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.

But one kind of food that spiders normally do not eat is plant material. The reason for this is that they have to liquify their food before they can ingest it and are no more able to digest the cellulose of plants than we are. Not so long ago there was a flurry of interest in the popular media over the discovery that the Central American salticid, Bagheera kiplingi, seems to be the world's only known plant-eating spider. It had been observed that this species lives on a species of Acacia tree and feeds on nectar and especially on tiny specialized leaf tip structures called Beltian bodies. These contain useful amounts of sugars, proteins and some fats and hence can be broken down by the spider's digestive secretions to a liquid form the spider can ingest. Whether or not this salticid can digest the cellulose components of the Beltian bodies is presently unclear but if they can do so it may be with the help of an ant species that also lives on this Acacia species and seems to interact with B. kiplingi in a mutually beneficial manner.

The majority of 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 those of a different species, especially ones 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, is said to be an obligate araneophage (spider eater), which means it rarely, if ever, feeds on creatures other than 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 in the bush whenever the opportunity arises. Lampona sometimes even succeeds in capturing 'impossible' prey such as Pholcus, the daddy-long-legs spider.

Similarly, salticids belonging to the genus Myrmarachne are excellent mimics of ants and prefer to feed exclusively on them if possible. To facilitate this they have developed an ant-like external appearance that allows them to join a stream of foraging ants without alarming them. But some spiders, including Hadrotarsine theridiids such as Phycosoma, seem to prefer ants as prey even though they do not have an ant-like disguise. The magnificent spider, Ordgarius magnificus, uses a very different kind of disguise: it has learned how to suspend a droplet of fluid containing a pheromone attractant matching that of a particular species of moth, which therefore becomes a major part of its diet.



What anatomical structures does a spider use for ingesting and digesting food?
Many people worry about being bitten by spiders but the reality is that spiders do not have jaws like those of a savage dog and cannot actually bite anyone or any creature. Instead of conventional mandibles (jawbones) they use a pair of chelicerae on the ends of which are fangs that can penetrate human skin if they are long enough. These are helpful for grasping and immobilizing prey and are usually assisted by a scissor action of the chelicerae. On some spider species there are also strong teeth on the chelicerae that help the spider tear open its prey to provide access to its tissues and sometimes the palps and even the front legs and the spider's actual mouthparts also play a holding and crushing role.

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 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 the more primitive mygalomorph spiders 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 abdominal 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, much of the available abdominal space then being taken up by a mass of eggs. 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 though 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 lack symbiotic microorganisms in their digestive systems to allow them to break down complex carboydrates such as cellulose. 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. Indeed, experiments with at least one spider species indicated that lipids are critical for maturation and ovarian development in the females. But spiders have a great need for proteins for silk spinning and other purposes and use proteolytic enzymes extensively for digestion. A variety of proteases, notably including collagenase, and probably some peptidases are used by spiders but some of these enzymes may only occur intracellularly in the gut caeca.

In 2017 A. Walter et al published a BMC Genomics paper (https:ref="//www.ncbi.nim.nih.gov>articles>PMC5553785) which seems to be the most recent comprehensive review of the digestive processes of spiders and hence is worthy of special mention here although their research only involves a few spider species and so may not be totally correct for all kinds of spiders. These authors concluded that extra-oral digestion is a prolonged process in spiders and involves the initial secretion of digestive enzymes combined with or immediately following the release of venom (which also seemed to contain digestive enzymes) then repeated sucking in and regurgitating of the liquified enzyme/prey tissue mixture. This was perceived as a way of allowing relatively small quantities of digestive enzymes to process larger amounts of prey tissue but it also has immunological benefits in that the secretions contain lysozymes and peptidoglycan recognition proteins and thus serve to destroy potentially harmful toxins as well as pathogenic bacteria, viruses and fungi.

How important is a supply of water for a spider?
Although spiders don't have the same need to drink water regularly that we do they have often been observed to ingest water droplets they happen to find. More important water sources are the contents of their prey as well as water formed as a normal byproduct of metabolism. But despite this conservative behaviour, most spiders are at some risk of desiccation, this being particularly true for 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. Similarly, male funnel-web spiders are often found in swimming pools and laundries or near leaking garden taps, which shows they have an ability to respond to changes in atmospheric humidity and to seek out habitats where the humidity is 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, a spider does not produce copious amounts of faecal material because the indigestible parts of its prey do not enter its 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 amounts of insoluble wastes are stored until excretion is convenient. Spider 'faeces' is usually whitish in colour because it also contains nitrogenous wastes, especially guanine, adenine, hypoxanthine and possibly 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 the liver-bile system, kidneys and urinary bladder that mammals have 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 numbers and kinds of potential prey that happen to be in their vicinity. 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. There will be weeks when the insect population in their part of the world is so low they have no opportunities to feed at all. However, because they are poikilothermic (cold-blooded) and inactive for much of each day a temporary loss of a food supply is not a problem. On the other hand, prolonged periods of enforced starvation will ultimately lead to death. This is probably one of the reasons why mature adult females of many spider species, especially orb weavers, drop from their webs and die as winter approaches. But most mygalomorph species live much longer than just 12 months and avoid starvation during the winter months by simply retreating into their burrows and 'hibernating'.

Can spiders store fuel the way a hibernating mammal does? To a small extent they probably can but those that live through several cold months in a what is effectiely the hibernated state seem to do so with no food intake and limited internal fuel reserves. Of course, some spiders paralyse their prey and wrap them in silk until they can be eaten conveniently but long-term food storage by this means does not seem to be common practice among spiders.

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.