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 may not be true for every spider species that exists that the present time. However, of the spiders that have been studied so far the majority seem to have similar dietary and digestive patterns with just a few variations to suit the particular habitat each species has chosen. It is important to understand that almost all spider species are not carrion feeders and only eat living prey or perhaps very recently killed ones. Their usual food is insects and other spiders but some of the larger species have been seen to eat other small animals such as millipedes, wood lice (slaters), and even small lizards, frogs, and birds.
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.
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 haslearned how to make and use a pheromone attractant matching that of a particular species of moth, which
therefore becomes a major part of its diet.
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.
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 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. 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 locate 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 the 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.
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.
Email Ron Atkinson for more information. Last updated 21 April 2015.