Some spiders have life spans of less than a year, while others may live for up to twenty years.
However, spiders face many dangers that reduce their chances of reaching a ripe old age.
Spiders as snacks
Spiders and their eggs and young are food for many animals. Animals which feed directly on spiders include birds, reptiles, mammals and many arthropods, including other spiders, centipedes, scorpions and even some insects. Spiders are considered a delicacy by people in parts of Asia and South America.
Spiders are often the target of hunting wasps seeking food for their young. Some wasps lay eggs onto the spider's abdomen. When the wasp larva hatches, it gradually eats the spider from the outside in, often starting near the spider's waist (pedicel), where the cuticle is softer and it is securely out of reach of the spider legs.
Mud dauber wasps sting and paralyse spiders, especially small orb weavers. They construct cell-like mud nests and pack them with one or several spiders before laying an egg on a paralysed victim and sealing up the mud cell. When the wasp's egg hatches there is plenty of fresh spider food for the hungry larva.
Spider eggs provide a highly nutritious source of food for the larvae of many species of wasps, flies and mantispid lacewings. Wasps and flies use their long ovipositers to penetrate into the egg sac and lay their eggs among those of the spider. Mantispids lay their eggs on bark; when the larvae hatch out, they disperse into bark crevices seeking and entering spider egg sacs.
Like us, spiders can suffer from diseases, fungal infections, mite infestations and worms.
Mermethid nematode worms are internal parasites of spiders. They eat the non-vital internal tissues, gradually weakening but not killing the spider. The worm needs to find moist soil or water to complete its life-cycle. When it is ready to leave its dying host, the worm somehow induces the spider to seek water.
Spiders use many strategies to protect themselves from their enemies. One of the most amazing of these is called autotomy. This is the spider's ability to self-amputate a leg that has been grabbed by a bird or other predator. Usually the leg breaks off close to the body, at the coxa-trochanter joint. Even more amazingly, juvenile spiders can regenerate their legs - a tiny, segmented leg grows within the coxal stump and appears at the next moult.
Other strategies include behavioural ploys, like direct threat displays of warning colours on the spider's body, or escaping a predator by dropping quickly away on a silk dragline and playing dead on the ground.
More devious strategies involve camouflage and mimicry. Looking like something you're not, such as a drop of dung or a dangerous or distasteful animal (mimicry), or simply merging into your background (camouflage) has great survival value, especially by day, in avoiding or deterring predators that would otherwise eat the spider. Some of these disguises also fool prey animals into approaching close enough to be ambushed.
The Wrap-around Spider (Dolophones sp.), spins its orb web at night, but by day wraps itself around a twig and 'disappears'. Other nocturnal orb weavers, like Poltys, sit on a branch or on bark by day, and look like broken off twig bases or buds. Although both of these spiders are very conspicuous on their webs at night, they are well hidden from predators like birds and wasps, which are mostly active by day.
Other spiders, like the bark-coloured Two-tailed Spider (Tama) and the leaf-coloured Green Huntsman, simply have colours that blend well into their backgrounds of bark or foliage. Again, this is especially valuable for spiders which are active during the day, like Tama. The Two-tailed Spider gets its name from its long tail-like spinnerets.
The Two-tailed Spiders are also called Rotating Spiders because when prey, such as an ant, comes near, these spiders burst into activity, running rapidly round and round the ant, surrounding it with a barrier carpet of entangling silk bands from the long spinnerets.
Another example of the superb camouflage that has evolved among the spiders is the number of unrelated spiders that imitate bird droppings on vegetation. Two examples are the Dung Spider (Phrynarachne decipiens) and the Bird Dropping Spider (Celaenia kinbergi).
Looking and behaving like an ant is a useful strategy for many spiders, notably jumping spiders (Myrmarachne) and the remarkable thomisid spider, Amyciaea albomaculata. Bird predators avoid this spider because they see it as a Green Tree Ant, which is a fierce biter and stinger. The spider catches and eats stray ants. The ants are easily caught as they seem to accept the spider as one of them - probably because the spider can mimic the ants' chemical scent signals (chemical mimicry). Having made its catch, the spider drops off on a silk thread so that it can eat its meal in safety.
Burrows provide refuge from predators like birds, bandicoots, centipedes and scorpions, as well as buffering climatic extremes for spiders and their young. Some spiders have a trapdoor at the top of their burrow, useful for disguising the burrows presence and ambushing prey. It can also be held shut by the spider or securely silked down when the spider is moulting. Some burrows have extra security within, in the form of additional chambers and doors, escape tunnels and burrow blocking devices like pebbles and loose silk collars. One trapdoor spider (Idiosoma nigrum) even uses its thick, hard abdomen as a plug against burrow invaders.
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Dr Mike Gray