Emma McCaughan, of Balclutha Primary School, asks :-

How are spiders poisonous?

Simon Pollard, a spider biologist at Canterbury Museum who is a RadioNZ science correspondent and the author of a number of books for children, responded.

Animals that secrete harmful chemicals that can be absorbed through the skin or gut are said to be poisonous, while animals that inject harmful chemicals through fangs or a stinger are said to be venomous. Poison-dart frogs are an example of a poisonous animal, while spiders are venomous.

Spiders produce venom inside glands that are in their heads near their fangs. The glands have muscles around them and when these muscles squeeze the venom glands, venom travels along two tubes that are connected to the sharp fangs that have an opening at their tip. It is very similar to the way a syringe works when you are given an injection. As the fluid in the syringe is squeezed, it comes out of the tip of the syringe and into your body. However, spiders produce venom to immobilize their prey so they can feed on them. All spiders are fluid feeders and by mixing the prey’s fluids and tissues with digestive enzymes, they turn the insides of the prey into a soup that they suck into their own digestive system. Of course, you need the prey to be still to feed in this way and spider venoms affect the prey’s nervous system and paralyse or kill the prey.

While most spider venoms, including those of the tarantulas, are only dangerous to their prey, some spiders produce venoms that can kill people. The Sydney funnel web and the black widow are spiders with venom that has killed people, but because anti-venoms have been developed, nobody has died from the bite of these spiders for a long time.

In New Zealand the katipo is the only spider with venom that could make you ill, but there is also an anti-venom for the very rare times when somebody is bitten.