Caught In a Sticky Situation:The Orb-Weaving Spiderweb
Emily Gastelum and Taylor Stinchcomb
Biology 342 Fall 2011
What is Mechanism?
An inquiry of mechanism examines behavior on an immediate time scale, looking at what stimulates the expression of the behavioral phenotype in the moment that it occurs. Mechanisms of behavior can include a description of the behavior as it is expressed by the animal, the animal’s morphology, physiological processes, and processes occurring at the molecular and genetic level.
The orb web is a foraging tool used to ensnare otherwise difficult-to-capture prey that fly at fast speeds above the ground. When building their webs, spiders can control details of the architecture such as overall web size, web stickiness, and spacing of silk threads, allowing for the selection of specific types of prey from the environment. Larger webs with a wider mesh are well-suited to capture larger and faster-flying prey that may be present at lower densities while smaller webs target small insects present at high population densities.
Several studies have found supporting evidence for the spider's use of environmental cues about prey availability in locating their orb web. Some forest-dwelling species seek well-lit foraging areas where insect prey densities are higher (Adams, 2000). One species, Parawixia bistriata, builds two sizes of web and synchronizes the timing of each web with the peak activity of target prey. Small webs built at sunset capture a high density of small nocturnal insects and large webs built during the day capture large strictly-diurnal termites (Sandoval, 1994).
There are three steps to effectively capturing prey in the orb web (Sensenig, et. al, 2011):
Most spiders sit in the central hub of the web awaiting prey. The impact or struggling motions of a captured insect causes vibrations on the web that are transmitted by the radial threads to the hub. Spiders sense these vibrations to determine the location on the web and the relative size of their prey, and respond to this information with a specified attack behavior (Blackledge, et. al, 2011).
A SPIDER CONSTRUCTING ITS ORB WEB
Silk Production and Spider Morphology
The orb web is composed of two different types of silk: dragline silk that provides a stiff framework of radial threads, and capture spiral silk that forms pliable yet tough concentric circles radiating out from the center hub of the web. The capture spiral silk threads are coated with aqueous drops of glue that stick to prey (C in the figure to the right).
Many species of orb weaving spiders are known to dismantle their webs daily, ingest the used silk, and build a new web the following day. This recycling of silk likely reduces the physiological costs of building a new web, as used silk proteins are conserved from fibers of the old web and thus do not have to be synthesized again within the silk glands. However, the recycling behavior is primarily observed in orb weaving spiders that produce viscid silk rather than cribellate spiders, suggesting that the primary target of recovery in silk recycling may in fact be the molecules that constitute the glue droplets of viscid threads (Blackledge, et. al, 2011).