Nervous and Sensory Systems of Spiders

This page examines the anatomical and functional characteristics of the nervous system as found in most spiders.

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.



Sensory receptors used by spiders
The human nervous system responds to mechanical stimuli (sound, touch and pressure and also internal proprioception), external and internal chemicals (taste, smell, pain and blood gas concentrations), and electromagnetic stimuli (visual and thermal). Spiders can also respond to most of these stimuli but many of the receptors they use are very different in structure and mode of action when compared with the vertebrate ones. The receptors used by spiders have now been located for most of these sensory modalities and the kinds of sensory information detected by spiders can be categorized as

Each of these will now be considered in more detail.

Touch and vibration
As a generalisation it can be said that although most spiders have eight eyes, touch/vibration and proprioceptor stimuli are considerably more important for their survival. On most the surfaces of the body and legs of an 'average' spider there are hairs of varying numbers, lengths and thicknesses. Most of these are there to detect touch and low-frequency vibrations but some have special functions associated with their location on the spider. A number of spider species have on their legs specialised hairs called trichobothria. These are so long and slender that they react to faint air movements, apparently including those we call sound waves. Spiders lack conventional auditory receptors but trichobothria provide a useful and effective alternative.

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.

Proprioception
Proprioceptors detect the position/posture of body appendages such as the leg segments and also the orientation of the chelicerae, cephalothorax and abdomen with respect to each other. There is also good circumstantial evidence that spiders are even able to recognize their orientation in space. In other words, they know whether they are upside down or not or at an angle other than the horizontal. Humans use the eyes and several different kinds of balance receptors for this purpose and the same may be true for spiders, although the nature of any balance receptors they may have (other than the eyes and leg joint proprioceptors) is presently unknown.

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.



Vision
Most spiders have four pairs of eyes. In a few families only two or three pairs are present and a few primitive, minute or cave-dwelling species have no eyes at all. The anterior median eyes (AME) are often the largest of the four pairs and can then be referred to as the primary eyes in that they detect things that are present directly in front of the spider. They are always dark and have the greatest visual acuity except in lycosids and deinopids where the posterior median eyes (PME) are the largest and the most useful visually. The other three pairs of eyes, which are known as secondary eyes, are much less efficient but still are useful to allow vision in dim light and to warn of movements in the spider's periphery.

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.



The chemical senses
These are known to be present in spiders but information about them is very limited. It seems likely that spiders can taste what they are about to eat and for this reason rarely attempt to digest anything that is not suitable as food for them. The nature and location of the receptors responsible for this awareness remain to be discovered but it appears that they are not in the digestive system itself but somewhere on the outer body surfaces. In addition, the distal end of the tarsus of each walking leg and also the palps of some spiders has been found to possess a minute entrance to a very small cavity called a tarsal organ. This is now considered to be the spider's main receptor structure for pheromones and for awareness to changes in temperature and humidity. For at least some species it may also serve as a taste receptor unit. Spiders probably also need to know when their tracheal system requires better ventilation and this implies a need for internal receptors that respond to changes in the concentration of oxygen, carbon dioxide, and hydrogen ions (acidity) in the haemolymph. Unfortunately, no one has so far managed to locate these receptors with any certainty.

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.



Some related sources of information
The pages on spider movements and growth and reproduction contain some information that is related to what is covered in the above paragraphs. In addition, the following articles are worth reading:

Barth, F.G. (2002) "A Spider's World: Senses and Behaviour" Springer-Verlag, Berlin (ISBN 3-540-42046-0)

Gingl E. and Tichy H. (2006) "Continuous Tonic Spike Activity in Spider Warm Cells in the Absence of Sensory Input" J. Neurophysiol., 96, 989-997.

DeVoe R.D., Small R.J.W. and Zvargulis J.E. (1969) "Spectral Sensitivities of of Wolf Spider Eyes" J. General Physiol., 54, 1-30.

Tiedemann K.B>, Ventura D.F. and Ades C. (1986) "Spectral Sensitivities of the Eyes of the Orb Web Spider Argiope argentata (Fabricius)" J. arachnology, 14, 71-78.

Blest A.D., Williams D.S. and Link Kao (1980) The Posterior Median Eyes of the Deinopid Spider Menneus" Cell Tissue Research 211, 391-403.

Williams D.S. and McIntyre P. "The principal eyes of a jumping spider have a telephoto component" Nature, 288, 5780580.

Seyfarth E.A., Eckweiler W. and Hammer K. (1985) "Proprioceptors and sensory nerves in the legs of a spider, Curiennius salei (Arachnida, Araneida)" Zoomorphology, 105, 190-196.

Evans T.A. and Main B.Y. (1993) "Attraction between social crab spiders: silk pheromones in Diaea socialis" Behav. Ecol., 42, 99-105.

Walcott C. (1969) "A Spider's Vibration Receptor: Its Anatomy and Physiology" American Zoologist, 9, 133-144.

Fabian-Fine R., Hoger U., Seyfarth E.A. and Meinertzhagen I.A. (1999) "Periperal Synapses at Identified Mechanosensory Neurons in Spiders: Three-Dimensional Reconstruction and GABA Immunochemistry" J. Neuroscience, 19, 298-310.