Spiders are living animals and they therefore need to eat just as you and I need to eat. Because spiders are predatory animals eating means first catching some other living animal. Most spiders are not fussy, though some have definite preferences and some have specific hunting techniques that catch them a particular type of prey.
Most spiders eat only living or freshly killed food, and most are not particular about their prey. Having said this many spiders will take dead prey in captivity and in some species it is not uncommon for spiders to be scavengers when the opportunity arises. For example the Mouse Spider (Scotophaeus) is known to steal dead insects in the wild. Also certain social spiders are known to scavenge the dead bodies of other colony members.
Spiders can taste their food and some items are rejected because of taste. Unlike humans however, spiders taste their food with their tarsi using chemosensitive hairs. Thus if you keep spiders you will notice that some spiders will not eat certain bugs and ticks. Different species of spider have different ideas about what is good to eat and what isn’t. For instance many spiders won’t eat woodlice although the house spiders in the genera Tegenaria will.
However there are some spiders, and groups of spiders, that do have particular prey items they specialise in. Among these are spiders in the genera Dysdera specialise in eating woodlice, preferring them to other foods. Other spiders with specialised tastes include the Pirate Spiders in the family Mimetidae which live exclusively on other spiders.
Pirate spiders protect themselves by having a potent quick acting toxin that immobilises their prey after just one quick bite to a leg extremity. They will also trick spiders out of their retreats by mimicking mates of prey caught in the web.
There are also ant Spiders in the family Zodariidae that specialise in eating ants. Ants are potentially dangerous prey and Zodariids such as those in the genus Zodarium that eat ants attack their prey quickly making a single bite and then moving away until the ant is overcome.
Ants are very common animals in most environments and it is not surprising therefore that there are ant specialists in other spider families, Callilepsis nocturna from the Gnaphosidae (on Formica spp.) and species of Salticidae on Pseudomyrmex spp.
For spiders that actively hunt their prey the first step in catching dinner is to locate it. Spiders that choose to sit outside their burrows or hideaways and wait for some suitable organism to wander past like some tarantulas and wandering spiders, rely on vibrations to tell them what is going on.
Such spiders as Cupennius can hunt just as effectively with their eyes covered as with them working. However spiders that go out actively looking for prey and hunt it down, such the wolf spiders and particularly the jumping spiders rely much more heavily on sight.
Nearly all spiders use venom to immobilise their prey before feeding. This makes it easy for them to feed on otherwise dangerous animals. Some Crab Spiders will catch Bumble Bees far heavier than themselves.
Whichever way the prey is caught it needs to be eaten and spiders practice what is called external digestion. This means that enzymes and other digestive juices are injected or spat into the prey’s body. The soft tissues are broken down by these juices and sucked up by the spider. For spiders, soup is the only thing on the menu.
Some spiders such as tarantulas and many of the orb-web spiders use the teeth on the basal segment of the chelicerae to mash their prey while they are feeding. In these cases all that remains after the spider has finished is a small dark blob of cuticle. Smaller spiders, especially those that feed on larger prey such as the Thomisidae bite only a small hole in the cuticle of their prey and suck the juices out through this. In this case what is left is a pretty intact shell of the prey animal.
While most spiders feed on invertebrates most of the time, they will take vertebrates when they can. Reports of Dolmedes catching small fish several times her own weight, of Leucorhestris taking small lizards up to its own weight and of Lycosids and Pisaurids catching tadpoles and small fish are fairly well documented. Evidence of large spiders taking small birds is also known in the tropics.
Tales of tarantulas taking snakes in the wild are harder to verify though the first description of them doing so was written by the Roman Pliny 2000 years ago. However there is no doubt they will take them in captivity and therefore probably would take them in the wild when the opportunity arrives. In captivity tarantulas have been recorded killing and eating 30cm pit vipers and 45cm rattlesnakes as well as frogs and lizards.
Stranger still, in 1924 Reginald Pocock described finding a Poecilotheria regalis feeding on a rat in India, though no mention is made of whether the spider actually killed the rat. Strangest of all is a tale from Australia written in 1919 by a Mr Chisolm.
He describes finding a chicken that had been killed and dragged 16 metres (50 feet) to a burrow by a Barking Spider Selenocosima spp.. The chicken was much too big to be pulled into the hole and was found with one leg down the hole with the spider hanging on to that leg.