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.

Foraging Strategy

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).   

Prey Capture

There are three steps to effectively capturing prey in the orb web (Sensenig, et. al, 2011):

  1. Intercept flying or jumping prey—the vertical orientation of the web above the ground and the ability of the spider to control the size of the web allow for the spider to target aerial, fast-moving prey.  

  2. Absorb and dissipate the prey’s kinetic energy—this is known as the web’s “stopping potential,” the maximum energy that the web can absorb per unit area.  This property dependent on the quality of silk and web architecture.  Radial dragline silk threads provide a stiff foundation that resists breaking under the impact of prey, and the rings of capture spiral silk threads are tough, but highly extensible, cradling prey on impact.  The ability to stretch and recoil results from a high mobility of structural molecules and the folding of structural proteins into spring-like helices.  In addition, the glue droplets that coat capture spiral threads stick to prey, preventing them from ricocheting out of the web.

  3. Retain insects long enough for the spider to subdue them—the web must be sufficiently sticky to hold a struggling prey until the spider can reach it.  Web retention is primarily influenced by the aqueous glue droplets that coat the capture spiral threads and stick to prey upon impact.  These droplets are also able to stretch, and generate more adhesive force as insects attempt to pull away. 

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).   



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).

Spiders have several different silk glands, each of which produces a specific type of silk.  The glands are contained in spinneret organs on the spider’s abdomen. Valve-like structures called spigots that protrude from the spinneret are connected to the internal silk glands and secrete the final silk fibers.   

Dragline silk is produced in the spider’s Major Ampullate silk gland as a single thread of paired fibers.  Capture spiral silk is produced from the Flagelliform gland and is excreted from a single spigot.  The aqueous glue droplets are produced by the Aggregate gland.  A pair of spigots from this gland flank the single spigot from the flagelliform gland, excreting glue droplets directly onto the core fibrils of the capture thread as it is spun (this triad of glands is shown in E on the figure to the right). 

Several species of orb weaving spiders do not produce the viscid, glue-coated capture spirals.  Instead, these spiders produce cribellate capture threads that contain core fibers similar to the flagelliform silk, but that are surrounded by puffs of dry cribellar silk fibrils (D).  Although they are dry, these cribellar fibrils are able to stick to prey from a combination of van der Waal’s and hygroscopicforces (absorbing water from the atmosphere via capillary action).  Cribellate spiders have a spinneret organ called the cribellum that contains a field of spigots (F in the figure to the right) from which cribellar fibrils are excreted.  The fibrils are attached to core fibers by a comb-like structure on the spider’s leg (G) that shapes the fibrils into puffs (Blackledge, et. al, 2011).

Silk Recycling

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).