This page provides information about the kinds of toxins found in spider venoms and the relative risk to humans posed by the venoms of some common Australian spider species.
The great majority of spider species are equipped with venom glands which are suggested to be derived from salivary glands and which serve not only to provide a means of defence against aggressors but also to immobilize prey and keep them in good condition to be eaten at a later date. The following will consider the chemical and toxicological nature of the main substances found in the secretions of these glands.
The anatomy and physiology of spider venom glands
In mygalomorph species the venom glands are normally located in the large chelicerae these spiders possess. There are probably muscle fibres in the walls of these glands and voiding of venom may also be encouraged by the many muscle fibres that fill the chelicerae. Venom secretion is under the control of the nervous system but can be induced in a spider that has been anaesthetized by brief exposure to an atmosphere of carbon dioxide gas. All that is needed is to apply electrical stimuli across the cephalothorax via a pair of small electrodes placed on each side of the sternum. Unfortunately, the venom that can be collected in this way will often be contaminated by gut fluid, regurgitation of which is also induced by this stimulation. For spiders with relatively large fangs this contamination can sometimes be avoided by placing small plastic tubes on the ends of the fangs. Araneomorph spiders typically have venom glands that are largely within the cephalothorax rather than in the chelicerae, but other than this they are similar to mygalomorphs and can be milked in much the same way.
What important substances are found in spider venoms?
To answer this question fully requires an understanding of what spiders use their venoms for. While it may seem reasonable to suggest that the primary role of a spider's venom is to provide a means of defending the spider from attack by some other creature, the reality is that spiders use their venoms at least as much for immobilizing their prey before they can cause major damage either to the spider or to its web (if it has one). It is a curious fact that many spiders paralyse insects with the same kinds of toxins that predatory insects such as wasps use against spiders. This situation is somewhat like two men fighting with matching swords or duelling pistols, the winner being the one who can use his weapon the more effectively. Thus araneid species produce venoms that contain substances called acylpolyamines to paralyse insect muscles and wasps such as the mud-daubers have similar polyamines in their venoms for paralysing spiders they want to store in their nests as food for their larvae.
Acylpolyamines, as the name suggests, are composed of a string of amino (-NH) groups attached via an amino acid linkage to an aromatic ring structure. There may also be an aryl side chain added, the purpose of which is variable and often uncertain. In the nervous systems of both insects and spiders polyamines generally bind to the postsynaptic surfaces of synapses within the nervous system and thereby block the neurotransmitter, glutamate, which normally allows nerve impulses to cross from one nerve cell to another or to a muscle fibre. In general, this binding and subsequent blocking of the nerve pathway is long-lasting unless the amount of polyamine present is very small. If this paralytic effect was exerted in a human or other large vertebrate it would be lethal because most vertebrates use muscles for breathing and for the circulation of blood.
However, small, cold-blooded insects and spiders are much less dependent on
cardiopulmonary functions and thus can remain alive but totally immobile for extended periods of time. Spiders such as the large orb weavers that have polyamines
as their major venom neurotoxin seem to be relatively harmless to humans and large vertebrates. If this is indeed the case the probable reason for this is that
vertebrate muscles are driven by peripheral nerves that use either acetylcholine or a catecholamine such as noradrenaline as their neurotransmitter. There is no
longer any doubt that glutamate is used as a neurotransmitter within the human nervous system but it plays this role only within the central nervous system (CNS)
which is protected from many circulating toxins by what is known as the blood-brain barrier. For this reason a polyamine will not have much effect on the nervous
system if injected under the skin but may still have potential for experimental or therapeutic use (for example, in cases of motor neurone disease) in humans.
Small peptides and some large proteins in spider venoms are also used as toxins. These molecules sometimes are particularly toxic to vertebrates, which suggests their main function is to provide an effective defence against large predators that cannot be rendered harmless by the injection of polyamines. This view seems logical when it is appreciated that large vertebrates are not creatures that spiders normally feed on but do have the capacity to overwhelm a spider unless swiftly dissuaded. Peptides and proteins are strings of amino acids and thus are substances for which the body's DNA strands contain sequence coding. In consequence of this fact no two spider species will have exactly the same protein or peptide toxins as those found in other species but related species may have molecules that act in exactly the same way in the vertebrate nervous system. Over the last 20 years there has been a great deal of interest in studying these peptides and it is now clear that there are an extraordinarily large number of them even in the venom of a single spider species and that they are active in the nervous systems of both large animals and small creatures like insects.
Curiously, the peptide toxins found in the venoms of the Australian funnel-web spider
species can be subdivided into those that are extremely toxic to vertebrates (apart from animals that possess some form of immunity against the toxins) and those that are insecticidal. There
is not much overlap between these two activities and in consequence there are now a number of research labs around the world attempting to create from spider
venom peptides new insecticides with a very low toxicity to vertebrates. Of course, the yield of venom from an individual spider is very small so it is not feasable
to collect venom for the commercial production of an insecticide. Instead, small peptides can now be systhesized in the lab or the genes that code for them are
incorporated into viruses specific for important insect pests or are added to the genomes of economically important plants such as cotton on which pest insects feed.
In addition to polyamines and protein or peptide neurotoxins spider venoms contain a variety of substances that are better called inflammatory mediators than
neurotoxins. Serotonin, gamma amino butyric acid (GABA), other strong organic acids, adenosine triphosphate and tyramine have all been found in some spider venoms as have also several
enzymes that have the potential to cause a strong inflammatory respone. Among these enzymes are hyaluronidase, collagenase, other proteases, sphingomyelinase and
some esterases, all of which can cause serious disruption of the architecture of the skin and other organs. These enzymes presumably are in spider venoms because
of their digestive activities but they also damage the skin of large vertebrates and in the process also cause local inflammation and pain. The latter may benefit
a spider (or at least the species to which it belongs) by causing the attacker to lose interest in the spider. As mentioned in more detail below, the bite site will
be an area of skin which has had its integrity compromised so the initially small bite site could eventually become a large area of inflammation if opportunistic
bacteria enter it and cause a secondary microbial infection.
Because of the very large number of different polyamine and peptide toxins that can be found in spider venoms it is a difficult task to establish the mechanism by which a spider toxin interferes with transmission across synapses within the human nervous system. Adding to this difficulty is the known complexity of the processes that occur at mammalian synapses. It is known that spider toxins can disturb synaptic transmission either by presynaptically by inhibiting release or causing excessive release of neurotransmitter, or postsynaptically by blocking the action of the neurotransmitter or by causing spontaneous stimulation of the postsynaptic surfaces.
At the present time it is accepted that there are three different kinds of openings through the two sides of a synapse:
calcium ion channels, which induce neurotransmitter release and also muscle contractions; sodium ion channels, opening of which is associated with stimulation of
the postsynaptic surfaces
and subsequent excitation of the attached nerve or muscle cell; and potassium ion channels, which have variable roles but mainly allow the synaptic
membranes to reset themselves in preparation for the next stimulus. The most dangerous of the Australian spider toxins are the funnel-web peptides and the
alpha-latrotoxins from redback spiders, all of which are said to act largely, if not entirely, by causing excessive and uncontrolled stimulation of many synapses
within the human nervous system. They could therefore kill by causing cardiac arrest, sustained apnoea (cessation of breathing because the breathing muscles are
in spasm) and perhaps asphyxiation if excessive amounts of fluid accumulate in the airways of the lungs.
Most dangerous Australian spiders cause their harmful effects on the human body by interfering with the functioning of the nervous system, but the
white-tailed spider has long been accused of causing a very different kind of injury. Its bites were believed to lead to
necrotising arachnidism, the characteristics of which are an area of inflamed and often ulcerated skin
which can continue to enlarge for many weeks. This skin damage is caused by digestive enzymes such as collagenase, hyaluronidase and sphingomyelinase and is generally unresponsive to antibiotics and antiinflammatory agents, although it may sometimes be
resolved by hyperbaric oxygen therapy. However, scientific reports and examination of clinical records by Dr Geoff Isbister and others have now established beyond
reasonable doubt that no Australian spider, including the white-tailed spider, has a significant capacity to cause more than a temporary and localised skin lesion
except perhaps indirectly.
The one possible exception to this is a Loxosceles species that is related to the American fiddle-back spiders, which have been shown
to be capable of inducing severe necrotic skin ulcers. Fortunately, the Australian Loxosceles species is found in an isolated area near Adelaide and almost nowhere
else and therefore presents and insignificant hazard to almost all Australian citizens. Many other spiders found in this country can produce bites that lead to
a small area of damaged skin and some associated inflammation and pain at the site but this does not persist for many months and neither does it continue to increase
in size or heal only to redevelop at a later time. Whenever this appears to have happened the lesion was actually secondary to some other disease such as diabetes
or to an invasion of the bite site by opportunistic bacteria (including the so-called flesh-eating Staphylococcus bacteria and also Mycobacterium ulcerans) or fungi. Similarly,
recurring skin lesions could be due to episodes of a variety of degenerative skin diseases or perhaps to an immune or autoimmune response.
While there are very few instances of genuine necrotizing arachnidism on record in Australia there are many victims of skin lesions that are mistakenly believed to
be examples of this disorder. What can be done to help such people? Firstly, a proper diagnosis if the lesions should be carried out to eliminate as far as possible the many
known causes that do not involve spiders. Clearly, each of these causes have their own best treatment regimens. If a spider bite was seen to occur at the lesion site
it is desirable to have this examined by a physician but there is probably nothing to be done about it other than to keep it clean and to wait a few days or weeks
until it heals spontaneously. At the present time there are no unquestionably beneficial treatments of ulcerating spiders bites in Australia including surgical
excision and repair of the bite site.
It is possible to reduce the numbers of undesirable spiders around a house by the application of pesticide sprays but this rarely provides a permanent solution to the problem and most of the sprays are non-specific for spiders and also potentially hazardous to the people who use them. Redback spiders are a significant threat to householders even though they are far more likely to build their webs outside houses and sheds than inside them. However, they are largely predictable in regard to the sites where they will build their nests, ledges outside the house as well as under the leaves of garden plants being their preferred places. They rarely venture into houses unless there is a convenient permanent opening such as a window.
It is also important to understand that most localities in Australia do not have funnel-webs and those that do will see them only during the breeding season or when
they have been accidentally excavated. Anyone who is concerned that they may have funnel-webs in their backyard should look for the characteristic burrow entrance
they build. If there is no sign of these burrows the chances that funnel-webs will be there are very low, though it is always possible some individuals might wander
in from a neighbouring property. And finally, while redback spiders will soon return to a property from which they have been eradicated, a thorough search for,
and excavation of, funnel-web burrows in a small area of land will leave that piece of land effectively free of funnel-webs for a very long time, if not forever,
because funnel-webs are not able to tolerate frequent disturbance of their habitat. It is for this reason they are found in bushland settings and also in lawns, rockeries
and hedges but not in frequently replanted garden beds or in acreage that is used for growing crops.
1. They do not set up colonies in houses because they can only survive in humid environments and therefore must spend most of their lives in underground burrows. If unable to find their burrows or to quickly build new ones, these spiders will die in a few days, either from dehydration or from attacks by birds, bandicoots or other predators. This would probably happen within the first 24 hours except for the fact that as daylight approaches funnel-webs usually find a dark, damp corner to hide in or build and occupy a retreat under low plants until night falls again. The latter tendency can make funnel-web males a short-term hazard for gardeners who don't take care where they put their hands!
2. Each spider, whether male or female, lives in a separate burrow which is made wider and deeper as the spider grows bigger during the first few years of its life. Funnel-webs rarely leave these burrows during the winter months but may be found waiting for insects at or near the top of the burrow during the evenings at other times of the year.
3. Male funnel-web spiders only develop the anatomical features of males during their breeding season, which on the Darling Downs is November to February. They usually perform their maturation moult only on rainy nights and then wander in search of a burrow that contains a female, never returning to their burrow. Whether they succeed in mating or not they inevitably die within a month or so after maturing.
4. Male funnel-webs cannot jump, though they will rear up impressively when provoked, and they mostly have little tendency to climb and so are unlikely to be found in any room that is accessed only via a flight of steps.
5. Because they must stay in humid conditions, funnel-webs are attracted to ground-level laundries, leaky garden taps, and backyard swimming pools.
6. It is unlikely that funnel-webs will be eradicated by pouring water (even boiling water) down their burrows because these are never vertical and often turn sideways to terminate under a buried stone or tree root. Funnel-webs are also slow to drown and have been known to recover even after 2 days submersion in a swimming pool.
7. Spraying the garden with potent insecticides will not kill funnel-web spiders but will destroy many other harmless small animals. Even spraying directly into each burrow has proven to be an inefficient way to kill these spiders. Somewhat more effective (but still environmentally inappropriate) is the pouring of petrol or diesel fuel down funnel-web burrows.
8. Digging up the spiders one by one will eliminate funnel-webs from a domestic garden and they will take many years to return. Regular cultivation of the soil will also prevent them from establishing burrows. Unfortunately, they can walk several hundred metres in one evening so they could still wander in from a neighbouring property, but of course this will only happen in the few months of their breeding season and even then only after rain.
Email Ron Atkinson for more information. Last updated 30 December 2009.