Anna-Christin Joel

Cribellate thread production in spiders: Complex processing of nano-fibres into a functional capture thread

Spider silk production has been studied intensively in the last years. However, capture threads of cribellate spiders employ an until now often unnoticed alternative of thread production. This thread in general is highly interesting, as it not only involves a controlled arrangement of three types of threads with one being nano-scale fibres (cribellate fibres), but also a special comb-like structure on the metatarsus of the fourth leg (calamistrum) for its production. We found the cribellate fibres organized as a mat, enclosing two parallel larger fibres (axial fibres) and forming the typical puffy structure of cribellate threads. Mat and axial fibres are punctiform connected to each other between two puffs, presumably by the action of the median spinnerets. However, this connection alone does not lead to the typical puffy shape of a cribellate thread. Removing the calamistrum, we found a functional capture thread still being produced, but the puffy shape of the thread was lost. Therefore, the calamistrum is not necessary for the extraction or combination of fibres, but for further processing of the nano-scale cribellate fibres. Using data from Uloborus plumipes we were able to develop a model of the cribellate thread production, probably universally valid for cribellate spiders.

Fighting for the web: competition between female feather-legged spiders (Uloborus plumipes)

Most spider species are solitary, and among the few social interactions among them, resource competition between females has received little attention. We discovered that females of the feather-legged spider Uloborus plumipes invade the orb webs of conspecifics and compete for webs. Following observations in the wild, intruder-defender interactions were studied in a terrarium and in controlled laboratory experiments. We found that contests for orb webs occurred spontaneously between adult females. Competitive interactions in U. plumipes were characterized by an escalation of ritualized behaviors. In 27% of the contests the winner was determined by interactions at a distance, which involved behaviors that caused vibratory signaling on the web. The remaining interactions escalated to physical contact, and in 78% of these a fight occurred between the contestants. Using multivariate logistic regression we determined the factors that predicted the outcome of the contests: (i) Web ownership did not give the defender a competitive advantage. (ii) The difference in physical size between the competing spiders was the most important predictor for the outcome of web contests. (iii) Independent of body size, the display of certain behaviors, specifically the ability to reach the hub before the contestant and the frequency of attacks, increased the probability of winning. (iv) Winning or losing a fight did not affect the chances of winning subsequent contests. The interactions reported here provide a promising approach to investigate communication in spiders and to test theoretical models of intraspecific competition.

Nanofibre production in spiders without electric charge

ABSTRACT Technical nanofibre production is linked to high voltage, because nanofibres are typically produced by electrospinning. In contrast, spiders have evolved a way to produce nanofibres without high voltage. These spiders are called cribellate spiders and produce nanofibres within their capture thread production. It is suggested that their nanofibres become frictionally charged when brushed over a continuous area on the calamistrum, a comb-like structure at the metatarsus of the fourth leg. Although there are indications that electrostatic charges are involved in the formation of the thread structure, final proof is missing. We proposed three requirements to validate this hypothesis: (1) the removal of any charge during or after thread production has an influence on the structure of the thread; (2) the characteristic structure of the thread can be regenerated by charging; and (3) the thread is attracted to or repelled from differently charged objects. None of these three requirements were proven true. Furthermore, mathematical calculations reveal that even at low charges, the calculated structural assembly of the thread does not match the observed reality. Electrostatic forces are therefore not involved in the production of cribellate capture threads. Summary: Although it has been hypothesized that nanofibre production in cribellate spiders involves an electrostatic charging of fibres, we refute this hypothesis, proving that spiders use no charge at all.

Moisture-Harvesting Reptiles: A Review

Reptiles can live in arid environments due to special adaptations of their integument to such habitats. So called moisture-harvesting reptiles show behavioral and morphological adaptations, as their diet often does not cover the complete water demand and rain is scarce. The collection of water from various sources by moisture-harvesting reptiles is often accompanied by a stereotypical behavior: snakes coil up in the open and show a dorso-ventral flattening of their body to increase the surface area. Lizards also show a flattening of their body, but additionally raise their abdomen by splaying and extending their legs and lowering their head and tail. A similar behavior is observed in tortoises. Though there are several behavioral descriptions of moisture-harvesting reptiles, there are only few investigations about the physical principles enabling a passive collection of water. Special skin structures, comprising a micro structured surface with capillary channels in between imbricate overlapping scales, enable lizards to collect water efficiently. In some lizards, such as the Texas horned lizard Phrynosoma cornutum, water droplets applied to their body surface show a preferred spreading direction, transporting the water towards their mouth for ingestion. This passive directional transport is enabled by asymmetric and interconnected channels between the scales. Elucidation of the physical principles behind the directional water spreading has inspired a biomimetic transfer to optimize future applications in liquid handling, e.g. in fields of microfluidics.

Morphological adaptation of the calamistrum to the cribellate spinning process in Deinopoidae (Uloboridae, Deinopidae)

Spiders are famous for their silk with fascinating mechanical properties. However, some can further produce, process and handle nano fibres, which are used as capture threads. These ‘cribellate spiders’ bear a specialized setae comb on their metatarsus (calamistrum), which modifies cribellate nano fibres to assemble a puffy structure within the capture thread. Among different species, the calamistrum morphology can differ remarkably. Although a model of thread production has been established for Uloborus plumipes, it is not resolved if/how different shaped calamistra influence the production process. We were able to transfer the model without restrictions to spiders with different shaped calamistra. Fibres are not locked between setae but are passing across a rather smooth surface-like area on the calamistrum. This area can be relocated, explaining the first morphological difference between calamistra, without changing the influence of the calamistrum on fibres. By performing an elongated leg movement, contact between fibres and calamistrum could be adjusted after finishing thread production. This movement has to bring the thread in contact with the second morphological peculiarity: cribellate teeth. We suggest these teeth are used to handle the thread independently of the spinnerets, a feature only necessary for spiders, which do not move during web construction.

Vibratory movements in contests between females of the feather-legged spider (Uloborus plumipes)

Females of the feather-legged spider Uloborus plumipes invade, and compete for, each others orb webs. In the context of these competitive interactions the question arose how the spiders communicate. Since substrate-borne vibrations are the most important component of the sensory environment of web-building spiders, we investigated vibratory movements that might serve as signals of communication. Three behaviors were found to be associated with female-female contests and to cause propagating vibrations in the spider webs: thread pulling, abdominal trembling, and web shaking. While thread pulling and abdominal trembling were also observed when prey insects were caught in the webs, web shaking occurred only in response to the presence of a competing conspecific. Caused by flexing of the first legs and a vigorous rotary movement of the opisthosoma, web shaking creates a short burst of strong oscillations of the orb web. This behavior always elicited a behavioral reaction by the competitor and may serve as an intraspecific signal in the mutual assessment of competing spiders. We suggest that web shaking communicates resource holding potential in U. plumipes.