Spider silk & spider senses

Spiders make my skin crawl, but it's always amazed me that, despite being mechanical and grotesque, they produce silk, which is not only one of nature's finest materials, but also one of the lightest and strongest.

The creator of the fictional superhero Spiderman wasn't too far off the mark when he decided that the character would have the ability to spin webs from his hands, because it is now known at least one species of spider - the Costa Rican zebra tarantula (Aphonopelma seemanni) - can secrete silk from its feet.

Stanislav Gorb and his colleagues filmed the spiders as they crawled up vertical glass surfaces, and observed that they left 'footprints' of silk threads up to 1 thousandth of a millimetre in diameter and 2.5 cm long. These silk threads, which are secreted from nozzle-like structures on the feet, act to tether the tarantula to the surface, and were secreted when the tarantula started slipping down the glass, preventing them from slipping further.

The findings have led the authors to reconsider the evolution of spider silk. Spiders evolved around 400 million years ago and are among the first terrestrial species. Silk may at first have been used to provide a protective covering for spiders' eggs or a lining for their nests. Later on, silk may have been used in the construction of trap doors for capturing prey.

Spiders' ability to weave webs probably evolved around the time when the diversification of insects and plants occurred. Until now, it was thought that an ancestral arachnid was able to spin silk from its feet, and that, during the course of evolution, this ability was eventually lost in favour of abdominal silk-secreting glands called spinnerets. In the light of this research, another hypothesis presents itself - that the tarantula independently evolved the ability to secrete silk from its feet, as a means of preventing them from falling to their death. 

Spider silk is a viscous, proteinaceous liquid produced in, and extruded from, the spinneret (left). Most spider species have six spinnerets (although some have four or two), located on the underside of the abdomen at the rear. Each spinneret is multi-pored and can move independently of the others; the muscles responsible for this movement also force the liquid silk to be extruded from the spinneret. Because the liquid is under high pressure as it is extruded, and because the spider pulls on the silk during its extrusion, the liquid solidifies as it is secreted, producing a thread.

Spiders produce at least 8 different types of silk, with individual species usually producing a five or six types. Spider silk is a natural high-performance fibre with mechanical properties which rivals, or betters, many man-made materials. It is waterproof and elastic, and can be stronger than steel, although silk strength varies according to type.

All spiders use silk to produce a 'dragline', which acts as a safety cord when they are moving. Dragline silk is known to be five times stronger than steel; it is also able to absorb five times the impact of Kevlar, the material used to make bulletproof vests, without breaking. The structure of the silk secreted by the feet of tarantula resembles the attachment that other species use to anchor the dragline to a surface.

About half of the known 50,000+ species of spiders weave webs to catch prey. An average web contains 20-30 metres of silk, but weighs less than 0.5 milligrams. Some species use silk to catch prey in other ingenious ways. For example, Bolas spiders (of the species Mastophora, below right) catch prey by dangling a silk thread with an adhesive blob on its end; they also secrete a pheromone which attracts male Armyworm moths (Spodoptera), which get stuck on the blob at the end of the silk thread.

Even more remarkable is the Ogre-eyed spider (Dinopis guatemalensis), which has large, extremely sensitive eyes. At meal times, the Ogre-eyed spider builds a silk platform from which it dangles. It then weaves a silk net which it holds open between its legs. When an insect crawls underneath it, the spider drops the silk web, entangling its prey. The trapped insect is then pulled up and eaten. The Ogre-eyed spider can even detect the vibrations created by insects flying nearby, and throw the net into the air to catch them.

Some spiders can also use silk to fly. By weaving a length of silk during strong winds, spiders can be lifted upinto the air and transported long distances. Spiders have been observed 1,500km from land and 4,500 above sea level! Hawaii is believed to have been originally populated by spiders who 'flew' over to the island in this way.

Spider brains & spider senses

The body of a spider is divided into two parts, called the cephalothorax (or prosoma) and abdomen. The spider's central nervous system (below), which consists of a single mass of tissue typically containing about 100,000 cells, is located entirely within the cephalothorax. The central nervous system takes up most of the cephalothorax body cavity, and is separated into two by the oesophagus. The supraesophageal region is devoted to vision, while the subesophageal region contains fused ganglia, from which project motor neurons that innervate the limbs.

The sense of vision varies greatly depending on the type of spider and the ecological niche it inhabits. Cave spiders, for example, live in the dark and have little or no sight, depending instead on mechanoreception, whereas spiders that do not use webs to catch prey, such as jumping spiders (Salticidae), wolf spiders (Lycosidae) and lynx spiders (Oxyopidae), have an acute sense of vision. In these species, vision is well developed; the jumping spider is known to see in colour, for example, and its vision is thought to be almost as good as that of humans.

For most types of spider, mechanoreception is the most important sense. It is this sense that guides many aspects of arachnid behaviour. It enables a spider to detect the vibrations caused by the presence of an insect which is has been caught in its web or is crawling near its nest. It enables the Ogre-eyed spider to detect the air movements produced by insects flying overhead. Mechanoreception also enables spiders to localize the source of a vibration.

Several types of mechanosensory organs contain mechanoreceptor neurons and enable spiders to detect vibrations and air movements. One type of mechanosensory organ, consisting of large tactile hairs, is shown below. Trichobothria are fine, smaller hairs which protrude vertically from their sockets and are particulary sensitive to currents of air and low frequency vibrations.

Lyriform slit sense organs, or sensilla, are found on the exoskeleton, movements of which they are responsive to; they consist of a slit, flanked by puckered 'lips' on either side, in the surface of the cuticle. The outer part of the groove contains lymph, and there is an opening in its centre containing a dendrite, which extends down through the outer layers of the cuticle. Movement of the exoskeleton causes compression of the slit, resulting in movements of the lymph which deforms the dendrite and makes it generate action potentials.

Sensilla can be found on their own but usually occur in small groups organized in parallel rows. They are also found around the spinneret, where they provide sensory information for the nervous control of silk secretion.

Mechanoreceptors are transducers - that is, they convert tactile stimuli into electrical signals. Mechanical vibrations cause a deflection of bristles on the surface of of the large tactile hairs. The action potentials produced by the mechanosensory organs are transmitted to large interneurons. In the case of the whip spider, each leg contains several of these interneurons, each of which receives sensory inputs from up to 1,500 bristles on the surface of the leg. These interneurons in turn transmit signals to the axon of a giant neuron, which carries the sensory information to the spider's brain.

Neurotransmission at peripheral synapses is mediated by octopamine and GABA, and probably other transmitters. Arachnids are the only invertebrates to have synapses outside of the central nervous system. The presence of these synapses suggests that much information processing takes place in the periphery.