Nervous and Sensory Systems of Spiders
This page examines the anatomical and functional characteristics of the nervous system as found in most spiders.
In general, spiders have a nervous system that resembles that of vertebrates except that it is much less elaborate, especially in regard to intellectual functions. The paragraphs below are an attempt to show that while spiders may have a more limited range of neurological responses to sensory input than humans they still are very good at helping them to survive.
Anatomy of a spider's nervous system
The structures that comprise the cellular masses of a spider's nervous system are almost entirely found in the cephalothorax with just a few relatively minor ganglia (clumps of nerve cells) in the abdomen. The piece of neurological tissue that serves as a spider's brain is the supraoesophageal ganglion which is positioned just above the digestive tract and in front of the sucking stomach. Present evidence indicates that the most important function of this brain is to process sensory information, especially that from the eyes, and to produce suitable responses, including movements of the chelicerae and the release of venom from the venom glands. These responses are obviously important for survival and a spider's brain does appear to have at least a small capacity to learn from past experiences and to respond more appropriately when dealing repeatedly with the same set of circumstances.
Below the digestive system and connected to the brain by lateral extensions is the suboesophageal ganglion. This is star-shaped because it has major branches leading to the legs and palps. It probably is equivalent to the human brainstem and has a primitive level of segmentation like that seen in other invertebrates and also in the human nervous system. Most of the spider's muscles and especially the leg muscles are controlled by the suboesophageal ganglion. In some species that have been studied electrophysiologically small peripheral ganglia have been found in the legs and these probably control whatever leg muscles are there as well as providing input pathways for the many sensory receptors each leg has.
The suboesophageal ganglion leads back to the pedicel through which a bundle of nerve fibres extends into the abdomen. There is no evidence of nerve cell masses
in the abdomen equivalent to the vertebrate spinal cord but there may be a few small ganglia in some abdominal structures, including the heart. As a
generalisation it can be stated that the brain is relatively larger than the suboesophageal ganglion in species such as salticids that are highly dependent on good vision
whereas the suboesophageal ganglion is proportionately larger than the brain in web-builders whose vision is not so acute or important.
Each of these will now be considered in more detail.
Experiments with the common theridiid, Parasteatoda tepidariorum, have led to the discovery of a very sensitive vibration receptor in the
vicinity of the tarsus and metatarsus of each leg on this spider. This is called a lyriform organ and consists of ten slit receptors that are tuned for
frequencies up to 1400 Hz and that can have their sensitivities adjusted by changes in the tension on the slits. These lyriform organs are
also used for detecting airborne sounds rather than vibrations of the spider's web or of the surface on which the spider is resting. There is mounting evidence that many
other kinds of spiders also have them or something similar.
The proprioceptors spiders use to the greatest extent are those associated with the leg joints. These allow them to know the extent of flexion or extension that exists at each joint at a given moment. Some extensive neurophysiological studies have been performed on the overseas ctenid species, Cupiennius salei, and we now know that this species has numerous hairs (or sometimes whole hair plates) of different sizes around each of its joints and that the extent of deformation of these as flexion or extension occurs at that joint is used by the spider's nervous system to monitor the posture of that leg.
The lyriform slit receptors which, like the musical instrument their name suggests, contain strips of tissue that
respond to changes in tension are also important as joint proprioceptors. These have been found associated with the metatarsi but there are also some simpler slit sensilla in other leg,
palp and probably spinneret joints. Similarly, slit sensilla in the pedicel are used to allow the spider to be aware of any changes in the
orientation of the abdomen with respect to the cephalothorax. Even the cuticle itself
seems to possess receptors that warn the spider of any deformation of those parts of the body that are flexible.
Anatomically, a spider's eye is reasonably similar to a human one. There is a curved cornea and an associated lens which does not have an adjustable focus. The focal distance of a spider's eyes therefore is fixed for most species. However, studies of the lenses of the anterior median eyes of salticids have revealed that they are not spherical but instead taper like a telephoto lens system on a modern camera. This produces a very narrow visual field but the salticids partly overcome this limitation by having small muscles at the back of the AME to allow the eye to have its axis turned sideways to some extent. These salticid eyes also have a cup-like front to the receptor cell mass and the vitreous material that fills this depression therefore serves as a secondary lens and gives the eye a telephoto capacity. This arrangement produces very good vision and is one of the reasons why salticids are harder to catch than many other kinds of spiders.
Also missing from spider eyes is a coloured iris to regulate the amount of light that enters the
eye unit. On the other hand, in the eyes of some species there are vitreous or pigment cells that form a ring just behind the cornea and thereby limit the
entry of peripheral light into the eye. A light-sensitive retina is present at
the back of the eye and nerve fibres from this conduct visual information down to the spider's brain.
The retina of a spider's eye has cellular arrangements that vary not only from species to species but also from eye to eye on an individual spider. In the primary (AME) eyes there is a layer (or sometimes several layers) of receptor cells, each with a nucleus nearest the light then a light-sensitive area called a rhabdome or rhabdomere, and finally a long fibrous tail which passes through some darkly pigmented cells to become part of the optic nerve. As in vertebrates, this nerve carries the visual image down to the spider's brain. In the three pairs of secondary eyes a typical spider possesses there is a layer of reflective material called the tapetum lying just deeper than the rhabdomere layer and this has the useful function of reflecting back onto the rhabdomeres light rays that have bypassed them. This increases the sensitivity of the secondary eyes in dim light. Except in just a few species such as the wolf spiders, they have very low visual acuity.
The presence of the tapetum in these eyes is the reason why many species have on the top of the
head region a pair of eyes with a silvery appearance (light is being reflected back out of the eye) and why wolf spiders have eyes that
glow when a torch is shone at them at night. Primary eyes normally look dark because they lack a tapetum. They have variable rhabdomere
distribution and orientation and at least one species is claimed to be able to detect plane-polarised light because it has some of its rhabdomeres
orientated at right angles to others. Some people who photograph spiders are surprised to observe that occasionally the eyes have a colour other
than silvery white or almost black. When the eyes appear to be green, blue, red, or yellow this is an artifact effect. It may be caused by diffraction
of the light used to illuminate the spider that is being photographed. Alternatively, it could be due to transmission of light forwards through a red, green, or orange area of the cephalothorax.
A considerable amount of research has now been performed to determine the wavelengths of light to which the visual receptors of spiders' eyes are most responsive. What is now clear is that in this respect there are some noteworthy differences between different kinds of spiders and also between primary and secondary eyes. In general it can be said that secondary eyes contain only one light-sensitive pigment and that this is responsive to light in the 500-540 nM wavelength range, which means green or blue light. On the other hand primary eyes generally seem to be able to detect visible light of a turquoise colour as well as light at a wavelength of approximately 360 nM, which is in the near-ultraviolet range.
A number of researchers have
claimed to have shown convincingly that some kinds of spiders do have colour vision, their eyes possessing receptors for orange, turquoise,
blue-violet and ultraviolet wavelengths and it has even been suggested that salticid anterior median eyes have three or four layers of receptor cells
because they use three or four different receptor types. However, other studies have failed to confirm these claims. It is probable that each species has eyes with a receptor system that is appropriate for the
habitat and behavioural patterns it has chosen to adopt and that good colour discrimination is relatively unimportant for most spiders.
Perhaps the most certain and best studied of the chemical receptor systems found in spiders are those that respond to pheromones, especially
those released by members of the opposite sex (generally the female). Pheromones are odours that can be detected in the air surrounding a spider
as well as being released onto the female's silk and possibly some body surfaces as well. These substances are difficult to study because
the concentrations present are extremely low, but they are low-molecular-weight lipid-soluble chemicals that may be volatile lipids or substances
somewhat like the aromatic terpenoids of plants. Several studies of pheromone responses by male spiders have suggested that the tarsal organs of the male palps
are particularly important for pheromone detection. Not only do they detect the presence of an adult female but they also trigger avoidance responses in competing males
of the same species. Some spider genera with a tendency towards social behaviour also have other pheromones that the adults (only) add to their silk
to persuade females to tolerate the presence of other females in their vicinity.
Email Ron Atkinson for more information. Last updated 23 December 2018.