Fine structural characteristics of the chorionic microspheres on the egg surface of the orb web spider Trichonephila clavata

The eggs laid by the orb web spider Trichonephila clavata must overwinter in bitterly freezing and dry conditions before hatching, but there does not seem to be any protection like a compact silk case covering the entire eggmass. Instead, the surface of the eggmass is completely coated with a milky coating called chorionic microspheres (CM). Therefore, we investigated the fine structural characteristics of CM to demonstrate their ecological importance. Although the diameter of CM in outer eggmass exhibits a significant variation, the chorionic surface is coated with a single layer of CM, characterized by a consistent diameter of approximately 2.3 µm. The surface structure of aggregated CM shows short papillary projections demonstrating segmental adhesion of mucous components. CM is insoluble in water but partially soluble in anhydrous ethanol, and its spherical structure is completely decomposed by hexafluoroisopropanol (HFIP), a strong organic solvent. Since our fine structural observations clearly show that CM is not derived from vitellogenic or choriogenetic processes, the CM adhesive coatings during ovipositional process appears to be equivalent to cocoon silk for various protective functions in silken eggcase.

Nephila edulis-breeding and care under laboratory conditions

Due to fascinating mechanical and biological characteristics spider silk is of great interest in many research fields. Among the orb-weavers Nephila edulis is one of the species used as source for natural spider silk in laboratories. Under appropriate conditions, animals can be kept and bred easily. This manuscript gives information about the spiders’ natural habitat, behavior, and breeding and compares them with the established methods and conditions within a research laboratory. Keeping conditions and methods of rearing are described in detail. Within a keeping facility with reliable supply of food, cannibalism rate is significantly reduced and spiders mate all year long. Cohabitants of the genus Steatoda are routinely found in laboratory keeping. While these small spiders do not pose a threat to Nephila edulis, cellar spiders (family Pholcidae) have to be extracted as they have been observed hunting for Nephila spiders.

The shield effect: nuptial gifts protect males against pre-copulatory sexual cannibalism

Several not mutually exclusive functions have been ascribed to nuptial gifts across different taxa. Although the idea that a nuptial prey gift may protect the male from pre-copulatory sexual cannibalism is attractive, it has previously been considered of no importance based on indirect evidence and rejected by experimental tests. We reinvestigated whether nuptial gifts may function as a shield against female attacks during mating encounters in the spider Pisaura mirabilis and whether female hunger influences the likelihood of cannibalistic attacks. The results showed that pre-copulatory sexual cannibalism was enhanced when males courted without a gift and this was independent of female hunger. We propose that the nuptial gift trait has evolved partly as a counteradaptation to female aggression in this spider species.

Adaptations and Predispositions of Different Middle European Arthropod Taxa (Collembola, Araneae, Chilopoda, Diplopoda) to Flooding and Drought Conditions

Simple Summary

This review summarizes adaptations and predispositions of different arthropod taxa (springtails, web spiders, millipedes and centipedes) to flood and drought conditions. The main focus sis directed to arthropod species, which are living in Middle European floodplain forests and wetlands, because of the fast change of flood and drought conditions in these habitats. Furthermore the effects of the predicted regional climate change like increasing aperiodic summer flooding and decreasing winter and spring floods are also discussed.

Abstract

Floodplain forests and wetlands are amongst the most diverse and species rich habitats on earth. Arthropods are a key group for the high diversity pattern of these landscapes, due to the fact that the change between flooding and drought causes in different life cycles and in a variety of adaptations in the different taxa. The floodplain forests and wetlands of Central Amazonia are well investigated and over the last 50 years many adaptations of several hexapod, myriapod and arachnid orders were described. In contrast to Amazonia the Middle European floodplains were less investigated concerning the adaptations of arthropods to flood and drought conditions. This review summarizes the adaptations and predispositions of springtails, web spiders, millipedes and centipedes to the changeable flood and drought conditions of Middle European floodplain forests and wetlands. Furthermore the impact of regional climate change predictions like increasing aperiodic summer floods and the decrease of typical winter and spring floods are discussed in this article.

Spatial and temporal demographic variation drives within-season fluctuations in sexual selection

Our understanding of selection in nature stems mainly from whole-season and cross-sectional estimates of selection gradients. These estimates suggest that selection is relatively constant within, but fluctuates between seasons. However, the strength of selection depends on demographics, and because demographics can vary within seasons, there is a gap in our understanding regarding the extent to which seasonal fluctuations in demographics may cause variation in selection. Here we use two populations of the golden orb-web spider (Nephila plumipes) that differ in density to examine how demographics change within a season and whether there are correlated shifts in selection. We demonstrate that there is within-season variation in sex ratio and density at multiple spatial and temporal scales. This variation led to changes in the competitive challenges that males encountered at different times of the season and was correlated with significant variation in selection gradients on male size and weight between sampling periods. We highlight the importance of understanding the biology of the organism under study to correctly determine the relevant scale in which to examine selection. We also argue that studies may underestimate the true variation in selection by averaging values, leading to misinterpretation of the effect of selection on phenotypic evolution.

The systematics and biology of the spider genus Nephila (Araneae:Nephilidae) in the Australasian region

Five species of the nephilid genus Nephila Leach are found in the Australasian region, which for the purposes of this study was defined as Australia and its dependencies (including Lord Howe I., Norfolk I., Christmas I., Cocos (Keeling) Is), New Guinea (including Papua New Guinea and the Indonesian province of West Papua), Solomon Is, Vanuatu, New Caledonia, Fiji, Tonga, Niue, New Zealand and other parts of the south-west Pacific region. All species are redescribed and illustrated. Nephila pilipes (Fabricius) occurs in the closed forests of eastern and northern Australia, New Guinea, Solomon Is and Vanuatu (through to South-East Asia); N. plumipes (Latreille) is found in Australia (including Lord Howe I. and Norfolk I.), New Guinea, Vanuatu, Solomon Is and New Caledonia; N. tetragnathoides (Walckenaer) inhabits Fiji, Tonga and Niue; N. antipodiana (Walckenaer) occurs in northern Australia (as well as Christmas I.), New Guinea and Solomon Is (through to South-East Asia); and N. edulis (Labillardière) is found in Australia (including Cocos (Keeling) Is), New Guinea, New Zealand and New Caledonia. Epeira (Nephila) walckenaeri Doleschall, E. (N.) hasseltii Doleschall, N. maculata var. annulipes Thorell, N. maculata jalorensis Simon, N. maculata var. novae-guineae Strand, N. pictithorax Kulczyński, N. maculata var. flavornata Merian, N. pictithorax Kulczyński, N. maculata var. flavornata Merian, N. maculata piscatorum de Vis, and N. (N.) maculata var. lauterbachi Dahl are proposed as new synonyms of N. pilipes. Nephila imperialis var. novaemecklenburgiae Strand, N. ambigua Kulczyński, N. sarasinorum Merian and N. celebesiana Strand are proposed as new synonyms of N. antipodiana. Meta aerea Hogg, N. meridionalis Hogg, N. adelaidensis Hogg and N. meridionalis hermitis Hogg are proposed as new synonyms of N. edulis. Nephila picta Rainbow is removed from the synonymy of N. plumipes and treated as a synonym of N. edulis, and N. nigritarsis insulicola Pocock is removed from the synonymy of N. plumipes and treated as a synonym of N. antipodiana. Allozyme data demonstrate that N. pilipes is distinct at the 80% FD level from N. edulis, N. plumipes and N. tetragnathoides. Nephila plumipes and N. tetragnathoides, deemed to represent sister-taxa owing to the shared presence of a triangular protrusion of the male pedipalpal conductor, were found to differ at 15% FD in the genetic study. No genetic differentiation was found between 10 populations of N. edulis sampled across mainland Australia. Species of the genus Nephila have been extensively used in ecological and behavioural studies, and the biology of Nephila species in the Australasian region is extensively reviewed and compared with studies on Nephila species from other regions of the world.

Genital structures in the entelegyne widow spiderLatrodectus revivensis (Arachnida; Araneae; Theridiidae) indicate a low ability for cryptic female choice by sperm manipulation

The female genital structures of the entelegyne spider Latrodectus revivensis are described using semithin sections and scanning electron microscopy. Apart from the tactile hairs overhanging the opening of the atrium, the contact zones of the female epigynum are devoid of any sensilla, indicating that the female does not discriminate in favor or against males due to their genital size or stimulation through copulatory courtship. The dumb-bell shape and the spatial separation of the entrance and the exit of the paired spermathecae suggest that they are functionally of the conduit type. Not described for other entelegyne spiders so far, the small fertilization ducts originating from the spermathecae of each side lead to a common fertilization duct that connects the spermathecae to the uterus externus. During oviposition, it is most likely that spermatozoa are indiscriminately sucked out of the spermathecal lumina by the low pressure produced by the contraction of the muscle extending from the epigynal plate to the common fertilization duct. As no greater amounts of secretion are produced by the female during oviposition, and no activated sperm are present within the female genital tract, the secretion produced by the spermathecal epithelium does not serve in displacement or (selective) activation of spermatozoa. These findings suggest that female L. revivensis are not able to exert cryptic female choice by selectively choosing spermatozoa of certain males.

The function significance of silk decorations of orb-web spiders: a critical review of the empirical evidence

A number of taxonomically diverse species of araneoid spiders adorn their orb-webs with conspicuous silk structures, called decorations or stabilimenta. The function of these decorations remains controversial and several explanations have been suggested. These include: (1) stabilising and strengthening the web; (2) hiding and concealing the spider from predators; (3) preventing web damage by larger animals, such as birds; (4) increasing foraging success; or (5) providing a sunshield. Additionally, they may have no specific function and are a consequence of stress or silk regulation. This review evaluates the strength of these explanations based on the evidence. The foraging function has received most supporting evidence, derived from both correlative field studies and experimental manipulations. This contrasts with the evidence provided for other functional explanations, which have not been tested as extensively. A phylogenetic analysis of the different decoration patterns suggests that the different types of decorations are as evolutionary labile as the decorations themselves: the analysis shows little homology and numerous convergences and independent gains. Therefore, it is possible that different types of decorations have different functions, and this can only be resolved by improved species phylogenies, and a combination of experimental and ultimately comparative analyses.

Genital morphology of Nephila edulis: implications for sperm competition in spiders

The genital morphology of the female is assumed to control the pattern of sperm priority. Spiders are divided roughly along phylogenetic lines into haplogyne and entelegyne types, the principal difference being in female genital morphology (cul-de-sac and conduit types of spermathecae). Nephila edulis is an entelegyne spider and we studied the genital morphology of both sexes by means of scanning electron microscopy and semithin sectioning. In the female, the copulatory ducts leading to the spermatheca were much longer and more differentiated than the fertilization duct. We question the common assumption that possession of genitalia of the conduit type with separate copulatory and fertilization ducts will automatically lead to first-male sperm priority because in our typical entelegyne species, the copulatory ducts were similar in arrangement to the combined copulatory/fertilization ducts in genitalia of the typical cul-de-sac type.