Hydropsychid Larvae Research Articles

<p class="ZootaxaTitle"><strong>Food Source for Hydropsychid Larvae during an Algae Bloom in Nan River, Nan Province, Thailand (Trichoptera: Hydropsychidae)</strong></p>

During November–March, blooms of Kai algae genera are commonly seen on rocks and cobblestones in the Nan River, providing habitat for hydropsychid larvae. This study attempted to determine a dietary relationship between the caddisflies and Kai algae by comparing gut contents of hydropsychid larvae between areas with and without Kai algae (Kai-blooming and Control sites). Fourteen specimens of Hydropsyche and Potamyia larvae were collected in the Kai-blooming and Control sites, respectively. Food items in the foreguts were classified as Kai algae (KA), other filamentous algae (OFA), diatoms (DT) and other items (OI). Although the main food type of larvae in both sites was Kai algae, the proportion of KA in larval foreguts from Kai-blooming sites was significantly higher than in those from Control sites (p < 0.05). In addition, larvae in the Kai-blooming area had a significantly lower proportion of OI than in the Control area (p < 0.05). Therefore, hydropsychid larvae tended to consume more Kai algae than diatoms and other filamentous algae during blooming periods. They tended to consume more Kai algae than would be expected by chance alone, even in the area with a small amount of Kai algae.

Insights into species diversity of the genus Hydropsyche Pictet, 1834 (Hydropsychidae, Trichoptera) from the Lake Kinneret catchment (Israel)

ABSTRACTSpecies identification based on larval stages of aquatic insects in Israel is difficult, given the paucity of taxonomic studies. In order to gain deeper understanding of biological processes in Israel's freshwater ecosystems and to develop a bioindication system, species have to be known and characterised at all life stages. Here, we applied an integrative morphological and genetic approach to characterise caddisflies of the genus Hydropsyche Pictet, 1834 from the Lake Kinneret catchment. Seventeen sites in the Lake Kinneret catchment were sampled in spring/summer 2015 and 2016. For 77 hydropsychid larvae and two adult specimens of H. yahfufah Malicky, 2001, we analysed a 658 bp fragment of the mitochondrial COI gene and compared the results of different genetic species delimitation methods to observed morphological variation. Our results suggest the presence of at least five species. This study is an important first step towards assessing stream biodiversity of a yet largely understudied region and highlights advantages of integrative taxonomic approaches.

Multiple predator effects on microdistributions, survival, and drift of stream hydropsychid caddisflies

We studied microdistributions, survival, and drift of larval hydropsychid caddisflies in the physical and/or chemical presence of 2 types of benthic predators with different foraging modes. Sculpins (Cottus bairdi, C. cognatus) are ambush predators, whereas perlid stoneflies (Acroneuria, Paragnetina) are stalkers, and stoneflies are an intermediate predator consumed by sculpins. Hydropsychid larvae in a northern Michigan stream were significantly more abundant in crevices than on flat surfaces on real cobbles in riffles with both predators. Larval colonization on experimental substrates (bricks) in 7 riffles was greater in crevices (grooves) than on flat surfaces, and the greatest differences in larval densities between these microhabitats occurred where predator densities were highest. Mean size (head capsule width) of larvae in both microhabitats was negatively related to densities of sculpins, but not stoneflies. A multifactorial experiment done in artificial stream channels revealed that caddisflies selected crevices over flat surfaces even in predator-free conditions. Sculpin and stonefly effects on hydropsychid survival were additive, suggesting a lack of multiple predator effects. Crevices provided a refuge from predators; however, stoneflies were more effective than large sculpins (>65 mm total length) at consuming hydropsychids in crevices. Like caddisflies, stoneflies predominantly occupied crevices, but stonefly crevice use and activity were not affected by sculpins, and no stoneflies were consumed by sculpins. Caddisfly drift was ∼3 to 4× greater in the physical presence of each predator than in predator-free channels. However, drift by caddisflies in the physical presence of both predators was lower than expected (i.e., nonadditive), suggesting it is less risky to remain in retreats (i.e., immovable cases) than to drift when both predators are present. Elevated chemical cues of either predator did not trigger drift responses by these sedentary prey. Crevice use and drift appear to be key mechanisms enabling larval hydropsychid caddisflies to coexist with a multispecies complex of predators.

Drifting or walking? Colonisation routes used by different instars and species of lotic, macroinvertebrate filter feeders

Many views of stream invertebrate populations centre on drift as the major route of larval dispersal, but few studies have presented unambiguous information about the role of drift. We present the results from an experiment designed to determine whether the major route of colonisation of substrata by hydropsychid larvae (commonly found in the drift) is by drifting directly onto substrata or by walking along the stream bottom. The experimental design contained four treatments: substrata open to drifters and walkers; fenced substrata open to drifters only; and two treatments open to drifters and walkers that provided forms of fence controls. Fifteen replicates of each treatment were set out at random locations within a riffle at each of three sites, with each site on a different river (the Little River, the Steavenson River and the Acheron River) in the Acheron River catchment. The experiment was run twice, once during autumn (April 1999) and once during early summer (January 2000). Both experiments were colonised by three species of hydropsychids, Asmicridea sp. AV1, and Smicrophylax sp. AV1 and AV2. We found that 2nd/3rd instars of Asmicridea sp. AV1 walked as well as drifted, whereas all others primarily drifted. No relation between numbers of recruits and water speed was found when substrata were open only to drifters, whereas substrata open also to walkers gained more recruits in faster flows. Additionally, larvae more frequently abandoned nets in slow than fast flows, indicating that drifting into unfavourable flow environments may result in mortality or redispersal of larvae. These findings demonstrate that, although drift is important, it is not necessarily the only method used by hydropsychids to colonise substrata. Larvae may have more capacity to choose substrata in fast flows when they colonise substrata by walking. Spot measures of hydropsychid distribution cannot distinguish between these explanations. The finding that walkers can sometimes comprise significant numbers of recruits raises the prospect that hydropsychids can be sourced locally and have not inevitably drifted in from upstream locations.

Hydropsychid (Trichoptera, Hydropsychidae) gill abnormalities as morphological biomarkers of stream pollution

1. The use of morphological gill abnormalities of hydropsychid larvae was assessed in Hydropsyche siltalai larvae exposed to cadmium in the laboratory and Cheumatopsyche lepida and H. pellucidula larvae collected from a polluted river. Two biomarkers were evaluated: (1) Hydropsychid abnormality incidence (HAI), referring to the proportion of individuals with at least some abnormalities, and (2) Hydropsychid gill abnormality indice (HYI), referring to the average number of abnormal gill tufts for all individuals.2. Abnormality–contaminant relations for both biomarkers were established by studying gill responses along gradients of increasing cadmium and organochlorine concentrations. A cadmium gradient was verified in laboratory exposures, whereas the concentrations of polychlorinated dibenzo‐p‐dioxin (PCDD), dibenzofuran (PCDF) and diphenyl ether (PCDE) of the aquatic moss Fontinalis antipyretica were used as measures of an organochlorine gradient in the field.3. Morphological abnormalities were easily distinguished as heavy darkening, malformation and/or reduction of single gill tufts. Darkening of the gills appeared to start either at the basal or distal ends.4. A marked increase of HYI values with increasing Cd concentration reflected a clear abnormality‐contaminant relation, whereas the mere dicotomic classification of larvae as normal or abnormal (HAI) was less informative. High values of both HAI and HYI were associated with high contamination. A significant positive correlation was found between organochlorine concentration in mosses and biomarker values for H. pellucidula, but not for C. lepida.5. We conclude that HAI indicates deleterious effects, but fails to quantify the severity of degradation. Use of individual gill tufts, as response units in deriving HYI, revealed a simple solution to the quantification problem. Further research into the ecological meaning, physiological background and patterns of gill abnormality is recommended for assessing the applicability and relevance of hydropsychid gill biomarkers.

Comparison of Diets of Prickly Sculpin and Juvenile Fall-Run Chinook Salmon in the Lower Mokelumne River, California

I compared diets of prickly sculpin, Cottus asper, and juvenile fall-run chinook salmon, Oncorhynchus tshawytscha, in the lower Mokelumne River, California, from January through June during 1998 and 1999. Prickly sculpin fed primarily on chironomid (Diptera) larvae and hydroptilid and hydropsychid (Trichoptera) larvae. Juvenile chinook salmon fed on zooplankton, plus chironomid, hydroptilid, and hydropsychid pupae. Both supplemented their diets with several other prey items, including larval Sacramento suckers, Catostomus occidentalis. Their diets did not overlap significantly any time during the study. Each species fed more as time progressed in both years. A significant relationship between prey item and fish size was observed for juvenile chinook salmon in 1998 and for both species in 1999. The pattern of growth for chinook salmon and prickly sculpin suggests increased feeding might be more related to water temperature than increase in size. I observed no predation by prickly sculpin on juvenile chinook salmon during this study, although sculpin eggs and larvae were infrequently observed in salmon stomachs.

Simultaneous measurement of uptake and elimination of cadmium by caddisfly (Trichoptera: Hydropsychidae) larvae using stable isotope tracers

The use of stable isotopes coupled with inductively coupled plasma-mass spectrometry provides a unique opportunity to investigate pathways of metals in aquatic invertebrates. In this study, the simultaneous uptake and elimination of cadmium (Cd) in stream-dwelling caddisfly (Trichoptera: Hydropsychidae) larvae was measured in laboratory experiments with stable isotopes of Cd. In one experiment, animals were exposed to low levels (0.5 microg/L) of 114Cd in filtered river water and uptake was measured for 9 d, followed by 9 d during which the larvae were exposed to either 114Cd, 113Cd, or filtered river water. In a second experiment, the initial exposure concentration of 114Cd varied from 0.1 to 1.6 microg/L for 5 d, followed by 5 d during which the larvae were exposed to the same respective levels of 113Cd. The purpose of the two experiments was to test whether or not elimination of 114Cd from Hydropsyche larvae was the same in the presence or absence of Cd in the water and also whether or not elimination rates were dependent on exposure concentration. The results indicated that elimination of Cd by Hydropsyche larvae (elimination rate constant approximately 0.21-0.24/d) in the presence of Cd in the water was the same as the depuration when no metal was present. Furthermore, the elimination rate was independent of exposure concentrations, ranging between 0.1 and 1.6 microg/L. Thus, the stable isotope tracer technique enabled the simultaneous measurement of uptake and elimination of Cd in hydropsychid larvae and may have the potential to facilitate the labeling of various compartments (e.g., water, sediment, and food) in the aquatic environment.

Anomalies on capture nets of Hydropsycheslossonae larvae (Trichoptera; Hydropsychidae), a potential indicator of chronic toxicity of malathion (organophosphate insecticide)

A laboratory study on the sublethal effects of malathion on the net-spinning behavior of the caddisfly larvae Hydropsyche slossonae was conducted in order to assess the potential of net anomalies as an indicator of chronic exposure to organophosphorus insecticides. Two anomalies were identified after chronic exposure to 0.01, 0.05, 0.1, 0.5 and 1.0 μg l −1 malathion. The first was a distortion of the midline meshes where the normal diamond shape structure was disrupted and the meshes were separated by extra strands (called ‘midline’ anomaly). The second aberration observed was a significant decrease in net symmetry. Both anomalies were highly correlated to the toxic action of malathion, i.e. inhibition of the acetylcholinesterase enzyme (AChE). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analyses of capture nets did not show any modification of silk polypeptides after exposure to malathion, indicating that net distortions were not related to silk composition. Both anomalies seem to represent the symptoms of the specific toxic action of malathion; nevertheless, they can serve as an index of the physiological condition of the larvae, especially the midline anomaly. The symmetry of the nets decreased significantly after exposure to 0.5 and 1.0 μg l −1. However, the toxicity curves (EC 50) showed that the sensitivity threshold for the midline anomaly ranged from 0.11 to 0.28 μg l −1, which reflect more realistic exposure to concentrations expected to occur in the field. Hence, the use of capture net anomalies of hydropsychid larvae could represent a valuable indicator of sublethal toxicity induced by malathion and other organophosphorus insecticides in running waters.

Ontogenic changes in prey consumption by the stoneflyParagnetina mediain relation to temporal variation in prey nutrient content

In Duffin Creek, Ontario, nymphs of the predatory stonefly Paragnetina media (Perlidae) commonly feed on detritus and three prey types: hydropsychid larvae (Hydropsyche sparna and Hydropsyche slossonae), nymphs of the mayfly Baetis tricaudatus, and chironomid larvae belonging to subfamilies Tanypodinae and Orthocladiinae. This study examined temporal changes in the nutrient (lipid and protein) content of these prey to determine if the predator tracks food resources by selecting prey on the basis of nutrient requirements at different stages in its own development. All three common prey types exhibited temporal variation in lipid levels, with peaks occurring at different times: June for the hydropsychids, August for the chironomids, and October for B. tricaudatus. Prey protein levels were less variable. The proportions of the prey types that were eaten varied throughout the year and according to predator size. For example, while small P. media ate mostly detritus, they also consumed early-instar hydropsychids during the larval recruitment period of the latter; mid-sized nymphs included more animal matter in their diet, primarily chironomid larvae; and larger nymphs primarily ate B. tricaudatus. Paragnetina media nymphs in all size categories showed an increase in body lipid level in the autumn, suggesting a general accumulation of lipid reserves in readiness for the winter, although dietary adjustment to accomplish this was detectable only in small P. media. There was no evidence to suggest that P. media selected prey on the basis of the latter's protein content. Male P. media nymphs preyed predominantly on chironomid larvae and included mites in their diet, whereas female nymphs preferred B. tricaudatus and hydropsychids. On a per milligram body mass basis, male nymphs had the higher nutrient gain, since, for both lipid and protein, intake by males was between two and three times that by females. However, male and female bodies had similar lipid contents. We conclude that whereas there is temporal variation both in the nutritional (lipid and protein) content of the common prey of P. media and in this predator's diet, there is only weak evidence for nutrient-resource tracking.

Anomalies on capture nets of Hydropsyche slossonae larvae (Trichoptera; Hydropsychidae) following a sublethal chronic exposure to cadmium

A laboratory study on the sublethal effects of cadmium (Cd) on the net-spinning process of the larvae of Hydropsyche slossonae was conducted in order to assess the potential of net anomalies as an indicator of chronic exposure to Cd. Two major anomalies with different frequency levels were identified after chronic exposure to 0.37, 1.2, 11.6, 21.4 and 43.3 μg l −1 of Cd. The first was a distortion of the midline meshes where the diamond-shape structure is disrupted and the meshes are separated by extra strands (called ‘midline’ anomaly). The second aberration consisted of a distortion of the rectilinear structure of net opening by strands being fused or added over the meshes (called ‘crossover’ anomaly). The midline distortion may be linked to a physiological stress caused by Cd, which can affect the control of the net-spinning process. It was not possible to relate the crossover aberrations to a specific toxic action of Cd, but data indicated that both anomalies are independent from each other and that two modes of action could be implicated. Protein analyses of capture nets have revealed silk polypeptide modifications at the highest Cd concentration tested, indicating a possible effect of Cd interaction with silk proteins. However, neither a gradient-concentration nor a time-dependent response could be established with both aberration frequencies. Silk protein modifications would rather play a secondary role in the appearance of both net anomalies, and mostly at a high concentration level. Finally, the toxicity curves (EC 50) show that the sensitivity threshold for both types of aberration ranged from 1 to 5 μg l −1 which is highly sensitive compared with other sublethal effects of Cd on other macroinvertebrate species. Hence, the use of capture-net anomalies of hydropsychid larvae would represent a valuable indicator of sublethal toxicity induced by Cd and possibly by other metals in running waters.

Ontogenic changes in prey consumption by the stonefly <i>Paragnetina media</i> in relation to temporal variation in prey nutrient content

In Duffin Creek, Ontario, nymphs of the predatory stonefly Paragnetina media (Perlidae) commonly feed on detritus and three prey types: hydropsychid larvae (Hydropsyche sparna and Hydropsyche slossonae), nymphs of the mayfly Baetis tricaudatus, and chironomid larvae belonging to subfamilies Tanypodinae and Orthocladiinae. This study examined temporal changes in the nutrient (lipid and protein) content of these prey to determine if the predator tracks food resources by selecting prey on the basis of nutrient requirements at different stages in its own development. All three common prey types exhibited temporal variation in lipid levels, with peaks occurring at different times: June for the hydropsychids, August for the chironomids, and October for B. tricaudatus. Prey protein levels were less variable. The proportions of the prey types that were eaten varied throughout the year and according to predator size. For example, while small P. media ate mostly detritus, they also consumed early-instar hydropsychi...

Interplay between dispersal determinants of larval hydropsychid caddisflies

We examined the interactive effects of predator (perlid stoneflies) presence, epilithic algal cover, and substrate size on drift and resettlement decisions by larval hydropsychid caddisflies in a series of experiments conducted in real and artificial streams. Predator effects on drift-entry depended on algal structural complexity. Stoneflies significantly increased hydropsychid drift from biofilm-covered rocks but not from rocks covered with a thick periphyton mat or with a macroalga (Cladophora glomerata, approx. 20% surface cover). When we manipulated degree of Cladophora cover, we found that Cladophora even in small amounts (4-13% cover) nullified stonefly effects on drift and that drift was inversely related to amount of Cladophora on rocks. However, Cladophora may only be important as a dispersal determinant until retreats (i.e., immovable cases) are constructed and vulnerability to predators decreases. Hydropsychid drift rates were positively related to gravel abundance, but Cladophora presence enhanced settlement rates on the substrate. Our study shows that multiple factors jointly influence dispersal patterns of hydropsychid larvae and that the relative strength of any one factor as a dispersal determinant varies depending on conditions of other determinants.

Metal concentrations in Hydropsyche pellucidula larvae (Trichoptera, Hydropsychidae) in relation to the anal papillae abnormalities and age of exocuticle

Metal concentrations in hydropsychid larvae inhabiting an acid, metal-polluted river were studied in relation to the incidence of morphological abnormalities and variation in their stage of development. The significantly higher Al concentrations in the abnormal larvae as compared to the levels measured in the normal larvae suggest that the aluminium accumulated in the Hydropsyche pellucidula larvae significantly contributes to the occurrence of morphological abnormalities. The comparison of metal concentrations between unmoulted and newly moulted larvae indicates that the main proportion of the iron is adsorbed on the body surface, whereas much of the aluminium, cadmium, copper, lead and zinc is also absorbed in the tissues. The average concentrations of Cd and Cu were significantly higher in the newly moulted than in the unmoulted larvae. It is suggested that either moulting enhances accumulation of Cd and Cu or these metals enhance moulting of the larvae. The results indicate that in bioaccumulation and monitoring studies utilizing aquatic insect larvae, more attention should be paid to the condition of the individuals. The incidence of morphological abnormalities in hydropsychid larvae appears to be a useful tool for impact assessment and the biomonitoring of metal-polluted streams.

APPLIED ISSUES Species‐ and population‐specific responses of translocated hydropsychid larvae (Trichoptera, Hydropsychidae) to runoff from acid sulphate soils in tin River Kyrönjoki, western Finland

SUMMARY1. The effects of runoff from acid sulphate soils on Hydropsyche angustipennis, H. siltatai and Ceratopsyche nevae larvae were studied. H. angustipennis occurs in both contaminated and uncontaminated rivers, whereas H. siltalai and C. nevae occur only in uncontaminated rivers. Larvae were translocated in PVC‐tubes to contaminated sites. Larvae returned to an uncontaminated river were used as a control.2. Heavy larval mortality and a high incidence of morphological abnormalities were observed in larvae translocated to the contaminated, acid dyers. In the control river pH remained near 6 and no larval mortality or morphological abnormalities were observed. In the most contaminated river nutrient concentrations, amount of organic matter and pH increased substantially after the flood.3. Mortality and incidence of abnormalities were significantly higher in H. siltalai and C. nevae than in H. angustipennis. As metal concentrations decreased and pH increased, the larvae of all species developed normally, except in the most contaminated river, where only H. angustipennis appeared to tolerate the increasing load of nutrients and organic matter.4. Reciprocal translocation of H. angustipennis larvae between contaminated and uncontaminated rivers revealed significant differences in larval development and the occurrence of abnormalities among sites and populations. At both sites, larvae originating from the contaminated site grew more slowly than did larvae from the uncontaminated site. Morphological abnormalities were detected only in H. angustipennis larvae translocated from the uncontaminated to the contaminated site.5. The results suggest that species‐specific differences in sensitivity to acidification, metal contamination and eutrophication are the major determinants of hydropsychid guild structure in the River Kyrönjoki. Further, population‐specific differences in growth and occurrence of morphological abnormalities in the H. angustipennis larvae may reveal either phenotypic adaptation or evolution of resistance to runoff from acid sulphate soils.

The role of grazers and shredders in the retention and downstream transport of a pcb in lotic environments

Field studies using flow-through artificial stream channels were conducted between May and October 1992 to study the role of the feeding activity of grazing and shredding invertebrates in promoting downstream transport of 2,2[prime],4,4[prime],5,5[prime]-hexachlorobiphenyl (HCBP). Plant material was labeled with [[sup 14]C]HCBP and fed to selected invertebrate species, and accrual of radioactivity by downstream collector species (Hydropsyche spp.) was measured. Downstream transport of HCBP was significantly increased by the presence of the grazer Elimia livescens in the upstream sections of the channels as demonstrated by significantly higher levels of radioactivity in hydropsychid larvae located downstream. Similarly, movement of HCBP downstream was significantly greater in channels containing the shredder Hydatophylax argus than in channels without shredders. These results suggest that the feeding processes of benthic invertebrates may play an important role in the downstream transport of particle-bound hydrophobic organic compounds.

Rapid behavioural and morphological responses of hydropsychid larvae (trichoptera, hydropsychidae) to sublethal cadmium exposure

Larvae of Hydropsyche contubernalis and H. siltalai were exposed to sublethal cadmium concentrations of 0, 0·012, 0·16 and 10 mg/litre −1 for 72 h. Linear logit models revealed a significant increase in the frequency and degree of damage of the anal papillae of both species with increasing Cd concentration. Hydropsyche contubernalis showed a stronger and earlier anal papillae response under Cd exposure than H. siltalai, whereas in the latter species darkening of the ventral sides of the abdomen was also observed. The lowest Cd level altered the competition behaviour of Hydropsyche contubernalis larvae after only 24 h exposure, yet no visual signs of morphological damage were detected. The exposure intruder larvae spent significantly less time trying to enter the nets of resident larvae than did unexposed intruders. In addition, both the exposed intruders and exposed residents pursued different behavioural tactics during the encounters compared to their unexposed counterparts.

THE ROLE OF GRAZERS AND SHREDDERS IN THE RETENTION AND DOWNSTREAM TRANSPORT OF A PCB IN LOTIC ENVIRONMENTS

Field studies using flow-through artificial stream channels were conducted between May and October 1992 to study the role of the feeding activity of grazing and shredding invertebrates in promoting downstream transport of 2,2′,4,4′,5,5′-hexachlorobiphenyl (HCBP). Plant material was labeled with [14C]HCBP and fed to selected invertebrate species, and accrual of radioactivity by downstream collector species (Hydropsyche spp.) was measured. Downstream transport of HCBP was significantly increased by the presence of the grazer Elimia livescens in the upstream sections of the channels as demonstrated by significantly higher levels of radioactivity in hydropsychid larvae located downstream. Similarly, movement of HCBP downstream was significantly greater in channels containing the shredder Hydatophylax argus than in channels without shredders. These results suggest that the feeding processes of benthic invertebrates may play an important role in the downstream transport of particle-bound hydrophobic organic compounds.

Feeding of an Iberian stream cyprinid assemblage: seasonality of resource use in a highly variable environment.

Food resource use by seven cyprinids from an Iberian stream was analysed over 9 months. Differences in food resource use were found both between species and within species between seasons. Plant material was a more important food for carp, nase, goldfish and barbel than for gudgeon, roach and chub, irrespective of the season. Chironomid larvae were the staple animal food for the former five species throughout the year. Roach and chub, especially the latter, displayed large seasonal variations in prey use, with chironomid larvae only being important during autumn. Ephemerellid nymphs and ephemeropteran imagos dominated the animal diet of chub during spring and summer, respectively. Dipteran adults and Formicidae were the most important prey for roach during spring and summer, with other common prey being ephemerellid nymphs and hydropsychid larvae. Food resource overlap among the three dominant species (roach, barbel and chub) displayed a large seasonal variation. High overlaps were observed during autumn when these species used the same resources. During summer overlaps were much lower with each species specialising on different prey. The remaining less abundant species had large diet overlaps amongst themselves and with barbel, over all seasons. It is suggested that morphological constraints, habitat partitioning and temporal changes in food resource limitation may be involved in producing these patterns of food resource use.

Effects of stone topography on abundance of net-building caddisfly larvae and arthropod diversity in an upland stream

We investigated whether small pits on the surfaces of stream stones affected abundances of net-spinning, hydropsychid larvae (Trichoptera) and species diversity in the Acheron River, southeastern Australia. On stream stones, nets occurred more often in small (at least 5 mm width) pits than expected by chance. However, proportionately high numbers of pits were not associated with high abundances of either of two species of hydropsychid larvae (both Asmicridea spp). A colonization experiment, using bricks with 0, 4, 12 or 30 small (diam. 5.5 mm) and large (13 mm) pits drilled into the top surfaces, showed that numbers of nets were related to numbers of pits. Small pits were preferred over large ones and the pits, as a group, were occupied almost entirely by late instars of both species. However, only Asmicridea sp. 1 was more abundant on pitted bricks; Asmicridea sp. 2 showed no response. The differences between the species, and between the stone and brick results, can be explained by different proportions of late instars in the samples. Many I instars probably did not make nets. Hence, higher numbers of larvae on pitted surfaces is expected only when late instars are relatively frequent, as was true for Asmicridea sp. 1 on bricks. There were no effects of pits per se or increased numbers of hydropsychid nets on species diversity or the abundances of other common species. These results contrast with those of other studies, which found stone topography or hydropsychid nets to increase species diversity on stones.

Effects of Microhabitat Selection on Feeding Rates of Net‐Spinning Caddisfly Larvae

Net—spinning caddisfly larvae of the family Hydropsychidae are known to prefer microhabitats with large, stable substrate and high water flow velocity. It is often assumed that net spinners in high—velocity microhabitats have higher feeding or growth rates than larvae in less preferred sites, but there is no direct evidence to support this assumption. We hypothesized that net—spinning caddisflies would select microhabitats that offered the greatest feeding rates. This hypothesis was tested by field experiments in which we determined if net—spining caddisfly larvae preferred high—velocity sites even when substrate size and type were held constant. We then measured feeding rates of net spinners in microhabitats with different flow characteristics. High—flow positions were selected by 96% of hydropsychid arvae colonizing artifical moss substrates. Artemia nauplii released into the water column were captured by individual larvae in high—flow sites at a rate of 0.016%/m, significantly higher than the capture rate in low—flow sites. Combing this rate of prey capture with mean hydropsychid densities of 1125 individuals/m2, we estimate that hydropsychid larvae in riffles remove drifting invertebrate prey at a rate of ° 18%/m. Assuming exponential prey removal, a prey item in the drift would travel an average of only 5.5 m before being consumed. This study is one of the first to show that the distribution of a stream filter feeder is related to the feeding rates obtainable in different microhabitats.