Spider Hazards and Defences

This page examines the many hazards spiders face in their normal environments and the various things they do to survive such hazards

1. Desiccation. Different kinds of spiders vary greatly in their tolerance of dry environments. In general, mygalomorphs need a habitat in which the humidity is relatively high and they therefore spend most of their lives in a burrow, venturing out only in the evenings and especially when it is raining. Some mygalomorph species, including the funnel-webs and also Cataxia pulleinei, can survive only in rainforest locations or at least in habitats where the annual rainfall is quite high. On the other hand, many Australian trapdoor and tarantula species are adapted to semi-arid inland locations. Araneomorphs generally are better able to survive in habitats that promote desiccation but even they tend to avoid exposure to strong sunlight and dry air currents. Although spiders are known to occasionally ingest small droplets of water and tend to be attracted to moist habitats, most species depend largely or entirely on water obtained from their prey and from their own metabolic activities, their behavioural patterns often reflecting their awareness of the risk of desiccation.

2. Extreme temperatures. Spiders are cold-blooded and therefore are inactive when their environment is very cold. Indeed, freezing conditions are lethal for spiders and most species die or hide in a protective retreat of some kind long before the prevailing temperature has fallen that far. As a generalisation it can be said that spiders are easy to find in tropical localities but the numbers and types of spiders diminish progressively as we move away from the equator and towards the poles. The majority of adult Australian orb weavers disappear from their webs as soon as the cold winds of winter arrive. Admittedly, they probably have already laid a batch of eggs and do not live longer than one year anyway. If spiderlings have hatched out in autumn many will also die as the environmental conditions deteriorate but a few will manage to hide in protective habitats and thus survive through winter to enjoy an early growth spurt in spring.

3. Starvation. Spiders do not need to eat every day but will slowly die of starvation if unable to find food for many weeks. This is usually not such a big problem for the long-living mygalomorphs and those araneomorphs that 'hibernate' during the winter months but it is a very real possibility for exposed species that fail to find any prey during late autumn and early winter. Similarly, in districts that are experiencing drought conditions the insect population will often be severely depleted even during the warmer months so the spider numbers will also reduced and those individuals that can be found will mostly look undernourished. There is not much spiders can do about such vagaries of the weather except perhaps to attempt to mate only when the environmental conditions seem to favour survival of themselves or their spiderlings.

4. Accidental mechanical damage. Although many humans are fearful of the prospects of a spider bite the reality is that spiders are more likely to be harmed by humans than humans are by them. Spiders have bodies that are really very fragile and, while they can tolerate the loss of a leg or two, accidental damage to the carapace and especially to the soft abdomen is usually fatal. Thus, if two tree branches or trunks rub together during strong winds some spiders (and particularly the larger ones) are at risk of accidental mechanical injury if caught between them. On the other hand, falling from a high position is almost never immediately harmful for spiders because they can extrude silk very quickly and this acts like a parachute or safety line so they hit the ground softly. And of course young spiders often deliberately jump from elevated positions and use extruded silk to 'balloon' away from their original perch to land on whatever relatively distant object they can find.

5. Attack by other spiders. As mentioned on the page about what spiders eat, most spiders prefer a solitary life and will attack other spiders, even members of the same species such as their own male counterparts, if they venture too close. Indeed, a number of genera, notably including the white-tailed spider Lampona, which is claimed to have a particular fondness for the black house spider, even choose to deliberately prey on other spiders and rarely eat anything else. Attacking or surviving an attack by other creatures is discussed further in later sections of this page.

6. Predation by parasitic animals, wasps, and other insect larvae. Large ground-dwelling spiders are at risk of attack by Gordian worms, which live and grow in their abdominal spaces until the damage they cause eventually kills the spider. In Australia there are also several species of wasps (some of them often referred to as hornets) that prey on spiders which they paralyse with their venom then store in their nests to be used as food by the wasps' larvae. Probably the best known examples of this are the mud-dauber wasps. These make oval nest capsules out of mud and fill them with spiders that are alive but totally inert. After laying at least one egg in each nest capsule the wasps close the capsule and leave the larvae to hatch out, feed on the spiders and then break out as adults. Mud-dauber wasps are both strong and skilful and are often seen carrying captured spiders that are much larger than the wasps themselves.

There are also some very small parasitic wasp species that attack the eggs of spiders rather than the spiders themselves. For example, in Australia there are a number of Baeus wasp species that lay their eggs in those of spiders, the spider eggs then hatching out wasps rather than spiderlings. A few Australian dipteran fly species have also been reported to lay their eggs either in spiders or in their eggs. Several common araneid spiders as well as some theridiid and linyphiid species are known to be subject to this form of parasitism it is likely members of other families are similarly parasitized. One frequently seen predatory wasp larva that attacks orb weavers is so much larger than a spider egg it is almost too big to fit inside the spider's body so it attacks its prey from the outside.

Similarly, a few worm parasites somehow find their way into the bodies of spiders, notably ground-dwelling species, and thus if you find a spider with an abdomen that is swollen in a non-symmetrical fashion this is probably because it contains a worm that has grown to the stage where it is now occupying a large part of the space within the spider's abdomen. What is more certain is that some spiders are prone to attack by small ectoparasites. Burrow-dwelling mygalomorphs are particularly likely to have on their body surfaces mites that do not penetrate the cuticle but feed off its surface, thereby gradually weakening it and compromising its integrity.

8. Predation by vertebrate animals. While parasites and insect predators are significant hazards for spiders, an even greater threat for those spiders that choose to live in relatively exposed habitats is attack by vertebrate animals. The latter include many species of birds but also amphibians, reptiles such as small lizards, and some mammals, especially bandicoots which seem to be able to eat even highly venomous male funnel-web spiders with impunity. It can be argued that spiders fear predation by these large animals at least as much as attack by invertebrates and that avoidance of attack by vertebrates is one of the reasons why many spider species prefer to search for prey during the evenings rather than in full sunlight. It is also a major reason why so many spiders have a strongly camouflaged appearance.

9. Microbial infections. Although spiders inhabit the same non-sterile world we live in they seem to be much less prone to bacterial, viral and fungal infections. One likely reason for this is that spiders are mostly solitary animals so contact between individuals is comparatively rare except during mating. They also cannot cough or sneeze and hence do not transfer pathogens via aerosols. Very few spiders live in aquatic habitats and those that do have water-repellent body surfaces so their chances of acquiring a waterborne pathogen are very small. It is possible a spider could acquire pathogens by accidental contact with faecal or other material from an infected spider but even this risk is minute because spiders have a very tough integument and do not ingest soil or plant material. They also don't eat the outer shell of their prey but simply ingest its liquified internal tissues. For these reasons it is highly unlikely that a spider disease epidemic will occur in the field.

On the other hand, the internet contains several references to a dyskinetic syndrome (DKS) that has been seen quite often in the colonies of tarantula breeders. The actual cause(s) of this syndrome remains uncertain and at least some instances of it could involve adverse environmental conditions rather than microbial pathogens. However, a European research group observed high death rates in their ctenid and diplurid colonies and the cause of this was eventually shown to be a pathogenic Mucor, fungus that was contaminating the lab-raised fruit flies (Drosophila melanogaster) they were using to feed their spiders.

Mention must also be made of the Gordian or horeshair worms belonging to the nematode-like Phylum Nematomorpha. Larval stages of these are found all over the world in freshwater ponds and puddles. There they enter the bodies of aquatic animals such as juvenile insects and form cysts. If an infected insect then matures and leaves the water to be captured and eaten by a spider the worm it carries make its way into the spider's abdomen. There it gradually consumes most of the spider's tissues before emerging from the spider when it is near water and mating in the water as an adult to create the next generation of worms. The adult worms are long and thin and form coils inside the spider's abdomen, hence their common names.

10. Neoplastic and other internal diseases not involving pathogens. There is even less published evidence that spiders develop cancers or suffer internal disease states that do not involve either microbial pathogens or parasites, but there is no reason why they could not occasionally develop such diseases because they use the same genetic code we use and are just as exposed to adverse environmental conditions. The fact that apparently no such spider diseases have been described in refereed scientific journals might perhaps be partly explained by the much shorter lifespan that most spider species have. Most cancers that occur in vertebrates are relatively slow-growing so for spider species that die from 'natural causes' in less than 12 months there probably is insufficient time for malignancies to become obvious.

Similarly, chromosomal diseases and neonatal malformations that occur all too often in human infants are unlikely to be noticed in early-instar spiderlings because many of the spiderlings that emerge from a typical batch of spider eggs quickly die from cannibalism or other environmental hazards. It seems likely that gene mutations occur at least as often in spiders as in vertebrate animals. Indeed, the formation of mutant genes combined with the very short life cycles of most spiders is a probable reason why the more successful spider families now have very large numbers of individual genera and species. Each new species evolving as a consequence of gene mutations can be viewed as a spider 'success story', the harmful mutant genes quickly disappearing because the individuals that carry them die prematurely.

What changes in appearance or behaviour have spiders developed to survive the various hazards mentioned above?
Well, there may not be much they can do about adverse weather conditions apart from hiding in a retreat or laying eggs that can remain in the dormant state until the weather improves. But there are many things that individual species have done to more efficiently catch their prey and to avoid being eaten by predators. Those species that do not make a large insect-trapping web often have ways of 'hiding in full view', as is described in more detail below, but when threatened by predators the majority of spiders simply try to escape by dashing into their retreat or any other convenient crevice, by running away, or by dropping to the ground and playing dead. The last of these is a risky defence in that it may be difficult to climb back up before being attacked by ants or other ground-level predators but many spiders reduce this risk by releasing a 'dragline' thread as they fall and ascending this as soon as they consider it safe to do so. Those species that do not spin this safety line or are normally on or near the ground anyway will probably use leaf litter as a convenient if temporary hiding place. And of course large orb weaving species such as Eriophora transmarina and Nephila edulis tend not to drop down but to climb further up their webs when threatened, presumably hoping to ascend to a height that puts them beyond the reach of any potential predator.

However, some spiders adopt fight-rather-than-flight behaviour when threatened. This is well demonstrated by the Australian funnel-web spiders and some other mygalomorph species, including the idiopid trapdoor Euoplos and even a few araneomorph species. These all rear up in an aggressive stance when approached, presumably hoping to discourage any further attacks by their aggressors. For the most part this behaviour is based more on bluff than on superior fighting prowess because most spiders do not have very good vision and rearing up causes most or all of the eyes to face away from the source of the threat. Species such as the araneid Araneus praesignis use a different kind of bluff. A. praesignis has a pair of large false eyes on the rear of its abdomen so by turning away from predators it can deceive them into believing the spider is a more dangerous creature that is watching every move they make. Of course, nearly all spiders do have active defences in the form of fangs and venom glands and they will bite if forced into battle. While it is true that the majority of spider species have venoms that can only cause minor localized harm to humans they are very effective against insects and other spiders. The nature and mode of action of spider venom components are considered on the venoms page of this website.

One defence used by many spiders to avoid detection by either prey or predator is to have an appearance that is an excellent match for that of the bark, leaves or flowers on which they are resting. By this means they effectively 'disappear' if they remain motionless. Excellent examples of this are the bark spiders Tamopsis, Pandercetes gracilis, and Stephanopis. Alternatively, They can be clearly visible yet be ignored because they are misidentified as objects of no interest. For example, Celaenia excavata looks like a bird-dropping, one Carepalxis species has the shape of a gumnut, and Poltys laciniosus and Miagrammopes appear to be dead twigs. An alternative approach is used by ant mimics such as Myrmarachne which can invade ant streams or colonies undetected because they have an appearance and perhaps scent that is very similar to that of the ants they are preparing to attack.



Why do many spiders have a distinctive color scheme?
While the majority of spiders are a dull yellow, brown or grey colour that probably helps them remain unnoticed by predators, many representatives of spider families such as the Araneidae, Tetragnathidae, Thomisidae and Theridiidae deliberately display brightly coloured and easily seen surface markings. There are several reasons why it can be advantageous for a spider to have strong and distinctive surface markings, such as

1. To attract a mate, especially in the case of species which display strong sexual dimorphism. The salticid Maratus volans is a good example of this, the males of this species even having brightly coloured abdominal folds that they can spread to attract the attention of their drab-coloured female counterparts.

2. To attract prey that perceive the patterning or bright colours as indicative of the presence of food in the form of fruit or nectar-containing flowers.

3. Conversely, to match the colour of leaves or flowers so insects such as honey bees will approach without realizing that the flower they are landing on contains a predatory spider. Perhaps the champions of this technique are members of the Family Thomisidae like Tharrhalea and Thomisus which are often referred to as flower spiders for this reason.

4. To warn potential predators that the spider should be considered too dangerous to approach. It is widely believed that this is at least one reason why the adult redback spider Latrodectus hasselti has a bright red stripe along the top of its abdomen.

5. To provide camouflage by making the spider look like something both predators and prey will perceive being of no interest. An example of this is probably the theridiid spider, Argyrodes antipodianus, which can scavenge in safety on the edge of a large araneid web because its silvery abdomen looks like a harmless dewdrop.

6. To protect internal organs from excessive exposure to UV light which easily passes through the almost transparent cuticle many spiders possess. No one seems to be sure why a few species (notably Austracantha minax) occasionally manifest a uniformly dark brown (melanic) body and legs with none of the bright colours that usually characterize this species but it possibly serves to make the spider harder to see. Melanic manifestations in spiders are suggested to be genetic variants of the usual colour scheme but are incorrectly described as melanic because melanin colours human skin and hair but is not a pigment found in spiders.

The chemical nature of the coloured pigments used by spiders is rather surprising. A spider's colour scheme is clearly determined yet individuals of some species vary considerably in colour. This is referred to as colour polymorphism and is well illustrated by the araneid Plebs eburnus. It is for this reason a spider's colour scheme is often an unreliable character to use for identification purposes. A spider's legs and cephalothorax are enclosed in a tough 'shell' called an exoskeleton and this is covered by a very thin, nearly transparent cuticle. These front parts of an individual spider are rarely seen to change much in colour except when the spider is passing through its moult stages or when surface hairs or scales have been rubbed off. On the other hand, the rear half of the spider, the abdomen, is also covered by a thin cuticle and is soft with some capacity to expand or shrink. On some species parts of the spider's body, and especially the mid-dorsal abdomen (which is where the spider's hearts are), have a green-blue tinge because of the haemocyanin in the spider's blood which can be seen through the cuticle.

Quite a few spider species are at least partly green or blue in colour but in most cases little or none of this colour is due to haemocyanin. It also is not caused by the presence of chlorophyll because spiders do not eat plants and if they feed on plant-eating insects any chlorophyll the insects have ingested has been degraded to a brownish colour by the time a spider acquires it. Similarly, the yellow carotenoids and the red, blue or purple xanthophyll pigments of flowers are not used by spiders to colour their body surfaces. And finally, red colours seen on spiders are not due to vertebrate haemoglobin, very few spiders feeding on vertebrate tissues.



So what kinds of integumentary pigments do spiders possess?
Although the integumentary pigments of only a very small number of spiders have been studied so far the ones that have been examined appear to belong to just three categories, though of course mixtures of two or more of these can produce a wide range of colours. The bilins are blue/green pigments, the best known example being micromatabilin which gives the huntsman Micrommata virescens its green colour. Though apparently not derived from vertebrate bile pigments the bilins of spiders are chemically related to them. However, we still are uncertain how spiders actually make or otherwise acquire them. Then there are the ommochrome pigments, which are synthesized from the amino acid tryptophan and may be red, gold, yellow, purple or brown-black. It is claimed that ommochromes are also used as screening pigments within spiders' eyes and may even provide some protection against integumentary damage by intense UV exposure since at least some of them fluoresce when irradiated with UV light.

The third pigment is the purine, guanine, which is matt white or silvery in colour depending on the way in which it has crystallized. It is not a 'conventional' pigment since it simply reflects white light. At least in a spider's abdomen guanine is stored in specialised cells called guanocytes. Guanine is a component of nucleic acids and therefore is produced as a waste product of the spider's food. Uric acid, the well known component of vertebrate urine, is also white, virtually insoluble in water and formed from dietary purines. Insects excrete uric acid in their faecal matter and the same is also true for the members of some arachnid Orders. However, there is some evidence that the majority of spiders either do not make uric acid at all or at least excrete most of their unwanted purines as guanine. Conversely, one research group found that the white colour of the prosoma (the front half of the spider) for at least one thomisid spider species appeared to be due to uric acid rather than guanine.

Guanine deposits in a spider's integument are not there by chance and neither are they just the random deposition of unwanted guanine under the cuticle because each spider species that has white markings has them in essentially the same shapes and positions on its body. The same is true for the bilins and ommochromes. In fact, the sharpness of the borders of coloured patches on the integument of many spider species indicates that most if not all integumentary pigments are located in pigment cells rather than dispersed through the tissue spaces. Of course, all spiders have guanine and/or uric acid to excrete so those species that are dark brown or black with no white surface patches must presumably be eliminating their unwanted purine breakdown products entirely in their faeces.

Can individual spiders change colour?
It is well established that some spiders can change colour substantially over a period of time and if this requires many hours or days it is probably hormonally controlled or is part of the maturation process. In general, coloured areas of the cuticle of a spider do not change either in shade or intensity except when the spider is moulting through its instar stages, juvenile spiders having less brilliant surface colours than adults. Some mygalomorphs moult even after reaching sexual maturity and these also are relatively pale for a day or two after moulting. But there are spiders that can reversibly change colour over quite short time intervals. For example, white crab spiders have been seen to become more yellowish if placed in a yellow environment. Similarly, some Leucauge species and a number of other spiders have silvery marks on their abdomen that quickly diminish in area and reflectivity when the spiders are threatened by a potential predator, but if a few minutes or hours later the threat appears to have disappeared the silvery patches return to their original size. Although this kind of rapid and reversible colour change has mostly been observed in those species that have guanocyte patches it probably is not restricted to them and may sometimes be manifested by other kinds of pigment cells.

Relatively rapid and reversible colour changes have been reported to occur in at least some members of the Araneidae, Tetragnathidae, Theridiidae and Linyphiidae. While some of these changes might be hormonal, neural mechanisms are likely to be more involved if a spider's appearance is seen to change in just a minute or two. But how does this happen? Do the pigment cells shrink and expand or do they perhaps convert their pigments into less brightly coloured forms? Well, at least for guanocytes recent research indicates that neither of these suggested mechanisms is correct. Instead, delicate muscle fibres attached to the guanocytes pull them into tight masses which therefore decrease their externally visible area. Hence, the spider rapidly changes to a dull colour when alarmed then regains its brighter, whiter appearance when no longer feeling threatened.

But integumentary pigments are not the only way spiders can manifest colours that will attract or deceive other creatures that wander into their vicinity. There are two other mechanisms they can use: fluorescence and iridescence. The first of these appears to be quite common among spiders, presumably because it can facilitate predator/prey interactions and/or sexual attraction. The principle of fluorescence is that when a fluorophore is exposed to light it reflects some of the light at one or more wavelengths that are longer than the incident light. This means that exposure of a spider to blue-violet or preferably UV light causes parts of the spider to reflect visible colours such as blue, green or even yellow-red. The chemical and physical nature of the fluorophores in spider integuments have not been comprehensively studied as yet. It is possible that in some instances they are present in special cells but there is also good evidence that they arrive in the blood and are simply sequestered under the spider's cuticle or in scales and setae (fine hair-like structures). Members of at least eight families have now been shown to exhibit fluorescence. In some instances this probably attracts insect prey but the fluorophores used by spiders are claimed to be family-specific and if this is the case then fluorescent reflections are also likely to be playing a role in attracting a mate.

Iridescence also involves reflection of incident light by spider surfaces but it does not require the presence of special fluorochrome substances. Instead, it is multi-coloured and depends on essentially the same diffraction process that creates rainbows on a rainy day. Only a few parts of a spider are likely to display iridescence. Most often it is manifested by hairs such as those in the claw tufts of barychelid spiders and by cuticular scales. It is a purely structural phenomenon involving microscopic ridges and grooves or multi-layered chitin-air-chitin arrangements.



Some related sources of information
The pages on what spiders eat, and venoms contain information relevant to what is covered in the above paragraphs.

Much of the information provided on this page can also be found in more detail in the following reference articles, all of which can be found on the internet:

Goodacre S.L. and Martin O.Y. "Endosymbiont Infections of Spiders", pages 93-106, and Evans H.C. "Fungal Pathogens of Spiders", pages 107-123 of "Spider Ecophysiology" (2013) Editor W. Nentwig, Springer-Verlag Berlin.

The article on Gordian worms by the Australian Museum

Wunderlin J. and Kropf C. (2013) "Rapid colour change in spiders" Chapter 26 of "Spider Ecophysiology" Springer-Verlag Berlin (Editor: W. Nentwig).

Herberstein M.E. and Wignall A. (2011) "Deceptive signals in spiders" pages 190-214 of "Spider Behaviour, Flexibility and Versatility" Cambridge University Press ISBN: 978-0-521-76529-9.

Thery M. and Casas J. (2009) "The multiple disguises or spiders: web colour and decorations, body colour and movement" Philosophical Transactions of the Royal Society London B 364, 471-480.

Insausti T.C. and Casas J. (2008) "The functional morphology of colour changing in a spider: development of ommochrome pigment granules" J. exp. Biol., 211, 780-789.

Andrews K., Reed S.M. and Masta S.E. (2007) "Spiders fluoresce variably across many taxa" Biol. Lett., 3, 265-267.

Lim M.L.M. and Li D. (2013) "UV-green iridescence predicts male quality during jumping spider contests", PLoS ONE, 8, e59774.