The venom in spiders helps them in several ways. It immobilize their prey, begins the process of digestion and is a defense against enemies. Venom is a complex mixture of substances, but the toxins are usually only a single substance. Venoms act in different ways and affect different parts of the bite victim. The main types of venom are

• Neurotoxins: affect the nervous system.
• Myotoxins: affect the muscles.
• Haemorrhagins: affect the blood vessels and cause bleeding.
• Haemotoxins: affect the blood.
• Nephrotoxins: affect the kidneys
• Cardiotoxins: affect the heart.
• Necrotoxins: affect tissue and cause necrosis.

Venom glands are located above the fangs or chelicerae. Venom ducts cross the chelicerae and open near the tips. The spider family Uloboridae are the only group in Australia that do not have venom glands. Venom glands originated as digestive glands, which aided in the external digestion of prey. 

Although funnel-web spiders are widely distributed throughout the southeastern Australia, including Tasmania, the only species so far proven to be dangerous to humans are largely limited to the eastern part of New South Wales and southeast Queensland. The only known killer is the Sydney funnel-web spider, which is found mostly in the Sydney region, north to Newcastle and south to the Illawarra region.

The antivenom for the Sydney funnel-web spider has also proved to be effective for various other species of funnel-web. The protein toxin, delta-atraxotoxin, present in the acidic venom is also thought to be the prevalent compound which causes the severe effects in humans. Other mammals seem to be unaffected by the funnel-web spider venom. The toxin produces a rapid effect on the nervous system.

Though there have been 13 recorded deaths from funnel-web spider bites, some cases do not always develop severe symptoms. However, the same precautions first aid should be administered because, if untreated, a major bite may cause death within an hour. First aid treatment involves the application of a pressure-immobilization bandage, the same treatment as applied to a snake bite. The entire affected limb is bandaged firmly and, wherever possible, is further restricted in movement by the application of a splint.

The large fangs and acidic venom make the bite very painful.  Bite symptoms start early, beginning with tingling around the mouth, twitching of the tongue, profuse salivating, watery eyes, sweating and muscle spasms. Hypertension and an elevated heartbeat occur which, when combined with respiratory distress may be very severe and potentially lethal. 

The antivenom from the red-back spider can also be used to treat bites from the black widow spider, an infamous killer from the Americas. The venom from the female red-back spider is known as a multi-component because it is made up of a family of protein toxins, the latrotoxins being the most prominent. One of these, alpha-latrotoxin, is effective in mammals, including humans, causing over stimulation of neural pathways throughout the body with a wide range of effects. Some of the effects include a stinging sensation when first bitten which can become excruciatingly painful, draining of the lymph nodes in the groin, pain throughout the abdomen, chest, neck and head, profuse sweating, mild to severe hypertension and nausea. The initial pain of the bite usually means the bite is detected immediately, however, the red-back bite is one of the few spider bites with which antivenom can be effective a few days after the bite occurs. First aid treatment for red-back bites is different to that for funnel-webs. Do not apply a constrictive bandage. In fact the only action taken should be to administer ice packs to the bite site to help reduce the pain and then seek medical attention immediately.

Only the female red-back spider is dangerous. While the female is large and distinctive with her shiny black body and bright red abdominal stripe (though not all specimens posses this marking), the male red-back is small and insignificant and has a complex pattern including white and, occasionally, yellow markings. As the red-back is not a wanderer, most bites occur when the spider's web has been pulled down or disturbed. Less than 20% of all red-back spider bites actually result in significant envenoming.

Red-back spiders usually make their webs under objects, with droplines to the ground or another flat surface. They are found most commonly under shelves, bottom rails of fence lines, under outdoor furniture, even in cupboards indoors. If you have red-backs in your area check thoroughly before putting your hands underneath items such as flower pot rims, bricks, tables, etc.