The evolution of cryptic spider silk: a behavioral test

Phylogenetic patterns of change in spider silk coloration provideinsight into the selective pressures directing evolution ofsilks. Trends toward evolution of silks with low reflectanceof ultraviolet (UV) light suggest that reduced UV reflectancemay be an adaptation to reduce visibility of webs to insectprey. However, a test of the visibility of primitive and derivedspider silks is lacking. Several genera of orb-weaving spidersinclude conspicuous designs of silk, called "stabilimenta,"at the center of their webs. Due to their large size, stabilimentapresent signals that insects can use to avoid webs. Unlikeother silks in the orb web, which reflect little UV light,evolutionarily derived stabilimentum silk retains a bright UV reflectance. But, unlike primitive silks, stabilimentum silkalso reflects large amounts of blue and green light. We comparedthe visibility of primitive tarantula silks and derived stabilimentumsilks to insects by using the ability of honey bees to learnto forage at targets of spider silk. We found that the uniquespectral properties of stabilimentum silk render it crypticto insects and that primitive silks are more visible to bees.Our findings support a hypothesis that the coloration of stabilimentumsilk is an adaptation to reduce the ability of insects to avoidwebs and that ancient biases in the color vision of insectshave acted upon the evolution of spider silk coloration throughsensory drive. But our findings question the emphasis on UVreflectance alone for visibility of spider silks to insects.     

Silk decorations: controversy and consensus

Although the occurrence of silk decorations has been noted in scientific literature for over 100 years, there is still little consensus as to their function. This is despite the proliferation of studies examining the five major hypotheses: (1) protection against predators, (2) increasing foraging success, (3) prevention of damage to the web, (4) providing shade and (5) mechanical support for the web. The first three of these hypotheses have received the most attention, and thus generated the most evidence (for and against) suggesting that web decorations are a type of visual signal. However, the effect of this signal on prey and predator receivers is unclear as the evidence is contradictory. Thus, the function of silk decorations may be context specific, depending on factors such as predators, prey, background colour and ambient light. A better understanding of how predators and prey see and process visual information from silk decorations, coupled with experiments examining the mechanisms behind the various hypotheses, are crucial in illuminating their function and resolving the controversy.     

Effect of abiotic factors on the foraging strategy of the orb‐web spider Argiope keyserlingi (Araneae: Araneidae)

Abstract Environmental conditions such as light level, background contrast and temperature might influence a spider's prey capture success and risk of predation. Thus it may often be advantageous for spiders to adjust web‐building behaviour in response to variation in these environmental conditions. This hypothesis was examined in a study of the construction of webs and web decorations (conspicuous strands of silk at the hub of the web) of the orb‐web spider Argiope keyserlingi. Web decorations are thought to have one or more separate functions. They may attract prey, deter predators or advertise the web to oncoming birds, thus preventing web damage. In this series of experiments, relationships between weather parameters and the construction of webs and web decorations were considered. In complementary laboratory experiments, A. keyserlingi spiders were exposed to two different light levels (700 and 90 lx), background contrasts (black and white) and temperature conditions (20 and 26°C). Of the available weather parameters, only temperature was significantly related to web decorating behaviour but not to web size. In the laboratory, temperature also influenced web‐decorating behaviour, and spiders in dim light (700 lx) constructed larger webs and longer decorations. Background contrast did not significantly alter web size or web decorations. These data suggest that when prey availability is reduced at low temperatures, spiders may use web decorations to attract prey to the web. Similarly, in dim light, spiders may build more and larger decorations to increase the visual signal to approaching prey or to advertise the web to oncoming birds.     

The evolution of novel animal signals: silk decorations as a model system

Contemporary animal signals may derive from an elaboration of existing forms or novel non-signalling traits. Unravelling the evolution of the latter is challenging because experiments investigating the maintenance of the signal may provide little insight into its early evolution. The web decorations, or stabilimenta of some orb web spiders represent an intriguing model system to investigate novel animal signals. For over 100 years, biologists have struggled to explain why spiders decorate their webs with additional threads of silk, producing a conspicuous signal on a construction whose function is to entangle unsuspecting prey. The numerous explanations for the maintenance of this behaviour starkly contrast with the absence of a plausible explanation for its evolutionary origin. Our review highlights the difficulties in resolving both the evolution and maintenance of animal signalling, and inferring the causative arrow-even from experimental studies. Drawing on recent research that focuses on physiological processes, we provide a model of the evolutionary progression of web-decorating behaviour.