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 contribution of axial fiber extensibility to the adhesion of viscous capture threads spun by orb-weaving spiders

The viscous capture threads produced by over 4000 species of orb-weaving spiders are formed of regularly spaced aqueous droplets supported by a pair of axial fibers. These threads register increased stickiness when spans of increasing lengths contact a surface, indicating that adhesion is recruited from multiple droplets. This study examined threads produced by five species to test the hypothesis that axial fiber extensibility is crucial for this summation of adhesion. It did so by comparing the stickiness of unstretched threads with threads that had been elongated to reduce the extensibility of their axial fibers. As stretching these threads also increased the distance between their droplets, we measured the stickiness of stretched threads with contact plates whose widths were increased in proportion to the degree of thread elongation. We then accounted for the actual thread elongation achieved for each individual's threads and for differences in the five species'absolute thread extensibility. The results showed that in four species thread extensibility contributed positively to adhesion. For three species, thread extensibility and droplet volume together explained the mean per droplet adhesion of threads. Models based on these three species show that, as threads were elongated, increasing amounts of potential adhesion were lost to diminished axial fiber extensibility. These models indicate that approximately one-third of an unstretched viscous thread's stickiness accrues from the adhesive recruitment made possible by axial fiber extensibility.

The effect of insect surface features on the adhesion of viscous capture threads spun by orb-weaving spiders

Spider orb-webs intercept a broad range of insects and their capture threads must adhere to a range of surface textures. In species of the Araneoidea clade, these capture threads are composed of viscid droplets whose size and spacing differ among species. To determine how droplet profile and insect surface texture interact, we measured the stickiness of viscous threads produced by four species using four insect surfaces that ranged from a smooth beetle elytra to the dorsal surface of a fly abdomen that was covered by large, widely spaced setae. The adhesion of threads to these surfaces differed by as much as 3.5-fold within a spider species and 2.1-fold for the same insect surface between spider species. However, 96% of these differences in stickiness was explained by four variables: the ratio of natural log of droplet volume to setal length, the natural log of droplet volume per mm of thread length, setal surface area, and the area of cuticle not excluded from thread contact by setae. Compared with previous measurements of primitive cribellar capture threads produced by orb weavers of the Deinopoidea clade,viscous threads performed more uniformly over the range of insect surfaces. They also held bug hemelytra, which were densely covered with fine setae, more securely, but held beetle elytra, fly wings and fly abdomens less securely than did viscous threads. Hemelytra may be held more securely because their setae more easily penetrate the viscous boundary layer to establish a greater area of interaction and, after having done so, offer more resistance as they are pulled through this layer. Finely textured surfaces may also have higher effective surface energies and therefore may interact more completely with viscous material.

Adhesive recruitment by the viscous capture threads of araneoid orb-weaving spiders

The sticky prey capture threads of orb-webs are critical to web performance. By retaining insects that strike the web, these spirally arrayed threads allow a spider time to locate and subdue prey. The viscous capture threads spun by modern orb-weaving spiders of the Araneoidea clade replaced the dry, fuzzy cribellar capture threads of the Deinopoidea and feature regularly spaced moist, adhesive droplets. The stickiness of a cribellar thread is limited by its tendency to peel from a surface after the adhesion generated at the edges of contact is exceeded. In this study we test the hypothesis that viscous thread overcomes this limitation by implementing a suspension bridge mechanism (SBM) that recruits the adhesion of multiple thread droplets. We do so by using contact plates of four widths to measure the stickiness of six species' viscous threads whose profiles range from small, closely spaced droplets to large, widely spaced droplets. The increased stickiness registered by an increased number of thread droplets supports the operation of a SBM. However, the accompanying decrease in mean per droplet adhesion shows that droplets interior to the edges of thread contact contribute successively less adhesion. Models developed from these data suggest that the suspension bridge mechanism is limited to a span of approximately 12 droplets.

Spider silk: thousands of nano-filaments and dollops of sticky glue

Some spiders use glues while others deploy strands of fine filaments for fixing flies. Recent work has provided new insights into the mechanical properties of these nano-scale ropes.

Efficiency of antlion trap construction

Assessing the architectural optimality of animal constructions is in most cases extremely difficult, but is feasible for antlion larvae, which dig simple pits in sand to catch ants. Slope angle, conicity and the distance between the head and the trap bottom, known as off-centring, were measured using a precise scanning device. Complete attack sequences in the same pits were then quantified, with predation cost related to the number of behavioural items before capture. Off-centring leads to a loss of architectural efficiency that is compensated by complex attack behaviour. Off-centring happened in half of the cases and corresponded to post-construction movements. In the absence of off-centring, the trap is perfectly conical and the angle is significantly smaller than the crater angle, a physical constant of sand that defines the steepest possible slope. Antlions produce efficient traps, with slopes steep enough to guide preys to their mouths without any attack, and shallow enough to avoid the likelihood of avalanches typical of crater angles. The reasons for the paucity of simplest and most efficient traps such as theses in the animal kingdom are discussed.

The effects of capture spiral composition and orb-web orientation on prey interception

Cribellar prey capture threads found in primitive, horizontal orb-webs reflect more light, including ultraviolet wavelengths, than viscous threads found in more derived, vertical orb-webs. Low web visibility and vertical orientation are each thought to increase prey interception and may represent key innovations that contributed to the greater diversity of modern, araneoid orb-weaving spiders. This study compares prey interception rates of cribellate orb-webs constructed by Uloborus glomosus (Uloboridae) with viscous orb-webs constructed by Leucauge venusta (Tetragnathidae) and Micrathena gracilis (Araneidae). We placed sectors of cribellar and viscous threads side by side in frames that were oriented either horizontally or vertically. The webs of both U. glomosus and L. venusta intercepted more prey when vertically oriented. In each orientation L. venusta webs intercepted more insects than did U. glomosus. Although this is consistent with the greater visibility of cribellar threads, the more closely spaced capture spirals of L. venusta may have contributed to this difference. Micrathena gracilis webs intercepted more prey than did U. glomosus webs, although web orientation did not affect the performance of this araneoid species. The stickier and more closely spaced capture spirals of M. gracilis may have enhanced the interception rates of this species and accounted for the greater number of smaller dipterans retained in its webs. The tendency for these slow, weak flight insects to be blown into both horizontal and vertical webs may account for similar interception rates of horizontal and vertical M. gracilis webs. These observations support the enhanced prey interception of vertically oriented orb-webs, but offer only qualified support for the contributions of lower visibility viscous capture threads.

Unraveling the mechanical properties of composite silk threads spun by cribellate orb-weaving spiders

Orb-web weaving spiders depend upon the mechanical performance of capture threads to absorb the energy of flying prey. Most orb-weavers spin wet capture threads with core fibers of flagelliform silk. These threads are extremely compliant and extensible due to the folding of their constituent proteins into molecular nanosprings and hydration by a surrounding coating of aqueous glue. In contrast, other orb-weavers use cribellate capture threads, which are composite structures consisting of core fibers of pseudoflagelliform silk surrounded by a matrix of fine dry cribellar fibrils. Based on phylogenetic evidence, cribellate capture threads predate the use of viscid capture threads. To better characterize how pseudoflagelliform and cribellar fibrils function, we investigated the mechanical performance of cribellate capture threads for three genera of spiders (Deinopis, Hyptiotes and Uloborus). These taxa spin very diverse web architectures, ranging from complete orbs to evolutionarily reduced triangle webs and cast nets. We found that the pseudoflagelliform core fibers of these webs were stiffer and stronger, but also less extensible, than flagelliform silk. However,cribellate capture threads achieved overall high extensibilities because the surrounding cribellar fibrils contributed substantially to the tensile performance of threads long after the core pseudoflagelliform fibers ruptured. In the case of Deinopis capture threads, up to 90% of the total work performed could be attributed to these fibrils. These findings yield insight into the evolutionary transition from cribellate to viscid capture threads.

Inter-specific sequence conservation and intra-individual sequence variation in a spider silk gene

Currently, studies on major ampullate spidroin 1 (MaSp1) genes of non-orb weaving spiders are few, and it is not clear whether genes of these organisms exhibit the same characteristics as those of orb-weavers. In addition, many studies have proposed that MaSp1 might be a single gene with allelic variants, but supporting evidence is still lacking. In this study, we compared partial DNA and amino acid sequences of MaSp1 cloned from different spider guilds. We also cloned partial MaSp1 sequences from genomic DNA and cDNA of the same individuals of spiders using the same primer combination to see if different molecular forms existed. In the repetitive region of partial MaSp1 sequences obtained, GGX, GA and poly-A motifs were present in all Araneomorphae and Mygalomorpae species examined. An extreme similarity in MaSp1 non-repetitive portions was found in sequences of ecribellate, cribellate and Mygalomorphae web-builders and such a result suggested that this sequence might exhibit an important function. A comparison of sequences amplified from the same individual showed that substitutions in amino acids occurred in both repetitive and non-repetitive regions, with a much higher variation in the former. These results suggest that the MaSp1 of Araneomorphae spiders exhibits several forms in an individual spider and it might be either a multiple gene or a single gene with a multiple exon/intron organization.

van der Waals and hygroscopic forces of adhesion generated by spider capture threads

Cribellar thread is the most primitive type of sticky prey capture thread found in aerial spider webs. Its outer surface is formed of thousands of fine fibrils that issue from a cribellum spinning field. The fibrils of primitive cribellar thread are cylindrical, whereas those of derived threads have nodes. Cribellar threads snag on insect setae but also adhere to smooth surfaces. A previous study showed empirically that cylindrical fibrils use only van der Waals forces to stick to smooth surfaces, as their stickiness is the same under different humidity. By contrast, noded fibrils are stickier under high humidity, where they are presumed to adsorb atmospheric water and implement hygroscopic (capillary) adhesion. Here, we model thread stickiness according to these two adhesive mechanisms. These models equate stickiness with the force necessary to overcome the adhesion of fibril contact points in a narrow band along each edge of the contact surface and to initiate peeling of the thread from the surface. Modeled and measured thread stickiness values are similar, supporting the operation of the hypothesized adhesive forces and portraying an important transition in the evolution of spider threads. Cribellar threads initially relied only on van der Waals forces to stick to smooth surfaces. The appearance of fibril nodes introduced hydrophilic sites that implemented hygroscopic force and increased thread stickiness under intermediate and high humidity.

Homology, behaviour and spider webs: web construction behaviour of Linyphia hortensis and L. triangularis (Araneae: Linyphiidae) and its evolutionary significance

Linyphiidae is the second largest family of spiders. Using Linyphia hortensis and L. triangularis, we describe linyphiid sheet-web construction behaviour. Orb-web construction behaviour is reviewed and compared with that of nonorb-weaving orbicularians. Phylogenetic comparisons and the biogenetic law are applied to deduce behavioural homology. Linyphia webs were constructed gradually and in segments over a period of many days and had a long lifespan. Two construction behaviours, supporting structure and sticky thread (ST) (within the sheet) were observed. ST construction behaviour in linyphiids is considered homologous to sticky spiral construction in orb-weavers. Overall web construction conformed to the pattern of alternate construction of sticky and nonsticky parts as observed in theridiids. Linyphiids had no problem in switching between structure construction and ST construction even during a single behavioural bout. Both web construction behaviours in linyphiids were nonstereotypic, which is unusual in orbicularians. This might be due to the loss of control mechanisms at genetic level, probably by macro mutation. Lack of stereotypic behaviour might have played a substantial role in the origin of the diverse web forms seen in nonorb-weaving orbicularians. This hypothesis is consistent with patterns observed in the orbicularian phylogeny.

Palaeontology: spider-web silk from the Early Cretaceous

The use of viscid silk in aerial webs as a means to capture prey was a key innovation of araneoid spiders and has contributed largely to their ecological success. Here I describe a single silk thread from a spider's web that bears glue droplets and has been preserved in Lebanese amber from the Early Cretaceous period for about 130 million years. This specimen not only demonstrates the antiquity of viscid silk and of the spider superfamily Araneoidea, but is also some 90 million years older than the oldest viscid spider thread previously reported in Baltic amber from the Eocene epoch.