The relative importance of reduced discharge, daily fluctuations in flow, flow constancy, and the distance to nearest rapid (as a measure of isolation) was evaluated in multiple regression analyses. These indicated negative effects of diel fluctuations on both abundance and richness, whereas flow constancy was favourable for richness. Neither the distance to nearest rapid nor reduced flow were significantly related to mayfly richness and abundance. In total, 26 mayfly species were recorded. Only Baetis rhodani was found at all sites, but another five species were present at more than 40 sites. The strongest effects were found for species within the Heptageniidae. Nineteen of 20 mayfly species present in both the regulated (with unreduced flow) and unregulated reference rapids were on average more common in the unregulated ones. Mayfly assemblage structure was primarily influenced by regional factors and nutrient status, although daily fluctuations in flow together with rapids dimensions also had a significant influence. No rare species appears to be threatened by hydropower regulation though it is conceivable that depressed abundances in regulated rivers indirectly influence predators and periphyton.

Life History and Productivity of Hexagenia limbata (Ephemeroptera: Ephemeridae) and Selected Physico-Chemical Parameters in Two Tributaries of the Wekiva River, Central Florida

Nymphal densities and size frequency distribution of Hexagenia limbata (Serville), a burrowing mayfly, were determined monthly for two years in two tributaries of the Wekiva River, central Florida, along with selected physico-chemical water parameters. Blackwater Creek supported a mean number of 112 (range 0-434) and Rock Springs Run 89 nymphs per m 2 (range 11-258). The mayfly species displayed a univoltine life history with adult emergence peaking in August. Productivity in Blackwater Creek was estimated at 4.688 g per m 2 and in Rock Springs Run at 3.123 g per m 2 . Productivity/biomass ratios were estimated at 4.09 and 4.59 in Blackwater Creek and Rock Springs Run, respectively. The nymphal densities in both streams were positively correlated with water pH and negatively correlated with water volume. Apparently, water volume was the overriding abiotic factor in both streams, influencing several measured water parameters and as well as nymphal populations during the study period.

Life histories, secondary production, and microdistribution of heptageniid mayflies (Ephemeroptera) in a tropical forest stream

The ecology of five species of heptageniid mayflies was investigated in Tai Po Kau Forest Stream (TPKFS), New Territories, Hong Kong, over a two‐year period (1977‐79). Quantitative benthic samples were taken from different microhabitats in the stream on 47 occasions and, in combination with light‐trap catches of adult mayflies, were used as a basis for investigating life‐history patterns and estimating secondary production. Electrogena sp. (49.8 individuals 0.5 m−2; 60% of total heptageniid densities) and Cinygmina sp. (15.7 individuals 0.5 m−2) were the most abundant heptageniids in TPKFS, followed by Epeorus sp. (9.1 individuals 0.5 m−2). Iron sp. and Paegniodes cupulatus, which were present in relatively low densities, made up &lt; 10% of total heptageniid abundance. In terms of mean biomass (1977‐79), Electrogena sp. and Cinygmina sp. were equally important (10.3 vs. 10.5 mg ash‐free dry weight (AFDW 0.5 m−2), while Epeorus sp. was ranked third (6.8 mg AFDW 0.5 m−2). There was, however, considerable inter‐year variation in heptageniid standing stocks: mean densities and biomass of Cinygmina sp. in 1977‐78 were (respectively) eight and nine times greater than in 1978‐79, while Iron sp. and P. cupulatus exhibited the converse trend, with densities more than twice as high and biomass more than three times greater in 1978‐79. Heptageniid abundance in TPKFS seemed to follow an annual pattern of wet‐season decrease and dry‐season increase, but this tendency was more apparent in some species (e.g. Iron sp.) than others (e.g. Cinygmina sp.). The life histories of TPKFS heptageniids were unclear. All species showed rather stable larval size‐frequency distributions throughout the study period, which was indicative of asynchronous growth and continuous recruitment. Adult flight periods did not exhibit strong seasonality. Calculations of secondary production were based upon the conservative assumption that each species was univoltine, but multivoltine life histories may have been possible for these tropical mayflies.Total heptageniid annual production over the period 1977‐79 was 212.0 mg AFDW 0.5 m−2, and was slightly higher during 1977‐78 than in 1978‐79 (225.0 vs. 202.1 mg AFDW 0.5 m−2). Inter‐year variations in production reflected variations in annual mean biomass: production of Cinygmina sp. was 7.9 times greater in 1977‐78 than in 1978‐79, but production of Iron sp. was 2.7 times higher during 1978‐79, and that of P. cupulatus was 4.6 times greater. As a consequence of these inter‐year variations, species‐specific production decreased in the order Cinygmina sp. &gt; Electrogena sp. &gt; Epeorus sp. &gt; Iron sp. &gt; P. cupulatus in 1977‐78, but the sequence was Electrogena &gt; Iron sp. &gt; Epeorus sp. &gt; P. cupulatus &gt; Cinygmina sp. in 1978‐79. For the study period as a whole (1977‐79), the order was Electrogena sp. &gt; Cinygmina sp. &gt; Epeorus sp. &gt; Iron sp. &gt; P. cupulatus. Production:biomass (P:B) ratios varied from 4.8 (Cinygmina sp., 1977‐78) to 11.1 (Iron sp., 1977‐78), and P:B ratios (1977‐79) decreased in the order Iron sp. &gt; Epeorus sp. &gt; Electrogena sp. &gt; P. cupulatus &gt; Cinygmina sp. Production calculations and P:B ratios are highly sensitive to the conservative assumptions made about heptageniid voltinism in TPKFS, and therefore may be significant underestimates. Epeorus sp., Iron sp., and P. cupulatus were most abundant in midstream microhabitats but Cinygmina sp. was more numerous close to the stream banks. The microdistribution pattern of Electrogena sp. was less distinct, but larvae seemed to favour midstream microhabitats. Multivariate statistics revealed that the microdistribution of all heptageniids (except Electrogena sp.) was influenced significantly by sediment grain‐size characteristics rather than by algae or detritus, but the proportion of variation (4‐14%) in the abundance of each heptageniid species accounted for by sediment characteristics was low.

Alternative Predator Avoidance Syndromes of Stream‐Dwelling Mayfly Larvae

Experiments were conducted to compare the patterns, mechanisms, and costs of predator avoidance behavior among larvae of five species of mayflies that co—occur with the predatory stoneflies, Megarcys signata and Kogotus modestus in western Colorado streams. Mayfly drift dispersal behavior, use of high vs. low food (periphyton or detritus) patches, microhabitat use, positioning, and activity periodicity were observed in the presence and absence of predators in circular flow—through chambers using natural stream water. Also, distances from predators at which prey initiated escape responses were compared among prey and predator species. Costs of predator avoidance behavior were assessed by measuring short—term (24 h) feeding rates of mayflies in the presence or absence of predatory stoneflies whose mouthparts were immobilized (glued) to prevent feeding. The intensity and associated costs of predator avoidance behavior of mayfly species were consistent with their relative rates of predation by stoneflies. Megarcys consumes overwintering generation Baetis bicaudatus &gt; Epeorus longimanus &gt; Cinygmula = Ephemerella; Kogotus consumes summer generation Baetis &gt; Epeorus deceptivus = Cinygmula; Megarcys eats more mayflies than Kogotus. While Megarcys induced drift by Baetis, Epeorus, and Cinygmula, this disruptive predator avoidance behavior only reduced food intake by Baetis and Epeorus. The morphologically defended mayfly species, Ephemerella, neither showed escape behavior from Megarcys, nor any cost of its antipredatory posturing behavior. Only Baetis responded by drifting from Kogotus. No mayfly species shifted microhabitats or spent less time on high—food patches in the presence of foraging stoneflies. However, predators enhanced the nocturnal periodicity of Baetis drift, which was negligible in the absence of stoneflies as long as food was abundant. Lack of food also caused some microhabitat and periodicity shifts and increased the magnitude of both day and night drift of Baetis. Thus, Baetis took more risks of predation by visual, drift—feeding fish not only in the presence of predatory stoneflies, but also when food was low or they were hungry. All other mayflies were generally nocturnal in their use of rock surfaces, as long as food was abundant. Finally, the distances at which different mayfly species initiated acute escape responses were also consistent with relative rates of predation. This study demonstrates alternative predator avoidance syndromes by mayfly species ranging from an initial investment in constitutive morphological defenses (e.g., Ephemerella) to induced, energetically costly predator avoidance behaviors (e.g., Baetis). Although the costs of Ephemerella's constitutive defense are unknown, experiments show that prey dispersal is the mechanism underlying fecundity costs of induced responses by Baetis to predators, rather than microhabitat shifts to less favorable resources or temporal changes in foraging activity. A conceptual model suggests that contrasting resource acquisition modes may account for the evolution and maintenance of alternative predator avoidance syndromes along a continuum from Baetis (high mobility) to heptageniids (intermediate mobility) to Ephemerella (low mobility). Prey dispersal (swimming) to avoid capture results in reduction of otherwise high fecundity by Baetis, which trades off morphological defense for enhanced ability to acquire resources. Thus, improved foraging efficiency is the selection pressure maintaining the highly mobile life style in Baetis, which increases resource acquisition and fecundity, offsetting the high mortality costs associated with this behavior.

Fine Structure of Flat-Tipped Antennal Sensilla in Three Species of Mayflies (Ephemeroptera)

A flat-tipped type of sensillum was studied with the scanning electron microscope in three species of mayflies (Ephemeroptera): Baetis rhodani, Rhithrogena loyolaea, and Epeorus sylvicola. This sensillum bears an apical pore and its slightly enlarged tip forms a flange. In B. rhodani, flat-tipped sensilla are distributed over the surface of the antennal articles, except on the flagellum, where they are arranged along the distal border of each article to form bridges between consecutive articles. In R. loyolaea and E. sylvicola, flat-tipped sensilla show a characteristic arrowhead shape, emerge farther from the distal border of each flagellar article, and are not present on scape and pedicel. They are irregularly distributed, fewer than those of B. rhodani, and rarely reach the adjoining antennal article. On the basis of morphology and location, we hypothesize that flat-tipped sensilla function both as mechanoand chemoreceptors. Additional key words: integumentary structures, Baetidae, Heptageniidae Insect sensilla are involved in monitoring the external environment. An exhaustive review of sensilla, including their categorization, organization, and function, has been published by Zacharuk (1985). Likewise, Mclver (1985) has given a detailed description of a variety of structures involved in mechanoreception. Among the wide range of sensilla, particular interest has been paid to antennal receptors (Mclver 1973, 1974; McIver & Siemicki 1979; Barlin et al. 1981; Solinas et al. 1987; Catala & Schofield 1994; Schmidt & Berg 1994; Nicastro et al. 1995). Sensilla of aquatic insects have been the subject of several studies (Rupprecht 1969; Kapoor & Zachariah 1983; Kapoor 1985a,b, 1986, 1988, 1991; Mclver & Beech 1986; Jensen & Zacharuk 1991, 1992). In mayflies (Ephemeroptera), the distribution of sensilla has been used as a taxonomic trait, but sensillum morphology has not attracted much attention. The first description of ephemeropteran sensilla, on gills of Caenis, was by Eastham (1936). As for antennal sensilla, a general account of those in Baetidae can be found in Muller-Liebenau (1969), but ultrastructural investigation is limited at present to the description of mechanoreceptors of the pedicel in a few species (Schmidt 1974). Adult mayflies are short lived and the aquatic immature stages, or nymphs, dominate the life cycle. As a consequence, most behavioral activity depends on the sensory mechanisms of immature stages, as in other insect groups characterized by a short adult phase (Kapoor 1985a). We compared the antennal sensilla of mayfly nymphs with those described in the literature for other insects. Our studies have revealed a sensillum with an

Acentrella feminalis new combination for an oriental Baetis (Ephemeroptera: Baetidae)

The Sri Lankan mayfly species Baetis feminalis is newly combined in Acentrella based on the distinctive anterior process of the mesothorax of the adult type material.

Microhabitat and Activity Periodicity of Predatory Stoneflies and Their Mayfly Prey in a Western Colorado Stream

Experiments were conducted to determine whether overlap between microhabitat preferences and activity periodicities of four mayfly species and their stonefly predators could explain species-specific differences in predator-prey encounter frequencies. Preferences for rock type (slate or granite), flow microhabitat (high or low), rock surface (top, bottom, upstream or downstream sides), and periodicity of drift and the use of rock tops were measured in a stream-side system of flow-through circular Plexiglas chambers receiving natural stream water and light levels. These parameters were compared among the predatory stoneflies, Megarcys signata or Kogotus modestus, and four species of mayflies that vary in their encounter rates with the stoneflies. Based on predator-prey encounter rates previously observed in similar chambers, we expected greater overlap between Megarcys and Ephemerella infrequens and the overwintering generation of the bivoltine mayfly, Baetis bicaudatus than with Cinygmula sp. Likewise, we expected Kogotus microhabitat use to overlap more strongly with that of summer generation Baetis than with later instars of Cinygmula and Epeorus deceptivus. Results ran counter to our predictions, indicating that microhabitats of the prey species with high predator encounter rates did not overlap more strongly with the stoneflies than did mayflies with low predator encounter rates. Most mayflies and stoneflies preferred the bottom surfaces of granite rocks, and showed few flow preferences. Most were nocturnal in their use of top rock surfaces, in drift and feeding activity periodicity. Therefore, nocturnal activity periodicities of both mayflies and stoneflies confirm that mayflies have not evolved feeding periodicity to avoid encounters with foraging stonefly predators. We conclude therefore, that neither temporal nor spatial microhabitat overlap is a reasonable explanation of differential encounter rates between predatory stoneflies and their mayfly prey. Alternative explanations for differential encounter rates are that more abundant or more mobile mayflies have higher encounter rates with predators, and effective pre-contact predator avoidance responses of other mayflies reduce their encounter rates with stoneflies.

Cadmium and methylmercury bioaccumulation by nymphs of the burrowing mayflyHexagenia rigida from the water column and sediment

Based on a three compartment microcosm-water column, natural sediment,Hexagenia rigida nymphs-an experimental study was set up to compare cadmium (Cd) and methylmercury (MeHg) bioaccumulation by a burrowing mayfly species, after exposure via the water column or the sediment as initial contamination sources. Results from a wide concentration range for each exposure condition revealed very marked differences between the two metals: MeHg was readily accumulated from the two contamination sources, leading to important metal concentrations in the nymphs after the 2 weeks' exposure; Cd bioaccumulation, on the other hand, was negligible when the metal was added to the water compartment, even though significant transfers were observed from the sediment source. The average Cd concentrations in the nymphs were proportional to the sediment contamination levels. Turbidity measurements in the water column, reflecting the bioturbation activity of the nymphs, revealed that the effect of Cd was significant, but only when the metal was initially added to the sediment. The results are discussed according to the uptake routes and the structural and functional properties of the biological barriers involved (gills and gut).

Size‐related shifts in the physical habitat of two mayfly species (Ephemeroptera)

SUMMARY1. The spatial niche shifts of the nymphs of two mayfly species (Rhithrogena semicolorata and Ecdyonurus sp, gr. venosus) in relation to their growth were investigated in two lowland springs of northern Italy where they are dominant species.2. Ordination of the different size classes of the two species revealed a canonical trend related to season and bottom roughness.3. Larger nymphs of both species colonized rougher substrata. Differences between species in the roughness of areas colonized by small nymphs, which were very numerous, may have been due to competition for space.

Two new Canadian species of the genusTricorythodesUlmer, with additional studies on other North American species (Insecta, Ephemeroptera: Leptohyphidae)

Two new mayfly species (Tricorythodes mosequs n.sp. and T. cobbi n.sp.) are described from the Assiniboine River, Manitoba, Canada. Taxonomic knowledge of the North American species of the genus Tricorythodes is summarized. The type material of T. stygiatus McDunnough, T. atratus (McDunnough), and T. peridius Burks was studied and additional characteristics of these species are described. New drawings of the genitalia of these species are presented. Tricorythodes peridius and T. atratus are considered to be junior synonyms of T. allectus (Needham). A key to the imagines of Tricorythodes in Canada is included.

Emergence and upstream flight of lotic mayflies and caddisflies (Ephemeroptera and Trichoptera) in a lake outlet, central Finland

Temporal and spatial occurrence and upstream flight of lotic mayflies and caddisflies were studied by means of kick netting, slit traps and floating emergence traps in Lake Konnevesi and its outlet stream (Siikakoski), mainly in 1983. Twenty species of mayflies and 78 species of caddisflies were recorded. Clearly lotic mayflies in the material were Baetis subalpinus, Heptagenia sulphurea and Ephemerella mucronata, which occurred only in the stream samples. Females of lotic Baetis rhodani had a long distance upstream flight in the area and occurred frequently in the floating emergence traps especially in sandy littoral habitats of L. Konnevesi. Procloeon bifidum and Nixe joernensis, which in some regions occur mainly in streams, were frequently found in the stony littoral belt of L. Konnevesi. Most of the mayflies recorded in the area were univoltine, but Baetis rhodani and Centroptilum luteolum at least had two generations in 1983. Strictly lotic species among caddisflies were Ceratopsyche nevae, Hydropsyche saxonica, Halesus digitatus, Ceraclea perplexa, Ylodes detruncatus, Athripsodes commutatus and Hydroptila forcipata. These were mainly found in stream samples. A short distance upstream flight of females (c. 0.1-0.4 km) was observed in the populations of H. siltalai, Psychomyia pusilla and Oxyethira frici and a moderate distance flight (at least 0.6 km) by females of Hydropsyche pellucidula, Cheumatopsyche lepida and Polycentropus irroratus. The most abundant caddisflies of the stream Siikakoski (Brachycentrus subnubilus, Neureclipsis bimaculata, Rhyacophila nubila, Agapetus ochripes, Micrasema setiferum and Hydroptila cornuta) were frequently found also in floating emergence traps in the lake (3. 7 km from the rapids). Females of Brachycentrus, Neureclipsis and Rhyacophila favoured traps on sandy substrata while females of Hydroptila were usually found in the stony belt. In the trap material of Agapetus ochripes, males were more abundant than females. According to emergence data, all caddisflies recorded in the area were univoltine though the emerging period of several species such as Rhyacophila nubila and Neureclipsis bimaculata was long. Maximum emergence was observed in early June when brachycentrids were swarming and another peak in August when limnephilids and leptocerids were abundant.

Distribution, diversity, life cycle and growth of a mayfly community in a prealpine stream system (Insecta, Ephemeroptera)

Within the framework of a study on the ecology of Dippers in a prealpine stream system, the macroinvertebrate communities were also investigated, especially Ephemeroptera. Ten stations were sampled qualitatively, of which two (Saane and Turbach streams) were investigated quantitatively for 16 months (April 1983–July 1984). Fifteen mayfly species were found. The faunistic affinities among the 10 streams investigated reflect the main abiotic characteristics. There was no significant difference in species richness, diversity and evenness for Saane and Turbach streams. Life cycles of nine species were studied in these two watercourses. Those of Rhithrogena degrangei, Rh. hybrida, Rh. savoiensis and Ecdyonurus helveticus are described for the first time. Baetis alpinus is the only species to exhibit a bivoltine cycle, whereas all others have a univoltine summer or winter cycle. Specific growth rates were measured for most species. The summer generation of B. alpinus presents the highest growth rate (4.51 % dry weight d−1), whereas the lowest values were recorded for Rh. degrangei (0.76% dry weight d−1). The importance of abiotic variables, especially water temperature, is also discussed.

Egg and Larval Development Times for 35 Species of Tropical Stream Insects from Costa Rica

We examined total development times for 5 mayfly species, 2 stoneflies, 10 caddisflies, and 18 chironomid midges collected from three streams that flow through tropical evergreen forest in northwestern Costa Rica. Most eggs, larvae, and pupae were reared in the laboratory in a photoperiod of 12:12 LD and at 20°C, which simulated field conditions. Algae, algal detritus, and leaves were provided as food for all species; predators were also given various animal prey. All study species had total development times that were rapid relative to the univoltine life histories observed or assumed for many temperate species. Egg development times ranged from a few days to approximately a week for chironomids and from 10 to 38 days for mayflies, stoneflies, and caddisflies. Most chironomids had short larval/pupal development times: development was completed in 19-29 d by seven species, in 30-40 d by nine species, and in >50 d by two species. Relatively short development times (including the pupal stage when present) were also observed for the mayfly Acerpenna sp. (28 d) and the caddisflies Wormaldia sp. (45 d) and Oecetis nr. prolongata (52 d). Larval/pupal development times were longer for the other four mayflies (76-159 d), two stoneflies (83-167 d), and eight caddisflies (72-209 d). No evidence of egg or larval diapause was observed. The combination of rapid development and absence of diapause suggests that all these species have multivoltine life histories. This finding has important implications for temporal changes in the structure and function of the aquatic insect assemblage in these streams.