The black widow builds its retreat in dark, sheltered spots, but builds its web for trapping prey between vegetation, and in most other places, even under stones. Unfortunately for humans, outdoor toilets are a favourite place, especially around the seat!
Usually it is found outside or in unheated buildings. It will only be found in heated buildings during cold spells, or prolonged rain. The adult female (above) is about 8 - 10 mm long and on the underside of the abdomen she usually has the characteristic red hourglass shape. The male is just 3 - 4 mm long, and his abdomen is elongated. The male does not bite humans.
Mating, reproduction and behaviour of the black widow spider
One mating can supply the female with enough sperm to last her a lifetime, and males are often, though not always, eaten after mating - it mainly depends on whether the female is well-fed or not. A male can tell if a female is well fed by touching her web with his feet, as he picks up scents through chemoreceptors in his feet. Given the choice of a hungry female and a well fed one, the male will always choose to mate with the well fed one as it is less likely that she will eat him afterwards. However if there is only a hungry female to mate with, the male is usually so desperate to mate that he will take his chances of escaping afterwards. Courtship can last hours, and consists mainly of the male wandering around the female's web tapping and plucking strands of silk and waving his legs in the air.
The drawing on the right shows a mating pair with the the male in black. The female lays around 400 eggs which hatch in 20 - 30 days. But cannibalism is rife among the spiderlings, and usually less than a dozen survive to meet the outside world.
In a good year a healthy female can produce around nine broods. Females are sexually mature at around 2 - 4 months. The average life span for a female is six months, and for a male, around three months.
The spider is timid and nocturnal. During the daylight it spends its time in the silken tunnel retreat. Generally when a female is on her web she will hang upside down.
Black widow venom and mode of action
The venom is neurotoxic, so it blocks the transmission of nerve impulses which leads to rigidity and cramp. It is said it is 15 times stronger than rattlesnake venom, but because of the small quantity injected it is not usually fatal to healthy humans on the rare occasions they are bitten, providing the bite is treated promptly and correctly.
Only about 0.5% of bites from the black widow to humans prove fatal. Strangely cats are very susceptible to the venom, but dogs are fairly resistant, as are sheep and rabbits