Marcelo O. Gonzaga

Individual specialization in the hunting wasp Trypoxylon (Trypargilum) albonigrum (Hymenoptera, Crabronidae)

Individual-level variation in resource use occurs in a broad array of vertebrate and invertebrate taxa and may have important ecological and evolutionary implications. In this study, we measured the degree of individual-level variation in prey preference of the hunting wasp Trypoxylon albonigrum, which inhabits the Atlantic Forest in southeastern Brazil. This wasp captures several orb-weaving spider genera to provision nests. Individuals consistently specialized on a narrow subset of the prey taxa consumed by the population, indicating the existence of significant individual-level variation in prey preferences. The population niche was broader in the wet season in terms of both prey size and taxa. In the case of prey size, the population niche expansion was achieved via increased individual niche breadths, whereas in the case of prey taxa, individual niches remained relatively constrained, and the population niche expanded via increased interindividual variation. The observed pattern suggests the possibility of functional trade-offs associated with the taxon of the consumed prey. The nature of the trade-offs remains unknown, but they are likely related to learning in searching and/or handling prey. We hypothesize that by specializing on specific prey taxa, individuals increase foraging efficiency, reducing foraging time and ultimately increasing reproductive success.

Parasitoid-induced mortality of Araneus omnicolor (Araneae, Araneidae) by Hymenoepimecis sp. (Hymenoptera, Ichneumonidae) in southeastern Brazil

All species included in the Polysphincta genus-group develop as ectophagous parasitoids of active spiders, killing their hosts prior to pupation. However, little information regarding natural history and ovipositing behavior of most species are available. In this study we inspected 85 webs of Araneus omnicolor to evaluate the frequency of parasitism and host size preferences of the wasp Hymenoepimecis sp. We also described the web characteristics of normal and parasitized spiders and the wasp ovipositing behavior. About 41% of the adult females of A. omnicolor inspected were parasitized. The highest incidence of parasitism was observed among relatively small females while no egg or larva was found in large individuals. Araneus omnicolor builds a strong web composed of an orb and barrier threads, where the spider rests within a curled leaf. The parasitoid larva builds its cocoon within this refuge, and modified cocoon webs were not observed. The ovipositing behavior of Hymenoepimecis sp. was very similar to that of Hymenoepimecis argyraphaga parasitizing Leucauge argyra, including the position of the sting, the killing of a previously attached larva, and the expelling of the egg from the base of the ovipositor.

Orb‐web spiders (Araneae: Araneomorphae; Orbiculariae) captured by hunting‐wasps (Hymenoptera: Sphecidae) in an area of Atlantic Forest in south‐eastern Brazil

Members of two hunting‐wasp families, Pompilidae and Sphecidae, are among the major predators of orb‐web spiders. In this study, we collected paralysed spiders from natural nests and trap‐nests provisioned by sphecids in an area of Brazilian Atlantic Forest, and compared these data with the composition of species collected by visual searching during one year. Prey preferences were analysed based on the relative abundance of spider species, their size and web characteristics. We also compiled a list of orb‐weavers captured by four sphecid genera reported in 40 other studies. A large number of prey was obtained from natural nests of Trypoxylon (Trypargilum) albonigrum in Parque Estadual Intervales, especially species of Eustala, Parawixia, and Araneus (Araneidae). Other prey, stored in trap‐nests by T. lactitarse and unidentified hunting‐wasp species, included Nephila (Tetragnathidae), Parawixia, Ocrepeira, Mecynogea, Acacesia (Araneidae), and other spider species that were less abundant. All the species that were heavily preyed upon had a relatively lower abundance in our samples of prey availability. The range of body sizes of spiders captured by Trypoxylon in our study area include the size of some abundant orb‐weavers always absent in their nests. These results indicate that factors other than abundance in the field and the spiders size influence prey selection or susceptibility to attack.

On the spider genus Oecobius Lucas, 1846 in South America (Araneae, Oecobiidae)

Three synanthropic species of Oecobius are recorded in South America: O. navus Blackwall, 1859, O. concinnus Simon, 1893 and O. marathaus Tikader, 1962. Oecobius fluminensis Mello-Leitão, 1917b, described from Brazil, and O. trifidivulva Benoit, 1976, from Zaire, are synonymized with O. navus and two other Brazilian species, O. hammondi Mello-Leitão, 1915 and O. variabilis Mello-Leitão, 1917 are confirmed as junior synonyms of O. navus. Oecobius marathaus, originally described from India, is considered a senior synonym of O. inopinatus Wunderlich, 1995b, from Australia. Distribution and habitat records for these species are provided.

On the sticky cobwebs of two theridiid spiders (Araneae: Theridiidae)

In this study we describe and illustrate a new species, Chrysso intervales n. sp., based on males and females collected in Parque Estadual Intervales, an area of Atlantic Forest in the State of São Paulo, Brazil. We present data on habitat selection, web architecture, thread adhesiveness, diet, and mortality due to fungi for both Chrysso intervales and the sympatric species, Helvibis longicauda. Both species build webs on vegetation close to river margins, but they were not found in forest sites away from these shaded and extremely humid corridors. The webs of both species are entirely composed of viscid silk lines, occupying the space between two or more large leaves. The construction of this web type by Helvibis and Chrysso, and the large amount of viscid droplets in their threads, indicate that the investment in adhesive components in theridiids may be dependent on the ambient conditions. We suggest that the costs of maintaining viscid silk lines in humid areas may be relatively low, explaining the habitat restriction observed in the species studied. By selecting humid habitats, however, these spiders are susceptible to attacks by fungi. The webs constructed by both species intercepted mostly Diptera, especially tipulids, but H. longicauda was also observed consuming a wide variety of prey types.

Spatio-temporal differentiation and sociality in spiders.

Species that differ in their social system, and thus in traits such as group size and dispersal timing, may differ in their use of resources along spatial, temporal, or dietary dimensions. The role of sociality in creating differences in habitat use is best explored by studying closely related species or socially polymorphic species that differ in their social system, but share a common environment. Here we investigate whether five sympatric Anelosimus spider species that range from nearly solitary to highly social differ in their use of space and in their phenology as a function of their social system. By studying these species in Serra do Japi, Brazil, we find that the more social species, which form larger, longer–lived colonies, tend to live inside the forest, where sturdier, longer lasting vegetation is likely to offer better support for their nests. The less social species, which form single-family groups, in contrast, tend to occur on the forest edge where the vegetation is less robust. Within these two microhabitats, species with longer-lived colonies tend to occupy the potentially more stable positions closer to the core of the plants, while those with smaller and shorter-lived colonies build their nests towards the branch tips. The species further separate in their use of common habitat due to differences in the timing of their reproductive season. These patterns of habitat use suggest that the degree of sociality can enable otherwise similar species to differ from one another in ways that may facilitate their co-occurrence in a shared environment, a possibility that deserves further consideration.

Influence of collective feeding on weight gain and size variability of anelosimus jabaquara levi 1956 (Araneae: Theridiidae)

Summary Differences in the size and weight of spiders within colonies may be an important factor for determining dispersion and food distribution among nestmates. In this study we report on the variation in size of spiders from 27 colonies of A. jabaquara , collected during one year. We also conducted an experiment under laboratory conditions to test if prey size, and consequently, collective or individual capture behaviours, ine uence the establishment of weight differences. Female size variation within colonies was high, increasing slightly from March to September. However, from November to January the variation was much lower, probably as a consequence of the emigration of large spiders during the reproductive period. Spiders that fed on large e ies in the laboratory experiment grew more and collective feeding seemed to induce a larger variation in weight among individuals. These results indicate that collective feeding promotes an unequal distribution of food in A. jabaquara colonies.

COLLECTIVE PREY CAPTURE AND FEEDING BEHAVIOURS OF ANELOSIMUS JABAQUARA LEVI 1956 (ARANEAE: THERIDIIDAE)

In this study we report on the predatory behaviour of Anelosimus jabaquara considering the frequency of bites in each prey body part during the immobilization phase, the proportion of the consumption time expended in each of these parts and the relative participation on capture and feeding processes by spiders of different body sizes. Generally, few individuals actively take part in prey immobilization events, biting mostly on flys legs (body segments which promote vibrations strong enough to attract spiders, but not to dissuade attacks). However, many spiders usually take part in the consumption of this medium size prey item. During the consumption phase appendages were rarely used, probably due to the higher nutritional value and biomass availability of central body segments (thorax, abdomen and head). Non-aggressive contests over displaced prey body parts were frequently seen during collective feeding in natural colonies, but not in small artificial groups in laboratory.

Variation in the Stabilimenta of Cyclosa fililineata Hingston, 1932, and Cyclosa morretes Levi, 1999 (Araneae: Araneidae), in Southeastern Brazil

We investigated the characteristics of the stabilimenta constructed by two species of Cyclosa, describing the variations within and among five populations. Both species constructed stabilimenta composed entirely of silk (linear and spiral types) or of silk and debris (linear, detritus clusters and complex types). The vertical linear detritus type was the most frequent structure for adult females of both species, whereas stabilimenta consisting of detritus clusters were more frequent for juveniles of C. morretes. The latter structures appeared to be an intermediate state towards the linear continuous type usually found in adults. The other types were rarely found, and silk stabilimenta were to be constructed only when detritus was not available. The substitution of silk by detritus indicated that both materials function as camouflage in C. morretes and C. fililineata webs. The positions occupied by the spiders within the detritus column (and in some cases the orientation of the stabilimenta) varied markedly within populations, and the unpredictability of their location could be important in reducing the risks of predation. The hypothesis that stabilimenta constitute defensive devices was indirectly corroborated by the observation that spider’s body width and length were, respectively, strongly correlated with the width and length of the stabilimenta.

Substrate selection and spatial segregation by two congeneric species of Eustala (Araneae: Araneidae) in southeastern Brazil

Abstract Habitat and microhabitat selection by spiders are influenced by abiotic and biotic factors, including vegetation structure, natural enemies, and prey availability. Some species are highly dependent on particular conditions, such as the presence of substrates where they remain camouflaged, constantly humid sites or the occurrence of plants bearing glandular trichomes. Others are distributed in areas that include a wide range of physical conditions and interact with several types of prey, predators and competitors. In the present study, we evaluated spatial distribution and substrate selection of two sympatric congeneric species with distinct body shapes and colors, Eustala taquara (Keyserling 1892) and E. sagana (Keyserling 1893), in an area of Atlantic Forest in southeastern Brazil. We focused on the following factors regarding habitat selection: i) distance from the border (forest edge or interior); ii) altitudinal distribution, ranging from 740 to 1294 m; iii) web height above ground level; and iv) plant species used for web attachment. All individuals of both species were located at the forest edge, especially on dry branches. However, they occurred preferentially in different host plants and altitudes. Eustala taquara individuals were strongly associated with Conyza bonariensis, and E. sagana with Hyptis suaveolens and C. sumatrensis. Dry branch preferences might be important to reduce species conspicuousness to visually oriented predators, such as birds and wasps. Spatial segregation between closely related species possibly minimizes interference interactions, such as competition for particular sites or prey items.