What Spiders Eat

This page outlines the kinds of things most spiders eat and how they ingest and digest their food and excrete any waste materials.

What do spiders normally eat?
This varies with each individual spider species. Indeed, most of the statements made on this page are generalisations that probably are not true for every spider species that exists that the present time. However, most of the spiders that have been studied so far seem to have similar dietary and digestive patterns with just a few variations to suit the particular habitat each species has chosen. The first thing that should be stressed is that spiders will only eat living prey (or perhaps very recently killed ones) and are not carrion eaters. The majority of spiders feed on insects but the larger species will also eat other small animals such as millipedes and wood lice (slaters), small lizards and frogs, and even small birds in the case of tarantulas and the largest of the golden orb weavers.

It should also be noted that most spiders prefer a solitary life and readily cannibalize each other if forced into close proximity. Victims of this tendency even include the male of the same species unless he is very cautious, has leg spurs to keep the female at bay while mating, or is so much smaller than his female counterpart as to go almost unnoticed by her.

While some adult female spiders display a small amount of maternal behaviour towards their newly hatched spiderlings it is very common for many individuals that hatch out of a single batch of spider eggs to be eaten either by the adult female or even by the stronger siblings in that hatching. And of course, spiders must always be cautious around spiders of a different species, especially those with long legs or robust fangs since these are likely to win any battle that ensues.

Some spiders could be described as fussy eaters in that they have a very restricted range of prey they are willing to eat. The so-called white tailed spider, Lampona, has been suggested to be an obligate araneophage (spider eater), which means it rarely, if ever, feeds on any creatures apart from other spiders. Inside houses it certainly seems to be very happy to play a role in keeping the numbers of other spiders down and it also is proficient at ambushing bark spiders whenever the opportunity arises. It has even been claimed that Lampona sometimes succeeds in capturing 'impossible' prey such as Pholcus, the daddy-long-legs spider.

Salticids belonging to the genus Myrmarachne are excellent examples of ant mimic spiders. They also seem to feed only on ants and have developed an external appearance so much like that of an ant that they can join a stream of foraging ants without alarming them. Hadrotarsine theridiids such as Dipoena also are believed to prefer ants to any other kind of insect even though they do not physically resemble an ant. The magnificent spider, Ordgarius magnificus, may be less restricted in the kind of insects it will eat than these ant-eaters but since it emits from its body a pheromone attractant matching that of moths of a particular species it is this kind of insect that forms the major part of the magnificant spider's diet.

What anatomical structures does a spider use for ingesting and digesting food?
Many people express fear of suffering spider bite but the reality is that spiders do not have jaws that act in the same fashion as those of a savage dog. Instead of conventional mandibles (jawbones) they have a pair of chelicerae on the ends of which are fangs that may be able to penetrate human skin. These are helpful for grasping and immobilizing prey but are not used for tearing insects apart or ingesting them. On the other hand, the palps of many species do possess teeth and these as well as the legs and even the spider's other mouthparts may partly crush insects, thereby making their tissues easier to access and digest.

The mouth opening is surrounded by the chelicerae in front and underneath the spider, a pair of maxillae on the sides, and a central labium. For most spiders there are fine hairs projecting inwards over the mouth entrance that strain solid particles out of any food the spider tries to ingest, only liquified materials actually entering the digestive system.

From the mouth the digestive tube passes backwards within the cephalothorax to a muscular expansion usually called the sucking stomach. This has a cross-section that can concertina and it has muscles attached to the roof and sides of the cephalothorax to increase its volume as well as encircling muscle bands that can compress it. Thus, it is able to drive fluid both forward and backwards by compression and suction. This arrangement allows the spider to pump digestive secretions into the captured prey and then to suck liquified food back into itself.

Present evidence indicates that most spiders lack conventional salivary glands, these probably having evolved as venom glands. It is possible that some species have other simple enzyme-secreting glands that secrete near the oral opening but these seem to be relatively unimportant. In mygalomorph species the salivary glands are confined to the chelicerae but in araneomorphs they typically extend into the front part of the cephalothorax. They may still secrete some digestive enzymes but the major source of these are almost certainly the midgut which is the part of the digestive system posterior to the sucking stomach.

Immediately behind the sucking stomach the digestive tube becomes the midgut and expands into a number of blind pouches called caeca. These sometimes take up a substantial amount of space in the cephalothorax and in some species even extend down into the coxae (the first segment of each leg). Similar but even more elaborate caeca are present in the abdomen, where they may occupy most of the space unless the spider is a gravid female. The cells that form the walls of these caeca are secretory and in many respects the overall abdominal caecal mass is functionally and sometimes even visibly similar to the mammalian liver. It is believed to secrete digestive enzymes that the sucking stomach then expels onto or into the spider's prey and also completes the digestion of liquified food, releasing nutrients and water into the tissue spaces of both major parts of the spider's body. It may even parallel the mammalian liver in adding waste materials to the hindgut for excretion.

What enzymes are important for digestion in spiders? Spiders seem to have relatively little use for carbohydrates despite the fact that there is a significant amount of glucose in their haemolymph. Presumably this serves as a rapidly available energy source when this is needed. Unlike white ants, spiders are not known to have symbiotic microorganisms in their digestive systems to allow them to break down complex carboydrates such as cellulose and rarely, if ever, feed on plant materials. Of course, those species that forage in flowers may sometimes ingest nectar-containing water and are then likely to metabolize any sugars acquired in this manner. The normal prey of most spiders do not have large amounts of stored body fat, which suggests that lipids are also relatively unimportant components of a spider's diet. On the other hand, all cell membranes in animals contain lipids so spiders must either acquire lipids in their diet or make their own. It is also worth noting that experiments with at least one spider species indicated that lipids are critical for maturation and ovarian development in the females. However, spiders have a great need for proteins for such purposes as the spinning of silk and therefore have proteolytic enzymes as their main aids for digestion. A variety of proteases, notably including collagenase, and probably some peptidases are used by spiders but some of these enzymes may only be used intracellularly in the gut caecal cells.

How important is a supply of water for a spider?
Few spiders exhibit compulsory ingestion of plain water, though most of them may be perfectly happy to absorb water droplets that happen to be handy. Instead, spiders make use of the water that is in their prey as well as water formed as a normal byproduct of metabolism. Despite this conservative behaviour, most spiders are at some risk of desiccation, this being particularly true for the more primitive mygalomorph species. Thus, female mygalomorphs spend virtually their entire lives in a burrow where the humidity remains reasonably high and adult males venture above ground only at night and especially during and after periods of rain. Male funnel-web spiders are often found in swimming pools and laundries or near leaking garden taps and this shows they have an ability to respond to changes in atmospheric humidity and to locate habitats where the humidity is relatively high. Araneomorph spiders are generally more tolerant of desiccating conditions but the majority of them still prefer to stay out of the midday sun and to forage for insects among green foliage or during the evenings if in exposed habitats.

How does a spider dispose of any waste products derived from its food?
Unlike many insects, spiders do not produce copious amounts of faecal material because the indigestible parts of their prey do not enter the spider's digestive system. Instead, they are discarded nearby. Burrow-dwelling mygalomorphs typically have the remains of insect exoskeletons scattered around their entrances and many web-building araneomorphs deposit strings of insect debris along strands of silk. However, all spiders do have a small amount of faecal material to dispose of from time to time. The posterior end of the digestive tube has an anal opening which is normally located just above (or behind) the spinnerets. Just before this opening is a blind sac called the cloaca or stercoral pocket and it is here that the spider's small amount of insoluble wastes are stored until excretion is convenient. Spider 'faeces' is usually whitish in colour because it also contains nitrogenous wastes, especially quanine, adenine, hypoxanthine and uric acid, all of which are white. at least for some spider species there may also be some sequestration of waste materials on the inside surfaces of the exoskeleton, perhaps to be 'excreted' when the spider moults.

Spiders lack a liver-bile system, kidneys and urinary bladder like those of mammals so they cannot excrete unwanted materials in bile or a liquid urine. However, the abdomen does possess some delicate tubular structures called Malpighian tubules which drain directly into the stercoral pocket and which are believed to serve many of the same functions as the nephrons of mammalian kidneys.

How often must spiders eat?
The dietary habits of spiders are largely determined by the prevailing environmental temperature and also by the behavioural patterns of their prey. Unlike humans they do not require three meals a day every day. Instead, they are opportunistic eaters and will feed on as many insects as they can catch in one short period of time. This means there will be weeks when the insect population in their part of the world is low so the spiders have no opportunities to feed for a while. Because they are poikilothermic (cold-blooded) and inactive for much of each day this temporary loss of a food supply is not a problem. However, prolonged periods of enforced starvation will ultimately lead to death. Indeed, this is probably one of the reasons why exposed mature adult females of many spider species, especially orb weavers, drop from their webs and die as winter approaches.

Can spiders store fuel the way a hibernating mammal does? To a small extent they probably can but their energy needs are very small in cold conditions so many spiders live through several cold months in a torpor-like state with no food intake and no need for substantial fuel reserves. Some species may paralyse prey and wrap them in silk until they can be eaten conveniently but long-term storage by this means does not seem to occur. Fortunately, a spider's abdomen is not enclosed in a hard exoskeleton so it has some capacity to expand or shrink. This allows an adult female to develop large numbers of eggs in its ovaries but an expandable abdomen also permits storage of water and dissolved nutrients at least for relatively short periods of time.

Some related sources of information
The pages on spider blood, spider venoms and spider growth and reproduction contain some information that is related to what is covered in the above paragraphs.

In addition, the website article by Robert Gale Breene III on spider digestion and storage is also worth reading.