Adhesive compatibility of cribellar and viscous prey capture threads and its implication for the evolution of orb-weaving spiders

Evolution of orb-weaving spiders that comprise the Orbiculariae clade involved a transition in the composition of prey capture thread that has been challenging to explain. The primitive cribellar threads spun by members of the Deinopoidea subclade resemble the capture threads of their non-orb-web-weaving ancestors and are formed of thousands of fine, dry, protein cribellar fibrils. In contrast, the derived viscous capture threads spun by members of the Araneoidea subclade have regularly spaced, aqueous adhesive droplets. When second instar deinopoid spiderlings emerge from egg sacs they are unable to spin cribellar threads, and, therefore, do not construct orb-webs; whereas second instar araneoids spin capture threads and construct orb-webs. If, as we hypothesize, viscous material evolved to enable second instar spiderlings to construct orb-webs, early araneoids may have spun composite cribellar-viscous capture threads. To examine the functional feasibility of such intermediate capture threads, we compared the adhesion of cribellar threads, viscous threads, and combined cribellar-viscous threads. The stickiness of these combined threads was greater than that of native cribellar or viscous threads alone. The viscous material of Araneus marmoreus threads exhibited a substantial increase in stickiness when combined with cribellar fibrils and that of Argiope aurantia threads a small increase in stickiness when combined with cribellar fibrils. Thus, if early araneoids retained their ability to spin cribellar threads after having evolved glands that produced viscous material, their composite threads could have formed a functional adhesive system that achieved its stickiness at no loss of material economy.     

Factors governing the stickiness of cribellar prey capture threads in the spider family Uloboridae

The surface of a cribellar prey capture thread is formed of thousands of fine, looped fibrils, each issuing from one of the spigots on an oval spinning plate termed the cribellum. This plesiomorphic capture thread is retained by members of the family Uloboridae, in which its stickiness differs among genera. An examination of five cribellar thread features in nine uloborid species shows that only the number of fibrils that form a thread explains these differences in thread stickiness. Neither the physical features of these fibrils, nor the manner in which they are combined to form threads differs among species. Threads produced by orb-weaving species contain fewer fibrils than those produced by species that build reduced webs. Relative to spider weight, the number of fibrils that form a cribellar thread is greatest in simple-web species of the genus Miagrammopes, less in triangle-web species of the genus Hyptiotes, and least in orb-weaving species representing five genera. A transformational analysis shows that change in the number of cribellum spigots is directly related to change in the stickiness of cribellar thread. This direct relationship between the material invested in a cribellar thread and its stickiness may have been a limiting factor that favored the switch from the dry cribellar threads of uloborids to the adhesive capture threads produced by other orb-weaving families. © 1994 Wiley-Liss, Inc.     

Population studies of two colonial orb-weaving spiders

Colonial spiders have individual capture webs (territories) within a communally shared web structure. I describe here the life histories and colony population dynamics of two communal species, Ctrtophora moluccensis (Doleschall) (Araneidae) in Papua New Guinea and Philoponella republicana (Simon) (Uloboridae) in the Panama Canal Zone. In both species, dispersal and foundation of new colonies are primarily by groups of immatures. Population growth of new-colonies was rapid during the first generation, but then colony population size decreased markedly. Colonies of P. republicana rarely lasted more than one generation, whereas those of C. moluccensis attained an equilibrium population size and often persisted for many generations at the same site. Reproduction occurred during the wet season in P. republicana colonies and year-round in colonies of C. moluccensis. Reproduction was synchronized in widely separated colonies of P. republicana. Factors controlling population growth and survival of colonies are discussed. Cyrtophora moluccensis colonies were probably regulated by density dependant factors, especially predation and parasitism, and perhaps a shortage of flying insects due to colony visibility. Philoponella republicana colonies were most likely limited by climatic conditions and instability of the habitat (i.e. density independent factors). Colonial social organization influences both dispersal and colony population growth. Coloniality is, however, compatible with various life history strategies.     

Co-adapted foraging traits in a guild of orb-weaving spiders

Summary From a previous two-species comparison, specific suites of morphological and web design traits are predicted to characterize foraging specializations in old-field orb-weaving spiders. Species with stout legs, large fangs, open mesh webs, high webs, and retreats should specialize on innocuous, rapidly escaping prey types. Species with long legs, small fangs, dense mesh webs, and low webs that rest in the hub should specialize on dangerous, clumsy prey that escape from webs slowly. Factor and discriminant analyses are used to test for these association patterns in eight co-occurring species of old-field orb-weavers. The association between most of the characters is supported, but the appearance of a new combination suggests that another dimension needs to be added to the model. Habitat use and diet predicted by the model are tested for species where data are available.     

Miniaturized orb-weaving spiders: behavioural precision is not limited by small size

The special problems confronted by very small animals in nervous system design that may impose limitations on their behaviour and evolution are reviewed. Previous attempts to test for such behavioural limitations have suffered from lack of detail in behavioural observations of tiny species and unsatisfactory measurements of their behavioural capacities. This study presents partial solutions to both problems. The orb-web construction behaviour of spiders provided data on the comparative behavioural capabilities of tiny animals in heretofore unparalleled detail; species ranged about five orders of magnitude in weight, from approximately 50-100mg down to some of the smallest spiders known (less than 0.005mg), whose small size is a derived trait. Previous attempts to quantify the 'complexity' of behaviour were abandoned in favour of using comparisons of behavioural imprecision in performing the same task. The prediction of the size limitation hypothesis that very small spiders would have a reduced ability to repeat one particular behaviour pattern precisely was not confirmed. The anatomical and physiological mechanisms by which these tiny animals achieve this precision and the possibility that they are more limited in the performance of higher-order behaviour patterns await further investigation.     

Foraging advantages of mixed-species association between solitary and colonial orb-weaving spiders

This study investigated association between solitary orb-weaving spiders and a colonial orb-weaving spider, Metepeira incrassata (Araneae: Araneidae). Spiders were sampled along transects and an index of species association showed that two of the species were associated more frequently than expected based on a null hipothesis of random co-occurrence. The potential advantages of mixed-species association were investigated by comparing prey-capture success of one of these associates, Nephila clavipes (Araneae: Tetragnathidae), when it occurs alone, in single-species groups, and when associated with M. incrassata colonies. Field observations of prey-capture success by all three of these categories of N. clavipes revealed that individuals in M. incrassata colonies captured significantly more prey than solitaries or individuals in single-species groups. The increase in prey capture by N. clavipes in M. incrassata colonies may result from utilization of a foraging niche which intercepts a diffirent spectrum of prey than that available to single-species groups or solitaries. Related to this enhanced prey consumption is greater fecundity of spiders in association with M. incrassata as compared to solitaries or individuals in single-species groups.     

Damping capacity is evolutionarily conserved in the radial silk of orb-weaving spiders

Orb-weaving spiders depend upon their two-dimensional silk traps to stop insects in mid flight. While the silks used to construct orb webs must be extremely tough to absorb the tremendous kinetic energy of insect prey, webs must also minimize the return of that energy to prey to prevent insects from bouncing out of oscillating webs. We therefore predict that the damping capacity of major ampullate spider silk, which forms the supporting frames and radial threads of orb webs, should be evolutionarily conserved among orb-weaving spiders. We test this prediction by comparing silk from six diverse species of orb spiders. Silk was taken directly from the radii of orb webs and a Nano Bionix test system was used either to sequentially extend the silk to 25% strain in 5% increments while relaxing it fully between each cycle, or to pull virgin silk samples to 15% strain. Damping capacity was then calculated as the percent difference in loading and unloading energies. Damping capacity increased after yield for all species and typically ranged from 40 to 50% within each cycle for sequentially pulled silk and from 50 to 70% for virgin samples. Lower damping at smaller strains may allow orb webs to withstand minor perturbations from wind and small prey while still retaining the ability to capture large insects. The similarity in damping capacity of silk from the radii spun by diverse spiders highlights the importance of energy absorption by silk for orb-weaving spiders.     

Adhesive efficiency of spider prey capture threads

Cribellar capture threads are comprised of thousands of fine silk fibrils that are produced by the spigots of a spider's cribellum spinning plate and are supported by larger interior axial fibers. This study examined factors that constrain the stickiness of cribellar threads spun by members of the orb-weaving family Uloboridae in the Deinopoidea clade and compared the material efficiency of these threads with that of viscous capture threads produced by members of their sister clade, the Araneoidea. An independent contrast analysis confirmed the direct relationship between cribellar spigot number and cribellar thread stickiness. A model based on this relationship showed that cribellar thread stickiness is achieved at a rapidly decreasing material efficiency, as measured in terms of stickiness per spigot. Another limitation of cribellar thread was documented when the threads of two uloborid species were measured with contact plates of four widths. Unlike that of viscous threads, the stickiness of cribellar threads did not increase as plate width increased, indicating that only narrow bands along the edges of thread contact contributed to their stickiness. As thread volume increased, the gross material efficiency of cribellar threads decreased much more rapidly than that of viscous threads. However, cribellar threads achieved their stickiness at a much greater gross material efficiency than did viscous threads, making it more challenging to explain the transition from deinopoid to araneoid orb-webs.     

The allometry of CNS size and consequences of miniaturization in orb-weaving and cleptoparasitic spiders

Allometric studies of the gross neuroanatomy of adults from nine species of spiders from six web-weaving families (Orbicularia), and nymphs from six of these species, show that very small spiders resemble other small animals in having disproportionately larger central nervous systems (CNSs) relative to body mass when compared with large-bodied forms. Small spiderlings and minute adult spiders have similar relative CNS volumes. The relatively large CNS of a very small spider occupies up to 78% of the cephalothorax volume. The CNSs of very small spiders extend into their coxae, occupying as much as 26% of the profile area of the coxae of an Anapisona simoni spiderling (body mass < 0.005 mg). Such modifications occur both in species with minute adults, and in tiny spiderlings of species with large-bodied adults. In at least one such species, Leucauge mariana, the CNS of the spiderling extends into a prominent ventral bulge of the sternum. Tiny spiders also have reduced neuronal cell body diameters. The adults of nearly all orbicularian spiders weave prey capture webs, as do the spiderlings, beginning with second instar nymphs. Comparable allometric relations occur in adults of both orb-weaving and cleptoparasitic species, indicating that this behavioral difference is not reflected in differences in gross CNS allometry.     

An organic coating keeps orb‐weaving spiders (Araneae,Araneoidea, Araneidae) from sticking to their own capture threads

More than 95% of orb‐weaving spider species ensure prey capture success by producing viscous threads equipped with gluey droplets. However, this trap may bear serious risks for the web‐inhabiting spider as well. The obvious question, how a spider avoids getting stuck in its own capture spiral, has gained little scientific attention up till now. In 1905, the French naturalist Jean‐Henry Fabre concluded from anecdotal observation that orb‐weaving spiders protect themselves by a fatty surface coating. Here, we test this hypothesis by indirectly measuring the force necessary to detach an autotomized spider’s leg from the capture spiral of its own web (here called ‘index of adhesion’, IOA). Three groups of legs, each of the species Araneus diadematus Clerck, 1757 and Larinioides sclopetarius (Clerck, 1757), were tested. One was left untreated, one was washed with distilled water (H₂O), and one was washed with the organic solvent carbon disulphide (CS₂). In both species, we found a weak IOA between the spider leg and the gluey capture spiral in untreated and water‐washed legs without significant differences between the two. The IOA approximately doubled, when spider legs had been washed with carbon disulphide prior to measurement, that is, the CS₂‐washed legs stuck significantly more strongly than the untreated and water‐washed legs. These results provide indirect evidence for a protective anti‐adhesive organic coating on the spider’s body surface and so support Fabre’s hypothesis.     

Individual differences in boldness positively correlate with heart rate in orb-weaving spiders of genus Larinioides

Recent years have seen a dramatic increase in the number of studies devoted to consistent individual differences in behavior, termed ＂behavioral syndromes＂ or animal ＂personality＂. In particular, ecologists and evolutionary biologists alike seek to explore the functional consequences of individuals＇ personalities and explanations for their general maintenance. Understand ing the mechanistic underpinnings to personality is arguably key to a deeper understanding of their functional consequences and maintenance. In particular, consistent individual differences in metabolic rate have been proposed as a general explanation for the maintenance of consistent individual differences in boldness, aggressiveness, and activity level. In this paper we explore whether consistent individual differences in boldness are associated with heat rate, a proxy for metabolic rate, in two species of orbweaving spider： Larinioides cornutus and L. patagiatus. We first documented consistent individual differences in boldness in L. cornutus but not L. patagiatus, as estimated by individuals＇ latency to resume movement following an aversive stimulus. We also measured individuals＇ heart rate twice in a separate situation. We then tested for an association between individuals＇ boldness and their heart rate during these two events. We found that increased boldness was associated with increased heart. Thus, indi viduals that resumed movement quickly following an aversive stimulus also exhibited faster heart rates. Our studies provide fur ther evidence that consistent individual differences in key physiological attributes, like metabolic rate, might be a general mecha nism underlying consistent individual differences in behavior .     

The phylogenetic basis of sexual size dimorphism in orb-weaving spiders (Araneae,Orbiculariae)

Extreme sexual body size dimorphism (SSD), in which males are only a small fraction of the size of the females, occurs only in a few, mostly marine, taxonomic groups. Spiders are the only terrestrial group in which small males are relatively common, particularly among orb-weavers (especially in the families Tetragnathidae and Araneidae) and crab spiders (Thomisidae). We used a taxonomic sample of 80 genera to study the phylogenetic patterns (origins and reversals) of SSD in orb-weaving spiders (Orbiculariae). We collected and compiled male and female size data (adult body length) for 536 species. Size data were treated as a continuous character, and ancestral sizes, for males and females separately, were reconstructed by using Wagner parsimony on a cladogram for the 80 genera used in this study. Of these 80 genera, 24 were female-biased dimorphic (twice or more the body length of the male); the remaining 56 genera were monomorphic. Under parsimony only four independent origins of dimorphism are required: in the theridiid genus Tidarren, in the distal nephilines, in the "argiopoid clade," and in the araneid genus Kaira. Dimorphism has reversed to monomorphism at least seven times, all of them within the large "argiopoid clade." The four independent origins of dimorphism represent two separate instances of an increase in female size coupled with a decrease of male size (involving only two genera), and two separate instances of an increase in female size with male size either remaining the same or increasing, but not as much as females (involving 30 genera). In orb-weaving spiders, far more taxa are sexually dimorphic as a result of female size increase (22 genera) than as a result of male size decrease (two genera). SSD in orb-weaving spiders encompasses several independent evolutionary histories that together suggest a variety of evolutionary pathways. This multiplicity strongly refutes all efforts thus far to find a general explanation for either the origin or maintenance (or both) of SSD, because the different pathways very likely will require distinctly different, possibly unique, explanations. Each pattern must be understood historically before its origin and maintenance can be explained in ecological and evolutionary terms. The most frequently cited example of male dwarfism in spiders, the golden orb-weaving spider genus Nephila (Tetragnathidae), is in fact a case of female giantism, not male dwarfism.     

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.     

Evidence for diet effects on the composition of silk proteins produced by spiders

Silks are highly expressed, secreted proteins that represent a substantial metabolic cost to the insects and spiders that produce them. Female spiders in the superfamily Araneoidea (the orb-spinning spiders and their close relatives) spin six different kinds of silk (three fibroins and three fibrous protein glues) that differ in amino acid content and protein structure. In addition to this diversity in silks produced by different glands, we found that individual spiders of the same species can spin dragline silks (drawn from the spider's ampullate gland) that vary in content as well. Freely foraging ARGIOPE: argentata (Araneae: Araneoidea), collected from 13 Caribbean islands, produced dragline silk that showed an inverse relationship between the amount of serine and glycine they contained. X-ray microdiffraction of the silks localized these differences to the amorphous regions of the protein that are thought to lend silks their elasticity. The crystalline regions of the proteins, which lend silks their strength, were unaffected. Laboratory experiments with ARGIOPE: keyserlingi suggested that variation in silk composition reflects the type of prey the spiders were fed but not the total amount of prey they received. Hence, it may be that the amino acid content (and perhaps the mechanical properties) of dragline silk spun by ARGIOPE: directly reflect the spiders' diet. The ability to vary silk composition and, possibly, function is particularly important for organisms that disperse broadly, such as Argiope, and that occupy diverse habitats with diverse populations of prey.     

Structure of a viscous exopolysaccharide produced by Lactobacillus helveticus K16

A viscous extracellular polysaccharide produced by Lactobacillus helveticus K16 has been investigated. Sugar and methylation analysis, 1H and 13C NMR spectroscopy revealed that the polysaccharide is composed of a hexasaccharide repeating unit. The sequence of sugar residues was determined by use of two-dimensional nuclear Overhauser effect spectroscopy and heteronuclear multiple bond connectivity experiments. The structure of the repeating unit of the exopolysaccharide from L. helveticus K16 is as follows: carbohydrate sequence [see text].     

Structural elucidation of the viscous exopolysaccharide produced by Lactobacillus helveticus Lb161

A viscous extracellular polysaccharide produced by Lactobacillus helveticus Lb161 isolated from raw milk has been investigated. Sugar and methylation analysis, and 1H and 13C NMR spectroscopy revealed that the polysaccharide is composed of a heptasaccharide repeating unit. The sequence of sugar residues was determined by use of two-dimensional nuclear Overhauser effect spectroscopy and heteronuclear multiple bond connectivity experiments. The structure of the repeating unit of the exopolysaccharide from L. helveticus Lb161 is as follows: carbohydrate structure [see text]. The polysaccharide contains approximately 0.6 equivalents of O-acetyl group per repeating unit (not located).     

Highly viscous polysaccharide produced by an Enterobacter isolate on a hemicellulose hydrolysate

Summary A new anionic extracellular polysaccharide (AEPS) with high molecular mass (1700 kD) was produced by an isolate tentatively identified as Enterobacter cloacae isolated from an acid hydrolysate of wood. The sugar composition of this AEPS is glucose, galactose, glucuronic acid, and fucose (5:4:4:11). Under specified conditions this AEPS is rheologically comparable to xanthan.     

Retention, capture and consumption of experimental prey by orb-web weaving spiders in coffee plantations of Southern Mexico

This study focuses on the predatory capacity of four sympatric species of web- building spiders that inhabit coffee plantations in Southern Mexico: Gasteracantha cancriformis, Cyclosa caroli, and the morphologically similar species pair Leucauge mariana and L. venusta which were considered as one species group. The retention capabilities of the webs of these species and the incidence of prey capture and consumption were measured using eight types of insect prey belonging to the orders Coleoptera (1 species), Hymenoptera (3), Diptera (2) Lepidoptera (1) and Homoptera (1). The different characteristics of each prey such as body weight, body size, defensive behaviour, etc., were recorded. The incidence of prey retention, capture and consumption were significantly higher in G. cancriformis than in any of the other species. The lowest rates of retention, capture and consumption were observed in C. caroli, while L. mariana/venusta were intermediate in their predatory capabilities. Significant negative correlations between prey size and percent consumption were detected in L. mariana/venusta and in G. cancriformis; in both cases, large prey were less likely to be immediately consumed than small prey items. The results can be interpreted in the light of the morphological characteristics of the spiders. G. cancriformis possesses long legs and a carapace and appeared to have few difficulties to manipulate all types of prey. In contrast, C. caroli showed lesser abilities to manipulate and subdue aggressive prey items, perhaps due to the short leg length and unprotected body of this species. The consumption of prey items may be related to the predatory strategy of each spider. G. cancriformis constructs a new web every morning and prey storage was never observed. The absence of prey storage behaviour could explain why this species consumes prey soon after capture. In contrast, C. caroli constructs a permanent web and stores captured prey on a stabilimentum that may explain the very low incidence of immediate consumption of prey observed in this species.