Functional organization of the spinning apparatus of Cyrtophora citricola with regard to the evolution of the web (Araneae,Araneidae)

Summary The spinning apparatus of Cyrtophora citricola closely corresponds to that of orb-weaving Araneidae, two peculiarities excepted. Firstly the spigots of the piriform glands differ extremely in size, the smallest of them being numerous and having a unique location on the anterior spinnerets. Secondly, the triad complex (on the posterior spinnerets) used by other Araneidae for producing gluey capture threads is lacking. Both these characteristics are correlated with the construction of a fine meshed sheet of dry silk by Cyrtophora instead of orbwebs with capture spirals. The sheet can be understood as being a very much enlarged central area of orb-webs. Since vestiges of triads could be found in early developmental stages of C. citricola, the origin of the meshed sheet from orb-webs with gluey capture threads is clearly demonstrated. The paper includes a study on how the spider produces thread attachments by means of the secretions of the piriform glands.     

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.     

The material cost and stickiness of capture threads and the evolution of orb-weaving spiders

Prey capture threads are essential to the operation of spider orb-webs because they prevent insects that have been intercepted from escaping before a spider can subdue them. The volume of material invested in a web's capture threads is related to spider weight and is the same for primitive orb-weavers that produce cribellar capture thread and modern orb-weavers that produce adhesive capture thread. However, as adhesive capture thread achieves greater stickiness relative to its volume, adhesive orb-webs have a greater total stickiness and, consequently, a greater prey capture potential than cribellate orb-webs. These differences appear to have favoured the transition from cribellate to adhesive capture threads and the success of adhesive orb-weavers, which include 95% of all orb-weaving species. Differences in the thread economy and the total stickiness of webs constructed by spiders of different weights also suggest that adhesive orb-weavers should grow more rapidly and be capable of attaining a larger size than cribellate orb-weavers.     

Webs of theridiid spiders: construction, structure and evolution

Understanding the web construction behaviour of theridiid (comb-footed) spiders is fundamental to formulating specific evolutionary hypotheses and predictions regarding the reduction of orb-webs. We describe for the first time in detail the web construction behaviour of Achaearanea tepidariorum , Latrodectus geometricus , Theridion sisyphium and T. varians as well as webs of a range of other theridiids. In our survey we distinguish four major web types. Among webs with gumfooted lines, we distinguish between webs with a central retreat ( Achaearanea -type) and those with a peripheral retreat ( Latrodectus -type). Among webs without gumfooted lines, we distinguish between those which contain viscid silk ( Theridion -type) and those with a sheet-like structure, which do not ( Coleosoma -type). Theridiid gumfoot-webs consist of frame lines that anchor them to surroundings and support threads which possess viscid silk. Building of gumfooted lines constitutes a unique stereotyped behaviour and is most probably homologous for Nesticidae and Theridiidae. Webs remained in place for extended periods and were expanded and repaired, but no regular pattern of replacement was observed. We suggest that the cost of producing and maintaining viscid silk might have led to web reduction, at least in theridiids.  © 2003 The Linnean Society of London. Biological Journal of the Linnean Society , 2003, 78 , 293−305.     

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 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.     

Evolution beyond the orb web: the web of the araneid spider Pasilobus sp., its structure, operation and construction

The araneid spider Pasilobus sp., common in the Morobe District, New Guinea, builds its web at night close to bushes and small trees. The more-or-less horizontal web has a triangular frame that is divided into halves by a midline thread running from the apical angle to bisect the base. From the midline thread hang 4–11 pairs of widely spaced spanning threads; these are the only adhesive elements in the web. The spanning threads are viscid for only part of their length and are strongly attached to the web only at their junction with the midline thread. The outer end of each spanning thread forms an easily ruptured, low-shear joint with the lateral frame thread of the web. When a flying insect strikes a spanning thread, the low-shear joint breaks and the thread drops below the web, leaving the insect tethered to the midline. The insect may continue to fly, on the tether, or may spin down to motionlessness. The spider rushes to the midline thread end of the tether, hauls up the spanning thread and then bites the insect. Experimental investigations of the low-shear joints and the adhesiveness and elasticity of the spanning thread are described and the results analysed. The web-building behaviour of Palilobus differs in several ways from that of most araneids and is described and compared with that of Gasteracantha and other species. The possible evolutionary origins of the Pasilobus web are outlined.     

Capture thread extensibility of orb-weaving spiders: testing punctuated and associative explanations of character evolution

Spider orb-webs contain sticky prey capture threads and non-sticky support threads. Primitive orb-weavers of the Deinopoidea produce dry cribellar threads made of thousands of silk fibrils that surround supporting axial fibres, whereas the viscous threads of modern Araneoidea orb-weavers produce adhesive threads with an aqueous solution that coalesces as droplets around the axial fibres. We have previously shown that the greater diversity of the Araneoidea is phylogenetically significant and attributed this disparity to a number of advantages, considered key innovations, that adhesive thread has over cribellar thread. An important putative advantage of adhesive thread demonstrated by Kohler and Vollrath in their 1995 study is its greater extensibility, a feature that better adapts it to absorb the kinetic energy of a prey strike. However, this conclusion is based on a two-species comparison that does not take advantage of the modern comparative method that requires hypotheses to be tested in a phylogenetic context. Using a transformational analysis to examine threads produced by nine species, our study finds no support for the punctuated explanation that adhesive thread has a greater extensibility than cribellar thread. Instead, it strongly supports the associative null hypothesis that capture thread extensibility is tuned to spider mass and to architectural features of the web, including its capture area, capture spiral spacing, and capture area per radius.     

Biomechanical variation of silk links spinning plasticity to spider web function

Spider silk is renowned for its high tensile strength, extensibility and toughness. However, the variability of these material properties has largely been ignored, especially at the intra-specific level. Yet, this variation could help us understand the function of spider webs. It may also point to the mechanisms used by spiders to control their silk production, which could be exploited to expand the potential range of applications for silk. In this study, we focus on variation of silk properties within different regions of cobwebs spun by the common house spider, Achaearanea tepidariorum. The cobweb is composed of supporting threads that function to maintain the web shape and hold spiders and prey, and of sticky gumfooted threads that adhere to insects during prey capture. Overall, structural properties, especially thread diameter, are more variable than intrinsic material properties, which may reflect past directional selection on certain silk performance. Supporting threads are thicker and able to bear higher loads, both before deforming permanently and before breaking, compared with sticky gumfooted threads. This may facilitate the function of supporting threads through sustained periods of time. In contrast, sticky gumfooted threads are more elastic, which may reduce the forces that prey apply to webs and allow them to contact multiple sticky capture threads. Therefore, our study suggests that spiders actively modify silk material properties during spinning in ways that enhance web function.     

Fine structure of sheet-webs of Linyphia triangularis (Clerck) and Microlinyphia pusilla (Sundevall), with remarks on the presence of viscid silk

We examined the webs of Linyphia triangularis (Clerck) and Microlinyphia pusilla (Sundevall) using light and scanning electronic microscopic techniques and compared them with the better known orb‐webs. The linyphiid sheet‐web consists of an unordered meshwork of fibres of different thicknesses. The sheet is connected to the scaffolding by means of attachment discs. Thin threads with globules, which appear similar to the viscid silk droplets of orb‐webs, are present in most webs examined. Webs of M. pusilla had a higher density of these globules than did webs of L. triangularis. Webs of both species possess five types of thread connections and contain no aqueous glue for prey capture. Instead, unlike orb‐webs, the sticky substances produced by the linyphiid aggregate glands cement the different layers and threads of the sheet by drying up after being produced. Due to their function, sheet webs may not require viscid silk, thereby leading to a more economic web. The assumption made in most previous studies, that the globules in linyphiid webs have the same properties and function as viscid silk in orb‐webs, is unfounded.     

The silk road of <Emphasis Type="Italic">Tetranychus urticae</Emphasis>: is it a single or a double lane?

Tetranychus urticae (Acari: Tetranychidae) is a phytophagous mite that forms huge colonies. All active members of a colony (immatures and matures, females and males) spin silken threads. These mites construct a common web that protects the colony from external aggression. The silk coverage is well-known to provide advantages to the colony but very little is known about the characteristics of the threads themselves. Here is the first quantification of the diameter of silken threads spun by two different stages (adult females and larvae) and its relationship with body size of the spinning individuals. Moreover, we observed how silk was deposited on the substrate through their two pedipalps. Threads were observed by means of transmission electron and fluorescence microscopy. Silken threads spun by larvae (0.055 ± 0.018 μm) were significantly thinner than threads spun by adult females (0.111 ± 0.038 μm). In the first step of the silk depositing behaviour, the mite attached the thread to the substrate by putting its pedipalps in contact with the surface (adhesion, double silken threads). When walking, silken threads became detached from the substrate and spitted up (silken threads were free). Finally, silken threads adhered to the surface. The presence of single and double threads makes thread diameter highly variable.     

Untangling the Tangle-Web: Web Construction Behavior of the Comb-Footed Spider Steatoda triangulosa and Comments on Phylogenetic Implications (Araneae: Theridiidae)

Theridiidae typically construct a three-dimensional web often described as irregular. The web consists of a supporting structure and lines under tension termed gumfooted lines. We used automated methods to observe web construction in the theridiid Steatoda triangulosa under laboratory conditions. Web construction lasted several nights. After orientation, spiders built a three-dimensional structure of several threads radiating sideways and downward from the retreat. To build gumfooted lines, they started from the retreat, moved along a structural thread, then dropped down to attach the thread to the lower substrate. On returning, they coated the lowest part of the thread with viscid silk before moving up along the same thread back to the structural thread. They then continued moving along the same structural thread to drop down again to build the next gumfooted line. This behavior was continued until the spiders had built a series of gumfooted lines (a bout). There were regular intervals between the construction of two bouts. Thus, a single web included many bouts built in different stages. We show that gumfooted lines are not homologues to sticky web elements of orb-weavers and present new synapomorphic characters that support the monophyly of Theridiidae + Nesticidae and the monophyly of araneoid sheet web weavers. Further, the time allocation pattern for different behavioral stages and the fine structure of a gumfooted line are presented.     

The silk-producing system ofLinyphia triangularis (Araneae,Linyphiidae) and some comparisons with Araneidae

Summary The spinning apparatus ofLinyphia triangularis, adult females and males, was studied with the scanning electron microscope and the main anatomical and histochemical characteristics of the silk glands, including the epigastric apparatus of males, are presented. The epigastric glands seem to be important for the construction of sperm webs. A detailed account of the use of the different kinds of silk in web building is given.The spinning apparatus ofLinyphia closely corresponds to the araneid pattern. Characteristic of linyphiid spiders is the poor development of the aciniform glands. Corresponding to the minor importance of capture threads forLinyphia, the triads (aggregate and flagelliform glands) are less developed than in Araneidae.Linyphia make much less use of the secretions of the piriform glands for connecting threads than Araneidae. Capture threads adhere to other threads by their own glue; other threads seem mostly to be bound to one another by the secretion of the minor ampullate glands whose chemical properties, inLinyphia, appear especially adapted to this function. Neither the anatomical and histochemical data concerning the spinning apparatus nor the structure of the webs provide any indication of close relationships between Linyphiidae and Agelenidae, as was recently claimed.     

Building a better insect trap; An experimental investigation of prey capture in a variety of spider webs

Summary Web-building spiders (Araneae; Theridiidae, Linyphiidae, Araneidae) are catagorized as searchers because they devote a large amount of energy to the construction of the web which constitutes the search phase in the foraging sequence. In this study search energy is equated with the density of threads in a web and the effectiveness of a variety of webs in three broad catagories (tangle webs, sheet webs & orb webs) is tested in the light of current foraging theory. Within each web type there is a distinct thread density at which the number of approaching Drosophila (Diptera; Drosophilidae) that are captured is maximized (Figs. 1, 2, 3). That maximum results from a combination of factors that are a function of the density of threads in the web. The visibility of the web to an approaching Drosophila increases which acts to decrease the number of flies that enter the web (Tables 2, 3, 4). The ability of the web to detain a Drosophila that contacts it (capture efficiency) increases to an asymptote as a function of thread density (Fig. 4). These data support an assumption of many optimal foraging models that with increasing investment in search the predator receives a diminishing return.More Drosophila intercept orb webs than intercept sheet or tangle webs. In addition orb webs detain a greater proportion of the flies that contact them (Fig. 4). Sheet webs are intermediate between orb and tangle webs in their relative abilities to contact and detain Drosophila.     

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.     

棒络新妇和悦目金蛛拖丝超微结构与力学行为

采用SEM对棒络新妇Nephila clavata腹部向上和向下在水平纱窗上爬行时纺出的拖丝、悦目金蛛Argiope amoena捕食拖丝与垂直向下缓慢纺出的拖丝及其圆网的铆钉丝进行了超微结构观察,采用电子单纤强力仪对棒络新妇拖丝与悦目金蛛圆网铆钉丝进行了力学拉伸试验.结果 表明棒络新妇和悦目金蛛拖丝均呈现出一至多根细丝纤维的多样化超微结构特征,其中悦目金蛛圆网铆钉丝还呈现出"S"形似弹簧的结构.两种蜘蛛丝的力学行为和性能与各自的功能要求相一致.蜘蛛能调节拖丝的超微结构、纤维组成和直径大小以适应其在不同环境条件下对力学性能和功能的瞬时需要.研究结果有助于拓宽和加深人们对蜘蛛丝超微结构、力学性能与生物学功能之间关系的认识和理解.     

THE WEB-SPINNING PROCESS AND WEB-STRUCTURE OF LATRODECTUS TREDECIMGUTTATUS, L. PALLIDUS AND L. REVIVENSIS

The web structure and the web spinning process of Latrodectus tredecimguttatus Rossi, Latrodectus pallidus Cambridge and Latrodectus revivensis Shulov have been investigated, particular attention being given to the structure of their catching webs.
The webs of Latrodectus represent a further development of the web-type of Archaeranea tepidariorum . The specific characters of the investigated webs concern the middle layer of their three-layer webs, where more or less regular structures appear.
In the middle layer of L. tredecimguttatus web paths of hexagonal meshes appear.
In the web of L. pallidus and L. revivensis more or less regular radially arranged catching platforms appear, connected with the distant retreat by a long bridge-web.
The lower layer of vertical threads, with viscid droplets on their far ends is spread-out in L. tredecimguttatus and consists of short threads as distinct from the concentrated bundle of high threads in the two other species.
Since L. pallidus and L. revivensis appear usually among vegetation, their restricted catching webs seem to be adaptive adjustment to the biotope.
The concentration of the catching webs brings about improvement as regards prey detection.
The differences in the web structure of L. tredecimguttatus and L. revivensis indicate that their behavioural characters are more distinct than the morphological ones. The different behavioural characters support the view that L. revivensis represents a species different from L. mactans tredecimguttatus .     

Evolution of adhesive mechanisms in cribellar spider prey capture thread: evidence for van der Waals and hygroscopic forces

Sticky prey capture threads are produced by many members of the spider infraorder Araneomorphae. Cribellar threads are plesiomorphic for this clade, and viscous threads are apomorphic. The outer surface of cribellar thread is formed of thousands of fine, looped fibrils. Basal araneomorphs produce non-noded cribellar fibrils, whereas more derived members produce noded fibrils. Cribellar fibrils snag and hold rough surfaces, but other forces are required to explain their adherence to smooth surfaces. Threads of Hypochilus pococki (Hypochilidae) formed of non-noded fibrils held to a smooth plastic surface with the same force under low and high humidities. In contrast, threads of Hyptiotes cavatus and Uloborus glomosus (Uloboridae) formed of noded fibrils held with greater force to the same surface at intermediate and high humidities. This supports the hypothesis that van der Waals forces allow non-noded cribellar fibrils to adhere to smooth surfaces, whereas noded fibrils, owing to the hydrophilic properties of their nodes, add hygroscopic forces at intermediate and high humidities. Thus, there appear to have been two major events in the evolution of adhesive mechanisms in spider prey capture thread: the addition of hydrophilic nodes to the fibrils of cribellar threads and the replacement of cribellar fibrils by viscous material and glycoprotein glue.  © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 77 , 1–8.