Protection, aggression and grip
Red-kneed Tarantula, Brachypelma smithi, showing irritant hairs on abdomen. Photo: R Bolzan © Australian Museum.
Barbed hairs
American tarantulas use the barbed irritant hairs (urticating hairs) on the abdomen to protect themselves against predators like lizards and mammals. When threatened, these spiders brush their back legs rapidly across the back of the abdomen. This sends clouds of loosely attached barbed hairs into the eyes, nose or mouth of the predator chasing them. The spider escapes while its pursuer is distracted by the highly irritant hairs.
A mass of barbed hairs.
Photo: S Lindsay © Australian Museum.
One type of barbed hair.
Photo: S Lindsay © Australian Museum.
A Whistling Spider (Selenocosmia crassipes) stridulating in a defensive pose. Photo: © R Morrison.
Spider sounds
Some spiders make noises to attract mates or as warning signals. Australian tarantulas are called Whistling Spiders because they 'whistle' when threatened. To make the whistling or whirring sound, they rapidly rub together modified hairs on their mouthparts. This is called stridulation. Both male and female whisting spiders have these organs. The sound may help scare off predators like small carnivorous marsupials when the spiders are caught hunting out of their burrows.
A triangular Spider (Arcys) seizes flies with its strong and spiny front legs. Photo: © D Gray.
Spiny legs
Most spiders have strong, thick modified hairs called spines on the limbs. Spines are often more numerous and larger on the front legs. They assist in capturing and holding prey or in helping males hold females during mating.
The underside of the foot of this Trapdoor Spider (family Barychelidae) is covered with scopula hairs.
Photo: S Lindsay © Australian Museum.
Getting a grip
Spiders like tarantulas, huntsman and jumping spiders have densely packed hairs called scopulae on their feet (tarsal and metatarsal leg segments). Scopulae give the legs lots of clinging power and allow the spiders to walk easily on smooth surfaces like tree trunks, leaves, ceilings and window glass. They also help these spiders to hold on to struggling prey.
There are two types of scopulae - brushes and claw tufts. Both may have hundreds of scopulate hairs. Each scopulate hair may have hundreds of little branches or 'end feet'. As a result, there can be more than a million of these tiny 'end feet' in contact with a surface, providing enormous frictional clinging power. This effect is aided by the capillary forces of an extremely thin layer of water on most surfaces.
Some of the hundreds of scopulate hairs.
Photo: S Lindsay © Australian Museum.
The tiny 'end feet' covering the surface of each scopulate hair.
Photo: S Lindsay © Australian Museum.
