Impact of starvation on the silk attractiveness in a weaving mite, Tetranychus urticae (Acari: Tetranychidae)

The two-spotted spider mite Tetranychus urticae is a silk producer known to live in groups. Its silk production plays an important role in protection against external aggressions (predators, rains, etc.). It is also used for group dispersal through the formation of silkballs or as a thread during individual migration by walking. Until now, the role of silk in enhancing migration has been poorly studied. In this paper, the influence of the silken thread presence on T. urticae’s locomotor activity is studied. One virgin female is placed at the centre of a cover glass partially covered by silk. Moving and resting time are studied on the silk or the clean part of the cover glass as a function of the starvation level of mites (fed vs. starved) and the age of the silk (30 vs. 60 vs. 90 vs. 120 vs. 150 min). Results show that a fed female spends more time on the silk-covered part than on the clean part as long as the silk is fresh (less than 120 min). Thus, the age-related changes in chemical and/or physical properties of the silk influence the spatial distribution of fed mites. Whatever the age of the silk, starved mites spend more time on the clean part of the set-up. Indeed, the silk freshly laid by conspecifics is attractive only for fed mites; starved mites probably prefer sites without silk (and conspecifics). This study shows that that the silk influences the spatial distribution of T. urticae according to its level of starvation and that silk is probably is an indicator of the presence of conspecifics.     

Microstructure of the silk spigots of the green crab spider Oxytate striatipes (Araneae: Thomisidae)

The genus Oxytate L. Koch, 1878 comprises a homogeneous group of nocturnal crab spiders that have silk apparatuses even though they do not spin webs to trap prey. We examined the microstructure of the silk spinning apparatus of the green crab spider Oxytate striatipes, using field emission scanning electron microscopy. The silk glands of the spider were classified into three types: ampullate, pyriform and aciniform. The spigots of these three types of silk gland occur in both sexes. Two pairs of major ampullate glands send secretory ductules to the anterior spinnerets, and another two pairs of minor ampullate glands supply the median spinnerets. In addition, the pyriform glands send ductules to the anterior spinnerets (45 pairs in females and 40 pairs in males), and the aciniform glands feed silk into the median (9–12 pairs in females and 7–10 pairs in males) and the posterior (30 pairs in both sexes) spinnerets. The spigot system of O. striatipes is simpler and more primitive than other wandering spiders: even the female spiders possess neither tubuliform glands for cocoon production nor triad spigots for web‐building.     

Microstructure of the silk apparatus in the coelotine spider Paracoelotes spinivulva (Araneae: Amaurobiidae)

The microstructural characteristics of the silk‐spinning apparatus and its ecological significance in the coelotine spider Paracoelotes spinivulva were examined by field emission scanning electron microscopy, with the goal of understanding the properties and the evolutionary origins of these silk constructs. The silk apparatuses of this spider were composed of four basic types of silk‐spinning spigot (ampullate, pyriform, aciniform and tubuliform), which connected with typical silk glands in the abdominal cavity. Of the three pairs of spinnerets, the posterior pairs were highly elongated along the body axis. Anterior spinnerets comprised two pairs of ampullate glands and approximately 70–80 pairs of pyriform glands in both sexes. Middle spinnerets had one to two pairs of ampullate spigots, three pairs of tubuliform spigots in females, and 50–60 (female) or 80–90 (male) pairs of aciniform spigots. An additional two pairs of tubuliform spigots in females and 70–80 (female) or 100–120 (male) pairs of aciniform spigots were counted on the spinning surfaces of the posterior spinnerets in both sexes. Although the coelotine spiders use their silk to catch prey, P. spinivulva characteristically do not have a typical “triad” spigot, including a flagelliform and two aggregate spigots, for capture thread production.     

Silk drives aggregation and following in the neotropical caterpillar Mechanitis menapis (Nymphalidae: Ithomiini)

Gregarious larvae that use chemical communication to feed and move together are widespread among folivorous insects, although social behaviour has been studied almost exclusively in a few temperate zone genera. The Menapis (or variable) tigerwing butterfly Mechanitis menapis mantineus Hewitson (Lepidoptera, Nymphalidae, Danainae, Ithomiini) is a neotropical species whose larvae feed gregariously on Solanaceae host plants. In laboratory experiments conducted in the Ecuador cloud forest, M. menapis caterpillars are attracted to silk produced by conspecifics and show no evidence of pheromone production. Indeed, caterpillars consistently choose arenas with silk over bare arenas but do not show a preference for arenas marked with abdominal cuticular surface residues. Mechanitis menapis caterpillars on silk‐coated plants are both more mobile and more cohesive than those on control plants. Nonetheless, caterpillars move independently over unmarked surfaces and groups do not make rapid collective choices between two food sources. Collective behaviour in M. menapis thus appears to be based on aggregation on collectively produced silk to facilitate feeding, as well as using this silk to maintain cohesion. Silk production is common in caterpillars, although M. menapis appears to be unique among species studied so far in using silk to maintain group cohesion.     

Structure,composition and mechanical properties of the silk fibres of the egg case of the Joro spider, <Emphasis Type="Italic">Nephila clavata</Emphasis> (Araneae,Nephilidae)

The silk egg case and orb web of spiders are elaborate structures that are assembled from a number of components. We analysed the structure, the amino acid and fibre compositions, and the tensile properties of the silk fibres of the egg case of Nephila clavata. SEM shows that the outer and inner covers of the egg case consist of thick, medium and thin silk fibres. The silk fibres of the outer cover of the egg case are probably produced by the major and minor ampullate glands. The silk fibres of the inner cover of the egg case from cylindrical glands appears to be distinct from the silk fibres of the major ampullate glands based on their micro-morphology, mole percent amino acid composition and types, and tensile behaviour and properties. Collectively, our investigations show that N. clavata uses silk fibres from relatively few glands in varying combinations to achieve different physical and chemical properties (e.g., color, diameter, morphology and amino acid composition) and functional and mechanical properties in the different layers of the egg case.     

Relation between habitat structure and development of the glandulae ampullaceae in eight wolf spider species (Pardosa,Araneae, Lycosidae)

Summary In eight Pardosa species studied there is a correlation between the structure of the habitat in which the species live, the quantity of silk (drag-lines) produced, the plane in which the silk is spun and the degree of development of the silk glands (the gl. ampullaceae). In general, species living in field-type vegetation have large gl. ampullaceae, produce large quantities of silk and spin proportionally more silk in the vertical plane, while the converse is true for species from open-ground type vegetation.     

Spinneret Microstructure of the Silk Spinning Apparatus in the Crab Spider, Misumenops tricuspidatus (Araneae: Thomisidae)

The silk spinning apparatus in the crab spider, Misumenops tricuspidatus was studied with the field emission scanning electron microscope (FESEM) and the main microstructural characteristics of the silk glands are presented. In spite of the fact that the crab spiders do not spin webs to trap a prey, they also have silk apparatus even though the functions are not fully defined. The crab spider, Misumenops tricuspidatus possesses only three types of silk glands which connected through the typical spinning tubes on the spinnerets. The spinning apparatus of Misumenops closely corresponds to that of wandering spiders such as jumping spiders or wolf spiders except some local variations. Anterior spinnerets comprise 2 pairs of the ampullates and 48 (±5) pairs of pyriform glands. Another 2 pairs of ampullate glands and nearly 20 (±3) pairs of aciniform glands were connected on the middle spinnerets. Additional 50 (±5) pairs of the aciniform glands were connected on the posterior spinnerets. The aggregate glands and the flagelliform glands which have the function of sticky capture thread production in orb‐web spiders as well as the tubuliform glands for cocoon production in females were not developed at both sexes of this spider, characteristically.     

Transgenic silkworms (<Emphasis Type="Italic">Bombyx mori</Emphasis>) produce recombinant spider dragline silk in cocoons

Spider dragline silk is a unique fibrous protein with a combination of tensile strength and elasticity, but the isolation of large amounts of silk from spiders is not feasible. In this study, we generated germline-transgenic silkworms (Bombyx mori) that spun cocoons containing recombinant spider silk. A piggyBac-based transformation vector was constructed that carried spider dragline silk (MaSp1) cDNA driven by the sericin 1 promoter. Silkworm eggs were injected with the vector, producing transgenic silkworms displaying DsRed fluorescence in their eyes. Genotyping analysis confirmed the integration of the MaSp1 gene into the genome of the transgenic silkworms, and silk protein analysis revealed its expression and secretion in the cocoon. Compared with wild-type silk, the recombinant silk displayed a higher tensile strength and elasticity. The results indicate the potential for producing recombinant spider silk in transgenic B. mori.     

Studies on silk secretion in the trichoptera (F. Limnephilidae)

Summary The ultrastructure and amino acid composition of the secreted silk of two species of trichopteran larvae, Pycnopsyche guttifer (Walk.) and Neophylax concinnus McL., were investigated. The spinnerets of these two animals were also examined by scanning electron microscopy. The silk consists of double-stranded, flat ribbons (1–4 wide), composed of bundles of 15–25 Å filaments. There are two components of the silk: the fiber proper and a surrounding coat thought to be a silk gum. Only the outer coat is positive to the EM PATP technique of Thiery (1967), which indicated the presence of neutral sugars. Amino acid analyses of Pycnopsyche silk show that, like other silks, two predominant amino acids are glycine and serine. Arginine, unexpectedly, is the third most abundant and there are a large number of basic and long side-chain amino acids. X-ray diffraction studies of the silk indicate that it has a less crystalline, more amorphous structure than that of other silks.Submitted to the Department of Biological Sciences of the State University of New York at Albany in partial fulfillment of the requirements for the degree of Doctor of Philosophy Acknowledgements. This study was supported in part by a National Institutes of Health Graduate Student Traineeship grant # GM-02014. The author would like to express sincere gratitude to Dr. Stephen Brown for his encouragement and help during the course of this study. I would also like to thank Dr. Curtis Hemenway and Mr. Douglas Halgren of Dudley Observatory for the use of their scanning electron microscope as well as Dr. Helen Ghiradella and Mr. William Radigan for help with the scanning electron microscopy. I owe special appreciation to Dr. Y. Myer of the Chemistry Department of SUNYA for doing an amino acid analysis of the silk and to Dr. K.M. Rudall of the University of Leeds for doing the X-ray diffraction studies of the silk samples     

Molecular studies of a novel dragline silk from a nursery web spider, Euprosthenops sp. (Pisauridae)

Various spider species produce dragline silks with different mechanical properties. The primary structure of silk proteins is thought to contribute to the elasticity and strength of the fibres. Previously published work has demonstrated that the dragline silk of Euprosthenops sp. is stiffer then comparable silk of Nephila edulis, Araneus diadematus and Latrodectus mactans. Our studies of Euprosthenops dragline silk at the molecular level have revealed that nursery web spider fibroin has the highest polyalanine content among previously characterised silks and this is likely to contribute to the superior qualities of pisaurid dragline.     

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.     

Silk Weave and Silk Glands in Aquatic Instars of Two Species of Helicopsyche von Siebold, 1856 (Trichoptera,Helicopsychidae)

Under SEM the silk weave in the snail-like cases of Helicopsyche crispata and H. shuttleworthi, the two species present in Italy consist of several types of meshes. The silk which connects the sand grains of the external wall is made up of multi-layered threads forming irregular meshes. The sand grains of the vertical pillars in the wall of the centr al columella are held together by very loosely woven silk and are supported by thick silken threads. The pupal silken membrane consists of concentric threads and the pupal case is attached to the substrate by a disordered mass of silken threads. The two glands which secrete the silk are long, double folded tubes.     

Cell cycle events during the development of the silk glands in the mulberry silkworm<Emphasis Type="Italic"> Bombyx mori</Emphasis>

Silk glands of the mulberry silkworm Bombyx mori are long and paired structures originating from the labial region and are anatomically and physiologically divided into three major compartments, the anterior, middle and posterior silk glands. The silk gland morphogenesis is complete by 8 days post egg laying. Extensive growth of silk glands during the larval stages is due to increase in tissue mass and not cell number. The cells in a completely formed silk gland pursue an endoreplicative cell cycle, and the genome undergoes multiple rounds of replication without mitosis or nuclear division. The expression patterns of cyclin B (mitotic cyclin) and cyclin E (G1 cyclin, essential for G1/S transition in both mitotic and endoreplicative cell cycles) in the course of silk gland development revealed that mitotic cell divisions take place only in the apex of the growing silk gland. However, the persistence of another mitotic focus in the middle silk gland even when the growing apex has moved well past this zone suggested the continued operation of mitosis for a while in this restricted region. The lack of cyclin B expression and abundance of cyclin E in the rest of the areas confirmed an alternation of the G1 and S phases of the cell cycle without an intervening mitotic phase. No expression of cyclin B was noticed anywhere in the silk glands after stage 25 of embryogenesis, indicating a complete switch over to the endomitotic mode of the cell cycle. The onset of expression of various genes encoding different silk proteins correlated with the onset of endomitotic events.Edited by D. Tautz     

Gene expression analysis in the larval silk gland of the eri silkworm Samia ricini

Insects produce silk for a range of purposes. In the Lepidoptera, silk is utilized as a material for cocoon production and serves to protect larvae from adverse environmental conditions or predators. Species in the Saturniidae family produce an especially wide variety of cocoons, for example, large, golden colored cocoons and those with many small holes. Although gene expression in the silk gland of the domestic silkworm (Bombyx mori L.) has been extensively studied, considerably fewer investigations have focused on members of the saturniid family. Here, we established expression sequence tags from the silk gland of the eri silkworm (Samia ricini), a saturniid species, and used these to analyze gene expression. Although we identified the fibroin heavy chain gene in the established library, genes for other major silk proteins, such as fibroin light chain and fibrohexamerin, were absent. This finding is consistent with previous reports that these latter proteins are lacking in saturniid silk. Recently, a series of fibrohexamerin‐like genes were identified in the Bombyx genome. We used this information to conduct a detailed analysis of the library established here. This analysis identified putative homologues of these genes. We also found several genes encoding small silk protein molecules that are also present in the silk of other Lepidoptera. Gene expression patterns were compared between eri and domestic silkworm, and both conserved and nonconserved expression patterns were identified for the tested genes. Such differential gene expression might be one of the major causes of the differences in silk properties between these species. We believe that our study can be of value as a basic catalogue for silk gland gene expression, which will yield to the further understanding of silk evolution.     

Fine structure of the silk gland in the mite Ornithocheyletia sp. (Prostigmata,Cheyletidae)

Silk spinning is widely-spread in trombidiform mites, yet scarse information is available on the morphology of their silk glands. Thus this study describes the fine structure of the prosomal silk glands in a small parasitic mite, Ornithocheyletia sp. (Cheyletidae). These are paired acinous glands incorporated into the podocephalic system, as typical of the order. Combined secretion of the coxal and silk glands is released at the tip of the gnathosoma. Data obtained show Ornithocheyletia silk gland belonging to the class 3 arthropod exocrine gland. Each gland is composed of seven pyramidal secretory cells and one ring-folded intercalary cell, rich in microtubules. The fine structure of the secretory cells points to intensive protein synthesis resulted in the presence of abundant uniform secretory granules. Fibrous content of the granules is always subdivided into several zones of two electron densities. The granules periodically discharge into the acinar cavity by means of exocytosis. The intercalary cell extends from the base of the excretory duct and contributes the wall of the acinar cavity encircling the apical margins of the secretory cells. The distal apical surface of the intercalary cell is covered with a thin cuticle resembling that of the corresponding cells in some acarine and myriapod glands. Axon endings form regular synaptic structures on the body of the intercalary cell implying nerve regulation of the gland activity.     

Durability of Simuliid Silk Pads (Simuliidae,Diptera)

A central aspect of simuliid adaption to life in moving water is a silky secretion which they produce in their salivary glands. This secretion is spread on the substrate surface thereby enabling adhesion of the posterior abdominal hooks of the larvae to the substrate surface. Therefore, silk pads are a prerequisit for simuliid filter‐feeding as well as any kind of locomotive activity. If silk pad adhesion is weak, larvae risk drifting off, either directly or during locomotion. Properties of the adhesive (=silk) as well as the substrate surface may cause weak adhesion. A specialist such as S. noelleri which has little chance of surviving after drifting off its lake outlet habitat, should have adaptations to reduce this risk. Such an adaptation could be very durable silk pads, giving larvae the chance to be safely attached for a relatively long time. In this study larval silk pads of S. ornatum and S. noelleri were stained using Giemsa's staining procedures. Changes in silk pad structure and traces of rot after 2–33 days of exposure to stream water were recorded and compared. Silk pads of S. ornatum and S. noelleri showed differences which indicate differences in ageing processes and biochemistry of this secretion.     

Behavioral manipulation of host caterpillars by the primary parasitoid wasp Cotesia glomerata (L.) to construct defensive webs against hyperparasitism

Many parasitoids control the behavior of their hosts to achieve more preferable conditions. Decreasing predation pressure is a main aim of host manipulation. Some parasitoids control host behavior to escape from their enemies, whereas others manipulate hosts into constructing defensive structures as barriers against hyperparasitism. Larvae of the parasitoid wasp Cotesia glomerata form cocoon clusters after egression from the parasitized host caterpillar of the butterfly Pieris brassicae. After the egression of parasitoids, the perforated host caterpillar lives for a short period and constructs a silk web that covers the cocoon cluster. We examined whether these silk webs protect C. glomerata cocoons against the hyperparasitoid wasp Trichomalopsis apanteroctena. In cocoon clusters that were not covered by silk webs (bare clusters), only cocoons hidden beneath others avoided hyperparasitism. In covered cocoon clusters, both cocoons hidden beneath others and those with a space between them and the silk web avoided hyperparasitism, whereas cocoons that contacted the silk webs were parasitized. The frequency of cocoons that were hidden beneath others increased with the increasing number of cocoons in a cluster, but the defensive effect of cluster size was thought to be lower than that of silk webs. However, the rate of hyperparasitism did not differ between covered and bare clusters when we allowed the hyperparasitoids to attack the cocoon clusters in an experimental arena. This result was thought to have been caused by low oviposition frequency by these hyperparasitoids. As a result, silk webs did not guard the cocoons from hyperparasitoids in our experiments, but would protect cocoons under high hyperparasitism pressure by forming a space through which the ovipositors could not reach the cocoons.     

Studies on silk secretion in the trichoptera (F. Limnephilidae): I. Histology,histochemistry, and ultrastructure of the silk glands

As part of a study on trichopteran silk secretion, the histology, histochemistry, and ultrastructure of the silk glands of two species of limnephilid trichopteran larvae, Pycnopsyche guttifer (Walk.) and Neophylax concinnus McL., were investigated. The silk glands consist of three anatomically distinct regions: a long, posterior silk-secreting region; a shorter, anterior conducting tube; and a terminal press/common duct. In Pycnopsyche, there is also a modified bulbous region between the secreting and conducting areas. Each anatomical region has a distinct cell type. There are two structurally and histochemically different components of the secretion in the glandular lumen: a core and a peripheral layer. Both components are produced all along the gland and are principally proteinaceous. However, the peripheral layer is also PAS and alcian blue (pH. 2.5) positive and shows β-metachromasia with toluidine blue (pH 3.5), indicating the presence of both neutral and acidic polysaccharides.     

Weaving resistance: silk and disease resistance in the weaver ant <Emphasis Type="Italic">Polyrhachis dives</Emphasis>

Social insects are at risk from a diverse range of parasites. The antibiotic-producing metapleural gland is an ancestral trait in ants which is thought to be one of their primary mechanisms of resistance. However, the metapleural gland has been lost secondarily in three ant genera, which include weaver ants that are characterised by the remarkable construction of their nests using larval silk. Silken nests may have allowed reduced investment in costly disease resistance mechanisms like the metapleural gland if the silk has antimicrobial properties, as in other insects, or is a hygienic substrate. Here we examine this hypothesis in the weaver ant Polyrhachis dives. We found no evidence of a beneficial effect of silk. The presence of silk did not improve the already high resistance of ants to the entomopathogenic fungus Metarhizium, the ants only rarely interacted with the silk regardless of whether they were exposed to Metarhizium or not, and silk also did not inhibit the in vitro germination or growth of Metarhizium. Furthermore, silk was found in vitro to be heavily contaminated with the facultative entomopathogenic fungus Aspergillus flavus, and many more ants sporulated with this fungus when kept with silk in vivo than when they were kept without silk. Further work is needed to examine the effects of silk on other parasites and of silk from other weaver ants. However, the results in combination suggest that silk in P. dives is unlikely to provide protection against parasites and that it is also not a hygienic substrate. Alternative explanations may therefore be needed for the loss of the metapleural gland in weaver ants.