Redback Habitat and Life Cycle

An unfussy habitat

Redbacks seem completely indiscriminate in their choice of a site for their web and build almost anywhere that can provide a reasonable amount of food. They seem to prefer locations in which the outer portion of the web has some exposure to sunlight and the inner portion is tucked away in a cool, dark recess.

Occasionally, webs may be found on exposed walls with only a narrow ledge as shelter. Along the outside of the house, Redbacks may be found under window sills, under the guttering and eaves, in the corners of windows and doors, among pot plants, under and on steps and verandahs, and, of course, under the seats of outdoor toilets.

In Brisbane, Redbacks have established themselves under the roofs of houses. From that point, they enter the living areas of houses through inset lights that are not sealed from the roof cavity. Inside houses, the possible locations for webs are even more numerous, but because Redbacks tend to stay near the sun, they are rarely found inside. Away from houses, their numbers increase as more hiding places are provided: piles of wood, iron, cans and old machinery are ideal hiding places.

Such is the tenacity of the Redback that they build in grass tussocks and the little shelters provided by sods of freshly ploughed fields. They are even found on and within regularly used motor vehicles; locations reported include inside motorcycle handlebars, on seat belts and behind bumper bars.

Redbacks are often absent in gardens that are well-kept, moist and shady, even in the summer. They are most common around houses that have few shrubs or trees to provide shade.

A hardy life cycle

Redback Spiders are difficult to control or eradicate from an area. They are hardy animals that breed prolifically, live in concealed places and disperse easily.

The female is able to store the sperm of a male and use it over a period of up to two years to lay several batches of eggs. Hence, the elimination or absence of males is no guarantee that the female cannot produce more young.

The following information is based upon studies of three species of American Black Widow Spiders. It is reasonable to expect that the information applies equally to their Australian relatives.

A female Black Widow Spider may lay eggs every 25-30 days. In one group of about 9-12 egg sacs (up to 29 have been recorded) a single female may lay up to 5000 eggs. The eggs hatch 13-15 days after being laid (often near the onset of rain).

Females mature in 60-325 days; on average in about four months. The smaller male matures in 37-167 days; on average in about 90 days. Females live for between two and three years; males for up to six or seven months.

Without food, spiders survived for about 100 days on average. The first to die lived for only 36 days, while the last lived for seven months.

The next generation

Redback Spider eggs are wrapped in tough layers of tightly bound silk that is very resistant to most sprays. These egg sacs are small, spherical and fawn-coloured and are hung near the back of the web.

Redbacks normally lay between 40 and 300 eggs in each sac, and usually make three to five sacs at one time. One spider may lay eggs several times in its life. Only one to three weeks need elapse before more eggs can be laid.

The adult female Redback remains with the eggs until they have hatched, which usually occurs at or near the onset of rain, when the air is cool and moist. The young emerge after about 14 days, when, if the weather is good, they disperse.

If conditions are not right and food is scarce, the female and her young become cannibals. Sometimes, the sole survivor is a male that has grown to maturity on its siblings. This male may then mate with the female so that eggs can be laid later when conditions have improved.

Leaving the web

Young Redback Spiders, like many other young spiders, are carried on the wind from the maternal web. Each spider holds its abdomen high in the air and produces a droplet of silk on the spinnerets. The wind draws the liquid silk out into a long thread that eventually has enough lift to carry the spider away. The silken thread eventually sticks to an object and this is where the young spider will establish its web.

What's on the menu?

Although insects are the usual prey of Redback Spiders, they are capable of killing and digesting larger animals, such as lizards and frogs.

When prey becomes entangled in the web, the spider climbs down and bites it, then retreats momentarily to wait for the venom to take effect. The spider then approaches the victim, swathes it in silk, and lifts it up into its retreat.

One Redback brought to the Museum had spun its web in some sheets hanging on a clothes line. It had captured a small lizard and lifted it over a metre into its web. In the United States, a Black Widow Spider was photographed with a small snake that it had captured, killed, bound head to tail and lifted into its web.