Microhabitat selection and life cycle patterns of two Rhyacophila species (Trichoptera: Rhyacophilidae) in southern Ontario streams

SUMMARY.

• 1 Two species of predatory caddisfly larvae—Rhyacophila fuscula (Walker) and R. melita Ross—have overlapping longitudinal distributions in southern Ontario streams.
• 2 The populations were studied at sites in areas of overlap (sympatry) and allopatry to determine what resources were partitioned, and if partitioning varied in the zones of overlap.
• 3 Life cycle and species size variation resulted in substantial physical differences between these univoltine species for most months of the year.
• 4 In May, when larvae of both species were not significantly different in size, a detailed microhabitat field study found no difference in microhabitat use between allopatric populations of R. fuscula and R. melita. Microhabitat selection by R. fuscula remained constant at sympatric sites but R. melita showed a significant change resulting in reduced microhabitat overlap with R. fuscula.

     

Stream microhabitat selectivity,resource partitioning,and niche shifts in grazing caddisfly larvae

Microhabitat selectivity, resource partitioning, and niche shifts in five species of grazing caddisfly larvae (Glossosoma califica, G. penitum, Dicosmoecus gilvipes, Neophylax rickeri, and N. splendens) were quantified by underwater measurement of microhabitat availability and utilization in three northern California streams. The microhabitat parameters water depth and velocity and rock size, roughness, and slope were measured. Comparisons of habitat available to habitat used revealed significant selection for at least two microhabitat parameters by each population, with depth and velocity being the most important. Comparisons of habitat used by different species showed significant partitioning of at least two microhabitat parameters at each site, with depth being partitioned at all sites. Non-parametric discriminant analysis revealed significant microhabitat partitioning on a multivariate level at two sites. Comparisons of habitat used at different sites quantified a major niche shift by D. gilvipes in its preference for riffles versus pools. Size-selective predation by dippers (Cinclus mexicanus) and steelhead (Salmo gairdneri gairdneri) is proposed as a hypothesis to explain the observed resource partitioning and niche shift.     

The behaviour and form of Psephenus herricki (DeKay) (Coieoptera: Psephenidae) in relation to water flow*

1. Psephenus herricki (DeKay) larvae were observed in aquaria and artificial streams using videomacroscopic techniques. Dye visualization was used to determine flow around the organisms. 2. The highly flattened larvae of Psephenus herricki are able to withstand and/or avoid the force of flowing water in streams through a number of mechanisms. The legs are vital: if they cannot gain purchase the larvae can be dislodged by the lifting forces acting on the body. At the flow rates tested (up to 50 cm s^?1), suction under the carapace is not needed to maintain position. 3. Active pumping of water through lateral slots of the carapace and out from under the body posteriorly greatly reduces turbulence around the body and may act to decrease drag at high Reynolds numbers. 4. An interpretation of the flattened body-shape of Psephenus herrickilarvae as an adaptation to fast currents and/or to prevent the larvae from being removed from the substrate by a predator is of questionable ecological significance because individuals are usually found in crevices and on the undersides of stones.     

Microhabitat utilization by two leptodactylid frogs in the Andes of central Chile

Summary Some basic parameters of the life history of Alsodes montanus and Alsodes tumultuosus (Anura-Leptodactylidae), were studied from 1977 to 1980 by periodic field observations at Farellones and La Parva (33–34° south lat.; 2,700–3,000 m above sea level). Special attention was paid to strategies of resource partitioning in relation to gross features of the environment. The latter was unstable with a relative short period favorable for activity of the animals. Physical environmental differences between the first and second season of this study, resulted in a decrease in total number of active adults, a reduction in the duration of larval activity and a shift in microhabitat preferences of larvae.During the favorable season, October to May, adults of both species showed spatial and temporal segregation, related to different physical features of the environment; larvae did not show temporal segregation. Larvae of both species were found in seven different microhabitats; only in one of these did they show significant difference in microhabitat preference, A. tumultuosus was found more often in crevices. Microhabitat dimensions were more important than time and food resources in the separation of the niches of the two species. The segregation of niche dimensions, microhabitat, diel and annual activity and food were not complementary.Coexistence was therefore observed with the species tending to use different resources. When the same resource was used, it was not limiting.     

Predator avoidance,microhabitat shift,and risk-sensitive foraging in larval dragonflies

Summary Dragonfly larvae (Odonata: Anisoptera) are often abundant in shallow freshwater habitats and frequently co-occur with predatory fish, but there is evidence that they are underutilized as prey. This suggests that species which successfully coexist with fish may exhibit behaviors that minimize their risk of predation. I conducted field and laboratory experiments to determine whether: 1) dragonfly larvae actively avoid fish, 2) microhabitat use and foraging success of larvae are sensitive to predation risk, and 3) vulnerability of larvae is correlated with microhabitat use. I experimentally manipulated the presence of adult bluegills (Lepomis macrochirus) in defaunated patches of littoral substrate in a small pond to test whether colonizing dragonfly larvae would avoid patches containing fish. The two dominant anisopteran species, Tetragoneuria cynosura and Ladona deplanata (Odonata: Libellulidae), both strongly avoided colonizing patches where adult bluegills were present. Laboratory experiments examined the effects of diel period and bluegills on microhabitat use and foraging success, using Tetragoneuria, Ladona and confamilial Sympetrum semicictum, found in a nearby fishless pond. Tetragoneuria and Ladona generally occupied microhabitats offering cover, whereas Sympetrum usually occupied exposed locations. Bluegills induced increased use of cover in all three species, and use of cover also tended to be higher during the day than at night. Bluegills depressed foraging in Tetragoneuria and to a lesser extent in Ladona, but foraging in Sympetrum appeared unaffected. Other laboratory experiments indicated that Sympetrum were generally more vulnerable than Tetragoneuria or Ladona to bluegill predation, and that vulnerability was positively correlated with use of exposed microhabitats. Both fixed (generally low use of exposed microhabitats, diel microhabitat shifts) and reactive (predator avoidance, predator-sensitive microhabitat shifts) behavioral responses appear to reduce risk of predation in dragonfly larvae. Evidence indicates that vulnerability probably varies widely among species and even among instars within species, and suggests that spatial distributions of relatively vulnerable species may be limited by their inability to avoid predation.     

Macroecology of Aquatic Insects: A Quantitative Analysis of Taxonomic Richness and Composition in the Andes Mountains of Northern Ecuador

To determine the spatial dynamics of Neotropical lotic insect species, specimens were collected from 41 streams on the eastern and western flanks of the Andes Mountains in Ecuador. We examined the manner in which taxonomic richness and composition differed with elevation, latitude, and versant. Statistical analyses were limited to 5 families (Ephemeroptera: Baetidae, Leptohyphidae, Oligoneuriidae; Heteroptera: Naucoridae; Coleoptera: Elmidae), comprising 32 genera and 85 species, for which identifications or morphospecies assignments were reliable. Assessment of taxonomic diversity was based on the richness of genera and species at each site. In addition, each site was characterized by species richness within each of 4 families with more than 10 species. The effects of versant and transect on composition and structure were family‐specific. Mean site differences between versants in elmid richness at generic and specific levels depended on transect. Only baetid richness was affected by versant and transect in a consistent manner. Variation among sites in composition based on all genera and species was captured using multidimensional scaling (MDS). Subsequent MANOVAs based on scores from MDS axes revealed that differences between versants were much stronger in the southern transect, although transects differed from each other at specific and generic levels. A Jaccard's similarity matrix was computed for each family to reflect the spatial organization of taxonomic composition. Mesogeographic patterns of species composition for each of the four families were correlated (Mantel analysis) at both the regional level and at the level of the entire study area. At the regional level, the only pair of families to exhibit correlated patterns of species composition was elmids and naucorids in the southwestern region. The pattern of species composition for each family was correlated with the patterns for one or more other families at the level of the entire study area. Thus, spatial dynamics of species composition was similar for the families examined, suggesting that the Andes exert a consistent influence on species distributions within families, regardless of ordinal affiliation. At a local scale, however, the way in which taxonomic composition changed with latitude and versant was family‐specific. Mayflies, the most vagile of the taxa studied, had the highest percentage of species overlap between versants. Of three genera of Naucoridae collected, species of Ambrysus, of probable Mexican origin, were found only on the eastern versant, corroborating other evidence that the genus is recent in South America. Moreover, dispersion by Ambrysus across the Andes Mountains may not have occurred, as it has for Cryphocricos and Limnocoris, which are of probable South American origin.     

Habits and relative population densities of some hydrophilids in California rice fields

Populations of the hydrophilids Berosus styliferus Horn, Hydrophilus triangularis Say, and Tropisternus lateralis (Fabricius) were observed in northern California rice paddies. Relative adult and larval density for each species was determined using aquatic light traps adjacent to the water margin, and at 5 m and 30 m distances from the levee. Egg cases were monitored by recording numbers per unit area.The seasonal patterns of abundance indicate each species has a univoltine life cycle. All stages of B. styliferus and H. triangularis are more abundant nearer the levees. T. lateralis adults are commonly found at the levees during the first 10 weeks post-flood, whereas T. lateralis larvae and egg cases are most numerous at the 30 m distance.Colonication of the paddies occurs through water connections, flight, and adult overwintering.     

Microhabitat use in coexisting desert rodents—The role of population density

Summary Among some species of Sonoran Desert rodents microhabitat differences are density dependent. I studied the differences in microhabitat use among four species of heteromyid rodents (Dipodomys merriami, Perognathus amplus, P. baileyi, and P. penicillatus) at low and at high population densities. Microhabitats are defined by the abundance and size distribution of desert shrubs. During a period of low population density the rodent species showed substantial microhabitat differentiation. Following a large increase in pocket mouse (Perognathus spp.) numbers differences in microhabitat use between species disappeared. The lack of microhabitat differentiation at high density is due to microhabitat shifts rather than an expansion in the number of microhabitats used. The shifts lead to increased similarity among species in microhabitat use. Microhabitat overlap is not constant but it is highly variable and sensitive to changes in rodent abundance.     

Risk of predation and behavioural response in three anuran species: influence of tadpole size and predator type

Many species alter their activity, microhabitat use, morphology and life history in response to predators. Predation risk is related to predator size and palatability of prey among others factors. We analyzed the predation risk of three species of tadpoles that occur in norwestern Patagonia, Argentina: Pleurodema thaul, Pleurodema bufoninum and Rhinella spinulosa. We sampled aquatic insect predators in 18 ponds to determine predator–tadpole assemblage in the study area. In laboratory conditions, we analysed the predation rate imposed by each predator on each tadpole species at different tadpole sizes. Finally, we tested whether tadpoles alter their activity in the presence of chemical and visual cues from predators. Small P. thaul and P. bufoninum tadpoles were the most vulnerable prey species, while small R. spinulosa tadpoles were only consumed by water bugs. Dragonflies and water bugs were the most dangerous tadpole predators. Small P. thaul tadpoles reduced their activity when they were exposed to all predators, while large tadpoles only reduced the activity in the presence of large predators (dragonfly larvae and water bugs). Small P. bufoninum tadpoles reduced the activity when they were exposed to beetle larvae and dragonfly larvae, while large tadpoles only reduced activity when they were exposed to larger predators (water bugs and dragonfly larvae). R. spinulosa tadpoles were the less sensitive to presence of predators, only larger tadpoles responded significantly to dragonfly larvae by reducing their activity. We conclude that behavioural responses of these anuran species were predator-specific and related to the risk imposed by each predator.     

Respiratory Metabolism of sub-Antarctic insects from different habitats on Marion Island

Oxygen uptake of the foliage-dwelling larvae ofEmbryonopsis halticella Eaton (Lepidoptera: Yponomeutidae) and adults ofEctemnorhinus marioni Jeannel (Coleoptera: Curculionidae), the litter-dwelling larvae ofPringleophaga marioni (Lepidoptera: Tineidae) and the wrack-dwellingParactora dreuxi Séguy (Diptera: Helcomyzidae) was examined over the range of temperatures experienced by these insects in their microhabitats. With the exception of the kelp fly,P. dreuxi, Q₁₀s and activation energies were generally lower than those found in temperate and Arctic insects, but were similar to values found in beetles from sub-Antarctic South Georgia Island. Q₁₀ and activation energy of each species reflected the temperature regime found in its microhabitat. Activation energies of the Marion Island species were intermediate between those found in temperate and polar arthropods, but towards the polar end of the range. The hypothesis that insects are capable of showing respiratory adaptation to temperature is supported.     

Interactive effects of interspecific competition and microhabitat on early post-settlement survival in a coral reef fish

Microhabitat type and the competition for microhabitats can each influence patterns of abundance and mortality in coral reef fish communities; however, the effect of microhabitat on the intensity and outcome of competition is not well understood. In Kimbe Bay, Papua New Guinea, surveys were used to quantify microhabitat use and selectivity in two live-coral specialist damselfishes (Pomacentridae), Chrysiptera parasema, and Dascyllus melanurus. A patch reef experiment was then conducted to test how intra- and interspecific competition interacts with two types of microhabitat to influence survival of recently settled C. parasema. Surveys demonstrated that C. parasema and D. melanurus recruits utilized similar coral microhabitats; 72% of C. parasema and 85% of D. melanurus used corymbose and bottlebrush growth forms of Acropora. One microhabitat type, Pocillopora sp. coral, was commonly used by D. melanurus but rarely by C. parasema. The patch reef experiment revealed that both microhabitat and interspecific competition influence abundance of recently settled C. parasema. Microhabitat had the strongest influence on survival of C. parasema. In the absence of interspecific competitors, ~85% of C. parasema survived for 5 days after transplantation to high-complexity bottlebrush Acropora reefs when compared to only 25% survival of Pocillopora reefs. In both microhabitats, interspecific competition with D. melanurus, but not intraspecific competition, significantly decreased the survival of C. parasema. Taken together, these results suggest that the observed distribution of C. parasema results from specialized microhabitat requirements and competition for space in those microhabitats. This study demonstrates that interspecific competition and microhabitat type can interact to influence early post-settlement survival in coral reef fishes, though, whether and how these factors influence survival will depend on the behavioural attributes and strength of habitat associations among potential competitors. Communicated by Environment Editor Prof. Rob van Woesik     

Segregation by species and size classes of rainbow trout,Salmo gairdneri,and Sacramento sucker,Catostomus occidentalis,in three California streams

Synopsis The hypothesis that Sacramento suckers, Catostomus occidentalis, compete with rainbow trout, Salmo gairdneri, for space in streams was examined by measuring microhabitat utilization of both species in three California streams. Two streams were similar in most respects except one contained only trout and one contained trout and a large population of suckers. The third stream, formed by the union of the first two, contained trout and a small population of suckers. The species overlapped in five of the six microhabitat variables measured: maximum depth, mean water column velocity, focal point velocity, surface water velocity, and substrate type. However, the species had strong vertical segregation; there was little overlap between species in focal point depth. Mean focal point velocities were also significantly different. Suckers roamed over and generally remained in contact with the bottom while trout held position in the water column. Microhabitat utilization by trout in the stream without suckers was similar to in the stream with a higher sucker density. Differences in microhabitat utilization by trout between the third stream and the other two was attributed to the larger size of the third stream. Both sucker and trout showed a similar within-species segregation of size classes - fish under 50 mm in length sought shallow water. Size-specific trends indicated ontogenic shifts in resource utilization which reduced overlap within species. These results suggest that competition for space between trout and suckers was not a major factor regulating microhabitat utilization of trout, although the possibility that larger suckers may displace small trout needs further study.     

Bryophyte Species Diversity in Secondary Forests Dominated by the Introduced Species Spathodea campanulata Beauv. in Puerto Rico

The introduced tree species Spathodea campanulata (Bignoniaceae) forms novel forests in Puerto Rico, these having emerged after the abandonment of fields in the mid‐20th century and resulting in forests with a new species composition. We assessed bryophyte species richness in these novel forests and sought correlations with geological substrate, past land use, forest edge and patch area, forest structure, elevation, microhabitat diversity, tree species richness, and microclimatic conditions. Transects were established (edge and forest interior) in nine moist forest patches dominated by Spathodea in north‐central Puerto Rico. These Spathodea forest patches ranged from 0.6 to 9 ha. ANOVA, Chi‐square, correlation, and cluster analyses were used in data analyses. We found 57 bryophyte species. There was a significant difference in bryophyte richness among patches. Those on karst exhibited highest bryophyte richness due to microhabitat diversity, past land use, and shorter hydroperiods. Alluvial sites scored lowest in bryophyte species richness, and forest structure was important for bryophyte communities on these sites. Significant differences in temperature, relative humidity, and light intensity were observed between edge and forest interior. These appeared important for establishing bryophyte species cover but not richness and composition. Microhabitat diversity, patch area, and forest age were more related to bryophyte species richness than elevation, exposed edge, and tree species richness, regardless of geologic substrate. Collectively, Spathodea patches were similar to mature forests on the Island with respect to bryophyte species richness and composition. Novel Spathodea forests have conservation value due to their habitat suitability for bryophyte communities.     

Macro and Microhabitat Associations of the Peter's Tent‐Roosting Bat (Uroderma bilobatum): Human‐Induced Selection and Colonization?

Understanding species‐specific habitat selection is essential to identify how natural systems are assembled and maintained, and how emerging natural and anthropogenic disturbances will affect ecosystem function. In the Neotropics, Peter's tent‐roosting bat (Uroderma bilobatum), known to roost in forests, has become abundant in human‐modified areas. To understand how habitat characteristics in both intact forest and human‐modified areas influence the presence and density of U. bilobatum, we characterized habitat use at two scales (macrohabitat and microhabitat) and used logistic and poisson regressions to determine which habitat characteristics best predicted the presence and density of U. bilobatum within each scale. Moreover, we performed a redundancy analysis to determine which habitat scale explained more variation. As these bats are obligate tent roosters, we used tent as a surrogate for bat presence and density. We found that both macrohabitat and microhabitat scales explained variation in presence and density. Characteristics of the microhabitat scale, however, had higher predictive power, revealing that U. bilobatum preferentially inhabits areas with high density of coconut palms. Coconut palms were introduced recently in the Neotropics and are found only in human‐modified areas. Therefore, we hypothesize that U. bilobatum is expanding its range into these areas following the expanded distribution of this exotic plant species.     

The macroinvertebrate fauna of an acid-stressed headwater stream system in the Adirondack Mountains, New York

SUMMARY.

• 1 The Silver Run system in the Adirondack Mountains of New York includes three first-order streams with very different pH regimes: one moderately acidic (pH 5.8-7.2), the others much more so (pH generally 4.4-5.0)
• 2 The less acidic (acidophilic) site yielded a relatively diverse, well balanced fauna, with at least 10% each of midges, mayflies, stoneflies and elmid beetles. Ephemerella funcralis (Ephemeroptera) and Oulimnius latiusculus (Coleoptera) were dominant in this community. They appear to be among the most acid-tolerant species of their respective families and may become abundant under moderately acidic conditions. Their absence or low abundance in the more acidic tributaries of the Silver Run system suggests that they arc eliminated as acidification intensifies, These species therefore may he useful indicators of advancing acidification.
• 3 The more acidic (acidobiontic) communities contained fewer Hum half as many taxa as the acidophilic communities, contained few elmid beetles or mayflies, and were heavily dominated In stoneflies. Leuctra feiruinea and Isopecrla sp. were dominant, comprising.56-86% of the individuals. Other common constituents included black flies (Simuliumn gouldingi and S. venustum) and the caddisfly Rhyacvphila fuscula.

     

Specialization of rainforest canopy beetles to host trees and microhabitats: not all specialists are leaf‐feeding herbivores

Reliable estimates of host specificity in tropical rainforest beetles are central for an understanding of food web dynamics and biodiversity patterns. However, it is widely assumed that herbivores constitute the majority of host specific species, and that most herbivore species feed on leaves. We tested the generality of this assumption by comparing both plant host‐ and microhabitat‐specificity between beetle communities inhabiting the foliage (flush and mature), flowers, fruit, and suspended dead wood from 23 canopy plant species in a tropical rainforest in north Queensland, Australia. Independent of host tree identity, 76/77 of the most abundant beetle species (N ≥ 12 individuals) were aggregated on a particular microhabitat. Microhabitat specialization (measured by S_m and Lloyd's indices) was very high and did not differ between flower and foliage communities, suggesting that each newly‐sampled microhabitat has a large additive effect on total species richness. In accordance with previous studies, host specificity of foliage‐inhabiting beetles was most pronounced among herbivorous families (Curculionidae, Chrysomelidae). By contrast, host specificity among flower‐visitors was equally high among herbivorous and nonherbivorous families (e.g. Nitidulidae, Staphylinidae, Cleridae). Effective specialization (F_T) measures showed that traditional correction factors used to project total species richness in nonherbivorous groups fail to fully capture diversity in the flower‐visiting beetle fauna. These results demonstrate that host specialization is not concentrated within folivores as previously assumed. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109 , 215–228.     

Intraguild predation, cannibalism, and microhabitat use in Calopteryx virgo and Somatochlora metallica larvae: a laboratory experiment

Intraguild predation (IGP) and cannibalism among co-occurring lotic odonate species was studied in Central Finland. A laboratory experiment was performed to assess the microhabitat use and cannibalism between intermediate and late instars of Calopteryx virgo larvae and predation by larger Somatochlora metallica larvae on the intermediate C. virgo instars. The experiment was run in small running-water aquaria where the larvae were able to divide their mutual habitat vertically by clinging onto artificial perches or crawling on the bottom. Life span of the small C. virgo larvae and attack rate on them were compared between the cannibalism and IGP treatments. The effect of predation on the activity, habitat use and spatial distribution of the small C. virgo larvae was examined. The IGP rate was 36%. The prey larvae spent the most of their time on the perches, whereas the S. metallica preferred the substrate. The large C.␣virgo larvae did not cannibalise smaller conspecifics. The presence of a predator (S. metallica) had no effect on the habitat use or activity of the prey (C. virgo) larvae. Habitat use differed more between those species than between conspecifics of different size classes of C. virgo. The spatial distribution between S.␣metallica and C. virgo showed a completely random pattern, whereas the two size classes of C. virgo aggregated in the vegetation. Absence of cannibalism and behavioural observations indicate that C. virgo may have a low tendency for intraspecific aggressions.     

Behavioural and chemical mechanisms behind a Mediterranean ant–ant association

1. Interspecific competition among ants is common, and so is competitive exclusion among dominant ant species. In contrast, specific associations between non‐parasitic ant species are rare, especially in the temperate zones. As an exception, the subordinate ant Camponotus lateralis frequently co‐occurs with the dominant Crematogaster scutellaris but rarely with other dominant ants. 2. This association is one of various associations between Camponotus and Crematogaster species across the world. However, the mechanisms behind these co‐occurences are largely unknown. 3. In the present study, we therefore investigated the association of Ca. lateralis and Cr. scutellaris. We studied the spatial association of the nests, interspecific aggression, both species' cuticular hydrocarbon profiles, and their propensity to follow the other species' pheromone trails. 4. Crematogaster scutellaris usually attacked and displaced the generally submissive Ca. lateralis, but was significantly less aggressive at jointly used trails. Camponotus nests were always in close proximity to Crematogaster nests. 5. The cuticular hydrocarbons of both species consisted of alkanes with chain lengths between C21 and C35. The two species had 25 hydrocarbons in common, including mono‐, di‐, and tetramethyl alkanes. Despite this qualitative similarity, however, the quantitative hydrocarbon composition differed between the two species. 6. Camponotus lateralis followed artificial trails containing trail pheromones of Cr. scutellaris, but the latter did not follow Ca. lateralis trail pheromones. Interspecific trail‐following by Camponotus, but not vice versa, has been observed in another Camponotus–Crematogaster association and may be a more general mechanism that facilitates associations between the two ant genera.     

Costs of reproduction and geographic variation in the reproductive tactics of the mosquito Aedes triseriatus

Intraspecific host discrimination is widespread in solitary parasitoids whose adult females forage for and evaluate host suitability, whereas interspecific discrimination is less common. In some parasitoid species, mostly Diptera and Coleoptera, the larva performs the last step of host searching. It has been suggested that host discrimination will rarely occur in such host-seeking larvae because their low mobility results in a low host encounter rate. We determined the extent to which the larvae of Aleochara bilineata Gyllenhal (Coleoptera: Staphylinidae), a solitary parasitoid of aggregated Diptera pupae: (1) discriminated between unparasitized hosts and hosts parasitized by conspecifics; (2) used semiochemical cues to discriminate; (3) were influenced by life expectancy, presence of conspecifics and host availability in their host acceptance decision; and the extent to which (4) A. bilineata and A. bipustulata L., a species exploiting the same hosts and occurring sympatrically, showed interspecific host discrimination. A. bilineata larvae were able to discriminate between unparasitized hosts and hosts parasitized by conspecifics in a choice experiment. Such behavior has never previously been described for a coleopteran parasitoid or for a parasitoid species whose larvae perform host searching. Host discrimination in this species was not based on the presence of visual or tactile cues (e.g., entrance holes) but rather on chemical cues. The life expectancy of A. bilineata larvae was significantly shorter in the presence than in absence of hosts, and older larvae had lower parasitism success than young larvae in a 24-h experiment. However, the host acceptance decision of A. bilineata larvae was not influenced by larval age or the presence of conspecifics when the ratio of hosts per larva was greater than or equal to 1. When hosts were scarce, the degree of superparasitism increased significantly with the number of foraging conspecifics and the age of the larvae. Both species of Aleochara showed intra- and interspecific host discrimination in a choice experiment. In contrast to A. bipustulata, A. bilineata larvae more frequently parasitized hosts parasitized by A. bipustulata than those parasitized by conspecifics. We suggest that host discrimination will be frequent in solitary parasitoids with host-seeking larvae when hosts are aggregated. Received: 4 June 1998 / Accepted: 1 September 1998     

Embryonic development of the false limpet Siphonaria lateralis from Atlantic Patagonia

Siphonariids are pulmonate gastropods inhabiting rocky intertidal habitats, and many studies have focused on these false limpets around the world. In the southern South Atlantic, studies on reproduction and development in species of Siphonaria are scarce. We studied the embryonic development and egg masses of Siphonaria lateralis at its northernmost distribution in Atlantic Patagonia. In S. lateralis, as in most species of Siphonaria, individuals spawn benthic egg masses that strongly attach to intertidal rocky substrata. A single spherical egg that measures ~120 µm develops inside the egg capsule of S. lateralis. Considering the relatively small egg size, and reports from previous studies, the developmental modality of S. lateralis might be expected to include a planktotrophic larval phase. However, we found that hatchlings emerged as 1‐mm crawling juveniles, probably owing to the presence of intracapsular fluid, which may provide the energetic requirements for direct development. The embryonic size changed little from the egg to veliger stages, and then increased rapidly until the hatchling stage. We compared development in S. lateralis with development in the sympatric Siphonaria lessonii, in which egg size was reported to be ~80 µm and hatching occurs as planktotrophic veliger larvae. In these two species, spawn and early intracapsular developmental modes are remarkably different; these differences represent contrasting ways to survive in the harsh and physically stressful intertidal Patagonian coasts.