Linden Higgins

VARIATION IN THE CHEMICAL COMPOSITION OF ORB WEBS BUILT BY THE SPIDER NEPHILA CLAVIPES (ARANEAE, TETRAGNATHIDAE)

Abstract The adhesive droplets in the orb webs of araneoid spiders contain, among other constituents, an aqueous solution of organic low-molecular-weight compounds. The chemical composition of this solution has been investigated for pooled web collections from several species, but little is known about how the composition might vary among individuals or among environments. To begin addressing these questions, we analyzed serial collections of orb webs spun by individual juvenile Nephila clavipes from three different populations held first under field conditions and then under laboratory conditions. Our results indicate that the composition of the organic low-molecular-weight solution is not fixed. We found significant differences in the droplet composition among individuals, among populations, and with the transfer of spiders to laboratory conditions. The possible origins and consequences of these differences are discussed.

The Nephila clavipes genome highlights the diversity of spider silk genes and their complex expression

Spider silks are the toughest known biological materials, yet are lightweight and virtually invisible to the human immune system, and they thus have revolutionary potential for medicine and industry. Spider silks are largely composed of spidroins, a unique family of structural proteins. To investigate spidroin genes systematically, we constructed the first genome of an orb-weaving spider: the golden orb-weaver (Nephila clavipes), which builds large webs using an extensive repertoire of silks with diverse physical properties. We cataloged 28 Nephila spidroins, representing all known orb-weaver spidroin types, and identified 394 repeated coding motif variants and higher-order repetitive cassette structures unique to specific spidroins. Characterization of spidroin expression in distinct silk gland types indicates that glands can express multiple spidroin types. We find evidence of an alternatively spliced spidroin, a spidroin expressed only in venom glands, evolutionary mechanisms for spidroin diversification, and non-spidroin genes with expression patterns that suggest roles in silk production.

Testing ecological and developmental hypotheses of mean and variation in adult size in nephilid orb-weaving spiders

Fecundity selection has been hypothesized to drive the evolution of female gigantism in the orb-weaving family Nephilidae. Several species of these spiders also exhibit large amounts of variation in size at maturity in one or both sexes. In this article, we attempt to detect correlations of mean and variation in adult size at a phylogenetic scale between the sexes and with latitude. We tested six predictions derived from three broad developmental, ecological, and age structure hypotheses, using independent contrasts and a recent species-level nephilid phylogeny as well as least squares and other conventional statistics: 1. In both sexes, species with larger mean size will have greater variation in size; 2. Males and females will show correlated changes in mean size and of variation in size; 3. In both sexes, mean size will be negatively correlated with the midpoint of the latitudinal range; 4. In both sexes, tropical species will be more variable; 5. In both sexes, more widespread species will be more variable; 6. Variation in male size will be positively correlated with mean female size. In no cases were male and female development correlated, suggesting that in this lineage male and female body size evolve independently. The only significant trend detected was a positive phylogenetic correlation between variation in female size and latitude, the opposite of prediction 4. Power tests showed that in all tests of the ecological hypothesis, sample sizes were more than adequate to detect significant trends, if present. Our results suggest that evolutionary trends in juvenile development among species are too weak to be detectable in such data sets.

Developmental response to low diets by giant Nephila clavipes females (Araneae: Nephilidae)

Abstract Female-biased sexual size dimorphism is common in arthropods, apparently driven by fecundity selection in females. Selective pressures that limit growth are less often considered. One factor that researchers have rarely considered is the possible role of energetic limits on growth. The orb weaving spider Nephila clavipes (Linnaeus 1767) is extremely sexually size dimorphic. Males are “normal” sized spiders and females are up to ten times longer, having passed through several additional juvenile instars. This extreme size dimorphism presents the opportunity to test for intrinsic energetic costs of gigantism. Prior studies have shown that males successfully reach maturity on a range of diets, while female dietary requirements increase rapidly with increasing size. We here examine the effects of variation in food availability on juvenile female development by randomly assigning spiderlings from six different families (from six distinct populations) to quantitatively varying but qualitatively identical diets. Based upon field observations, we expected that dietary restrictions would have the greatest effect on duration of instars, particularly later instars, and on instar number (because longer total development would lead to curtailment of growth at an earlier stage), with relatively little effect on growth per molt. Because the diets ranged from higher than mean intake observed in the field to well below mean intake, we expected females to mature at a wide range of instars (and sizes). Our results support the functional relationship among food intake, instar duration, and fixed growth per molt (although growth per molt was less canalized than suggested by field observations). However, we observed no variation in number of instars, and we suggest that these data provide additional support for the importance of rare, large prey in the diets of web-building spiders.

Nephila clavipes females have accelerating dietary requirements

Abstract Nephila spiders are famous for extreme sexual size dimorphism, with females an order of magnitude larger than males. The proximal developmental mechanism for the sexual size dimorphism is extended development in females: they have many more juvenile instars than males. During an experimental rearing of Nephila clavipes (Linnaeus 1767) from two populations, we discovered that females cannot reach sexual maturity on diets that are qualitatively and quantitatively sufficient for male maturation. Here we describe the dietary regimes that produced sexually mature females and the life history implications of these requirements.

QUANTITATIVE SHIFTS IN ORB-WEB INVESTMENT DURING DEVELOPMENT IN NEPHILA CLAVIPES (ARANEAE, NEPHILIDAE)

Abstract When there are direct conflicts in resource allocation to foraging effort versus growth and development, the relative allocation to foraging may change in a predictable manner with development. Orb-webs provide a physical record of resource allocation to foraging, and their synthesis requires the investment of physiologically important resources. Spiders in strongly seasonal habitats must complete development prior to the end of the season, and may be expected to alter foraging effort to maximize the probability of successful reproduction. Comparison of populations of the orb-weaving spider Nephila clavipes (Araneae, Nephilidae) in very seasonal versus less seasonal habitats allows testing for changes in allocation of resources to foraging effort during development. Orb-web size increases with increasing spider size, with little variation in slope among populations. However, in univoltine populations inhabiting strongly seasonal habitats, the size of the orb web is not a simple function of spider size: the rate of increase in orb-web size decelerates abruptly at a relatively small juvenile stage. Spiders in a less seasonal habitat did not decelerate foraging investment, and the pattern cannot be explained by changes in other aspects of orb-web structure. I postulate that the decline in relative investment into foraging is related to increased investment into juvenile female growth and development in circumstances where delayed maturation carries heavy fitness penalties.

Juvenile Nephila (Araneae, Nephilidae) Use Various Attack Strategies For Novel Prey

Abstract Nephila clavipes (Linnaeus 1767) and N. pilipes (Fabricius 1793) juveniles exposed to a novel and potentially dangerous prey item frequently attack using thrown silk. To quantify the frequency with which N. clavipes opt to use thrown silk, naïve hand-reared small N. clavipes juvenile females were observed attacking a new prey type, stingless bees. Repeated exposure to the stingless bees suggests that the spiders incorporate prior experience into prey attack strategies, as experienced spiders attacked using the more usual Nephila long-bite.