Next time you see a spider and reach for a can of insecticide, think again. Not only could they be killing all the mosquitoes, cockroaches, flies and other unwanted pests in your home and garden, but they are also highly intelligent.
South Africa has about 2 000 identified spiders but almost all are harmless.
Last week arachnid authority, Norman Larsen, from Devil’s Peak gave a talk at Kirstenbosch about the mythological spider bite. Mr Larsen has published several articles and field guides on spiders, scorpions and solifugaes in southern Africa and is the author and photographer of Iziko Museum’s Bioweb page on spiders.
Solifugaes are an order of animals in the class arachnida and commonly known as haarskeerders or baardskeerders because they collect hair from animals to line their nests.
In opening his talk, Mr Larsen said if you take away all the invertebrates (an animal lacking a backbone) you will die, but if you take away the vertebrates (large group distinguished by the possession of a backbone or spinal column, including mammals, birds, reptiles, amphibians, and fishes) from the world, life will continue.
“If you look closely in your home, you’ll have the Kruger National Park in miniature in your garden,” he said.
What do spiders eat?
“Actually they don’t eat but can only drink and have to liquidise their meal, such as Neoscona hirta often seen making their webs at night in gutters. Their prey gets wrapped into their web and only then does the spider bite into it and suck out the liquefied juices,” said Mr Larsen.
Rain spiders prey on moths and cockroaches, but they are able to catch geckos.
Spitting spiders produce a silk and venom compound which is spat in zigzag or longitudinal strands to capture prey. “Should you find them in your home, leave them as they will eat the fish moths that can destroy your book collection,” Mr Larsen said.
Another spider guest to leave alone are daddy long legs as they will eat the mosquitoes in your home.
Hikers will be familiar with spiders’ wardrobe.
“A bag of leaves woven into a silken ball, made by the common and Cape rain spider, Palystes superciliossus and P. castaneus. The spiderlings hatch inside the protective sac and chew their way out about three weeks later. Females will construct about three of these egg sacs over their two-year lives.”
Spider bites and what to do
After the talk Mr Larsen introduced a baboon spider, Theraphosidae, endemic to Africa, of which the tarantulas are a subfamily. A bite from this hairy beast causes intense, local burning pain which can last from 10 to 40 hours returning to normal via pins and needles before disappear. If a dog is bitten by a baboon spider, it will probably die.
Mr Larsen keeps a few live spiders for lecture purposes, or to help arachnophobes referred to him by clinical physiologists get over their fear.
He said spiders have fangs large enough to pierce human skin, however, they regulate how much venom they inject. In fact, it is often a dry bite with no venom injected.
The diagnosis of spider bite, especially when the patient is unaware of having been bitten, can be difficult.
The medically important spiders of southern Africa can be divided into a neurotoxic group which includes button spiders, and cytotoxic groups, represented by violin spiders. Mr Larsen said sac spiders do not have the enzyme to cause necrosis and should now be considered as being of no medical importance.
Treatment for a bite is a tetanus injection, mainly to counteract dirt from the spider’s fang or from bacteria introduced after scratching the bite site.
Some interesting facts
“Some spiders have the ability to catch fish, frogs and tadpoles in fresh water. Others will go into the intertidal zone while many will find hunting from and taking refuge in sand.
“During the course of their lives they can lose some of their eight legs, but not to worry as these do regenerate in the following moult.
“Jumping spiders have eyes like telescopes, the eye so long that it takes up the length of the spider’s head, providing it with the ability to see its prey in full detail. The layered retina in these anterior median eyes forms a focus that allows the spider to recognise other species and male from female – a useful trait if it is looking for a mate. The other eyes help the spiders to locate prey.
“The jumping spiders’ prey technique is to learn the rhythm of the other spider so it can prey on it – these spiders show an intelligence by going out of sight and working on a secondary technique of prey capture.
“Spiders likely to be seen in forests are bark spiders, Caerostris sexcuspidata, which make webs attached at three points and are so well camouflaged on bark their day time retreat, that they are totally invisible.
“Spiders come in many guises, from the ability to camouflage themselves against bark, or as in the case of Thomisus the flower crab spider which chameleon-like takes up to two days to change its colour.
“Scorpions give birth to live young that ride on the parents’ back for about two months until the young start eating each other – a sign for them to move on.
“Common banded spiders, Argiope australis, formerly called by the ridiculous name black-and-yellow banded garden spider, can be seen among low base vegetation. Look carefully, you can see the difference between the sexes, the male is tiny and has boxing gloves.
“Nephila fenestrata, black-legged nephila (also known by the common name black-legged golden orb-web spider) emigrated across the Hottentot Holland mountains during hot berg winds a few years ago and is now the most commonly seen diurnal orb-web spider on the Cape Peninsula.”
The scorpion-tailed spider, Arachniura scorpionoides, a spider that re-sembles a leaf and has a scorpion type tail, is known from only this species in southern Africa but Mr Larsen has recorded two further spices.
As someone who had attention deficit hyperactivity disorder (ADHD) at school, Mr Larsen is a humorous and knowledgeable speaker who prefers to use scientific names rather than the clumsy common ones. “Ask any child for the common name of a dinosaur. They know the scientific name,” he said at the talk.