I-Min Tso

EVALUATION OF FORMULAE TO ESTIMATE THE CAPTURE AREA AND MESH HEIGHT OF ORB WEBS (ARANEOIDEA, ARANEAE)

Abstract We evaluated several formulae to estimate the capture area (the area of the web covered by capture spirals) and the mesh height (the distance between capture spirals) of orb webs constructed by Argiope keyserlingi Karsch. The accuracy of the various formulae was estimated through regression analyses. Accordingly, we propose two new formulae specifically suited for asymmetric orb webs, which provide accurate estimates of capture area and mesh height.

Giant wood spider Nephila pilipes alters silk protein in response to prey variation.

SUMMARY Recent studies have demonstrated that orb-weaving spiders may alter web structures, foraging localities or silk output in response to prey variations. In this study we conducted field surveys and food manipulations to examine whether orb-weaving spiders may also adjust the protein of silk to prey variations. A comparison of dragline silks collected from nine giant wood spider Nephila pilipes populations in Taiwan showed a spatial variation. The percentage of all amino acids (except alanine and glycine) exhibited significant differences among populations. A survey of prey composition also revealed a significant spatial variation among N. pilipes populations. To determine whether prey variation was responsible for silk protein variation, we fed N. pilipes with different types of prey (dipteran vs orthopteran) then compared the percentage of five major dragline amino acids and secondary structures. The results showed that dragline of N. pilipes fed with orthopteran prey contained significantly higher proline and glutamine but lower alanine. Congruent with this result were those from FTIR spectroscopy, which showed that dragline of N. pilipes fed with crickets exhibited significantly higher percentage of proline- and glutamine-containing β turns, and lower percentage of alanine-containing β sheet structures. Since the results of feeding manipulations showed that diet significantly affected the compositions of dragline silks, the observed spatial variation seemed to reflect the different types of prey these spiders had consumed. Results of this study thus indicated that orb-weaving spiders can alter dragline protein in response to prey variations.

Colour-associated foraging success and population genetic structure in a sit-and-wait predator Nephila maculata (Araneae: Tetragnathidae)

Giant wood spiders, Nephila maculata (Fabricius 1793), typically have a greenish cephalothorax and a dark abdomen decorated with striking yellow bands and spots. However, in Taiwan and neighbouring coastal islands we also found some morphologically indistinguishable individuals that were totally dark. As insects are attracted to ultraviolet (UV) light, we compared the UV reflectance property and insectcatching ability of the two morphs to see whether variation in colour affected foraging success. We also examined the population genetic structure to estimate indirectly the level of gene flow between these two colour morphs. Body surface UV reflection rate was measured from six areas of the spider with a spectrometer. To compare the insect-catching ability of different morphs, we recorded the spiders’ body colour, orb size and insect-interception rates. The typical morph of N. maculata reflected significantly more UV in four of the six areas examined and caught significantly more insects than the melanic morph. We estimated population genetic structure by allozyme electrophoresis, using 20 loci from 17 enzymes. The population differentiation index (FST) derived from all eight polymorphic loci was 0.023, indicating a minimum level of genetic differentiation. These results indicate that the two morphs of N. maculata may be members of an interbreeding population, and melanics have lower foraging success because of a lower body surface reflectance.  2002 The Association for the Study of Animal Behaviour

Diurnal and nocturnal prey luring of a colorful predator

SUMMARY While animal color signaling has been studied for decades, we have little knowledge of the role conspicuous body coloration plays in the nocturnal context. In this study we explored animal color signaling in both diurnal and nocturnal contexts to arrive at a more comprehensive understanding of its function. We quantified how the brightly colored giant wood spiders Nephila pilipes are viewed by nocturnal insects, and performed field manipulations to assess the function of a spiders coloration in both diurnal and nocturnal conditions. Seen through the eyes of moths, the conspicuous body parts of spiders are quite distinctive from the vegetation background. The presence of N. pilipes significantly increased the diurnal as well as the nocturnal prey interception rates of their webs, but these rates were significantly reduced when the conspicuous color signals of N. pilipes were altered by black paint. A comparison of the diurnal and nocturnal hunting performances of spiders showed that their conspicuous coloration had a higher luring effect under dim light conditions. These results demonstrate that the conspicuous body coloration of N. pilipes functions as a visual lure to attract both diurnal and nocturnal prey. It seems that nocturnal insects are the major target of this colorful sit-and-wait predator. We suggest that the selection pressure to effectively exploit the color vision of nocturnal prey could be one of the major forces driving the evolution of spider coloration.

Multiple prey cues induce foraging flexibility in a trap-building predator

Predators must be behaviourally flexible to counter the temporal and spatial stochastic fluctuations and response variability of their prey. To ensure behaviours are adequate across environments, animals must regularly assess environmental cues. Spider orb webs are an example of a flexible foraging trait in a predator, as web architectural components vary in response to exposure to different prey types and prey traits. The cues used by orb web spiders to initiate changes in web architecture are not known. Current research predicts that prey nutrients and vibratory stimuli are potential candidates, but how they combine to affect spider foraging decisions is not clear. We performed experiments exposing the giant wood spider, Nephila pilipes, to different prey nutrients and vibratory stimuli. Spiders were fed either large profitable prey with high kinetic energy (crickets) or small prey with low kinetic energy (flies). In two treatments the prey nutrients and vibratory cues came from live prey, but in the other two treatments spiders received dead crickets with webs stimulated by flies and vice versa. The spiders fed on live flies built larger webs with more radii that were less stiff and had greater vibration damping. These web characteristics did not differ between the other three treatment groups. Our results show that in the absence of nutrient and vibratory cues from profitable prey, spiders alter their web architecture to build webs better able to capture the less profitable prey at a cost of more material investment, greater web visibility and reduced vibratory signal clarity.

Silk stealing by Argyrodes lanyuensis (Araneae: Theridiidae): a unique form of kleptoparasitism.

In this study we used field surveys and experiments to investigate silk stealing, a unique form of kleptoparasitism. We studied the role that silk stealing plays in the interactions between a kleptoparasitic spider Argyrodes lanyuensis and its giant wood spider host Nephila maculata by testing: (1) if silk stealing generates any cost to the hosts; (2) if the amount of web lost to kleptoparasites varies with the size of the host; and (3) how prey stealing by A. lanyuensis affects N. maculata. Compared with uninvaded webs, N. maculata webs invaded by A. lanyuensis suffered an average orb reduction of 21%. There was a significant negative correlation between the percentage web reduction and body length of N. maculata, indicating that the impact of silk stealing was greater on younger hosts. No significant correlation was found between the number of A. lanyuensis on orbs and orb loss, but there was a significant negative relationship between percentage orb loss and silk diameter of N. maculata. These results suggest that the finer silk of smaller hosts may be at least partially responsible for their higher percentage orb loss. While prey stealing by other Argyrodes species greatly affects the foraging gain of hosts, A. lanyuensis did not seem to affect N. maculata. This was because the host took larger prey than the kleptoparasite did, and less than 3% of the hosts biomass intake came from prey that A. lanyuensis would steal. Copyright 1998 The Association for the Study of Animal Behaviour.

Biogeography and Speciation Patterns of the Golden Orb Spider Genus Nephila (Araneae: Nephilidae) in Asia

The molecular phylogeny of the globally distributed golden orb spider genus Nephila (Nephilidae) was reconstructed to infer its speciation history, with a focus on SE Asian/W Pacific species. Five Asian, two Australian, four African, and one American species were included in the phylogenetic analyses. Other species in Nephilidae, Araneidae, and Tetragnathidae were included to assess their relationships with the genus Nephila, and one species from Uloboridae was used as the outgroup. Phylogenetic trees were reconstructed from one nuclear (18S) and two mitochondrial (COI and 16S) markers. Our molecular phylogeny shows that the widely distributed Asian/Australian species, N. pilipes, and an African species, N. constricta, form a clade that is sister to all other Nephila species. Nested in this Nephila clade are one clade with tropical and subtropical/temperate Asian/Australian species, and the other containing African and American species. The estimated divergence times suggest that diversification events within Nephila occurred during mid-Miocene to Pliocene (16 Mya-2 Mya), and these time periods were characterized by cyclic global warming/cooling events. According to Dispersal and Vicariance Analysis (DIVA), the ancestral range of the Asian/Australian clade was tropical Asia, and the ancestral range of the genus Nephila was either tropical Asia or Africa. We conclude that the speciation of the Asian/Australian Nephila species was driven by Neogene global cyclic climate changes. However, further population level studies comparing diversification patterns of sister species are needed to determine the mode of speciation of these species.

Salticid predation as one potential driving force of ant mimicry in jumping spiders

Many spiders possess myrmecomorphy, and species of the jumping spider genus Myrmarachne exhibit nearly perfect ant mimicry. Most salticids are diurnal predators with unusually high visual acuity that prey on various arthropods, including conspecifics. In this study, we tested whether predation pressure from large jumping spiders is one possible driving force of perfect ant mimicry in jumping spiders. The results showed that small non-ant-mimicking jumping spiders were readily treated as prey by large ones (no matter whether heterospecific or conspecific) and suffered high attack and mortality rates. The size difference between small and large jumping spiders significantly affected the outcomes of predatory interactions between them: the smaller the juvenile jumping spiders, the higher the predation risk from large ones. The attack and mortality rates of ant-mimicking jumping spiders were significantly lower than those of non-ant-mimicking jumping spiders, indicating that a resemblance to ants could provide protection against salticid predation. However, results of multivariate behavioural analyses showed that the responses of large jumping spiders to ants and ant-mimicking salticids differed significantly. Results of this study indicate that predation pressure from large jumping spiders might be one selection force driving the evolution of nearly perfect myrmecomorphy in spiders and other arthropods.

Prey type, vibrations and handling interactively influence spider silk expression.

SUMMARY The chemical and mechanical properties of spider major ampullate (MA) silks vary in response to different prey, mostly via differential expression of two genes – MaSp1 and MaSp2 – although the spinning process exerts additional influence over the mechanical properties of silk. The prey cues that initiate differential gene expression are unknown. Prey nutrients, vibratory stimuli and handling have been suggested to be influential. We performed experiments to decouple the vibratory stimuli and handling associated with high and low kinetic energy prey (crickets vs flies) from their prey nutrients to test the relative influence of each as inducers of silk protein expression in the orb web spider Nephila pilipes. We found that the MA silks from spiders feeding on live crickets had greater percentages of glutamine, serine, alanine and glycine than those from spiders feeding on live flies. Proline composition of the silks was unaffected by feeding treatment. Increases in alanine and glycine in the MA silks of the live-cricket-feeding spiders indicate a probable increase in MaSp1 gene expression. The amino acid compositions of N. pilipes feeding on crickets with fly stimuli and N. pilipes feeding on flies with cricket stimuli did not differ from each other or from pre-treatment responses, so these feeding treatments did not induce differential MaSp expression. Our results indicate that cricket vibratory stimuli and handling interact with nutrients to induce N. pilipes to adjust their gene expression to produce webs with mechanical properties appropriate for the retention of this prey. This shows that spiders can genetically alter their silk chemical compositions and, presumably, mechanical properties upon exposure to different prey types. The lack of any change in proline composition with feeding treatment in N. pilipes suggests that the MaSp model determined for Nephila clavipes is not universally applicable to all Nephila.

Wind induces variations in spider web geometry and sticky spiral droplet volume

SUMMARY Trap building by animals is rare because it comes at a substantial cost. Using materials with properties that vary across environments maintains trap functionality. The sticky spiral silks of spider orb webs are used to catch flying prey. Web geometry, accompanied by compensatory changes in silk properties, may change across environments to sustain web functionality. We exposed the spider Cyclosa mulmeinensis to wind to test whether wind-induced changes in web geometry are accompanied by changes in aggregate silk droplet morphology, axial thread width or spiral stickiness. We compared: (i) web catching area, (ii) length of total silks, (iii) mesh height, (iv) number of radii, (v) aggregate droplet morphology and (vi) spiral thread stickiness, between webs made by spiders exposed to wind and those made by spiders not exposed to wind. We interpreted co-variation in droplet morphology or spiral stickiness with web capture area, mesh height or spiral length as the silk properties functionally compensating for changes in web geometry to reduce wind drag. Wind-exposed C. mulmeinensis built webs with smaller capture areas, shorter capture spiral lengths and more widely spaced capture spirals, resulting in the expenditure of less silk. Individuals that were exposed to wind also deposited larger droplets of sticky silk but the stickiness of the spiral threads remained unchanged. The larger droplets may be a product of a greater investment in water, or low molecular weight compounds facilitating atmospheric water uptake. Either way, droplet dehydration in wind is likely to be minimized.