Tuesday, 6 March 2007

#11 A venomous backyard Australian

Although Redback Spiders are common residents of our urban backyards, they are not aggressive, and if disturbed by something larger than meal-sized are more likely to retreat to a safe haven than go looking for trouble.

A Redback guarding her egg sac

But Redbacks should not be messed with. Thirteen deaths have been recorded in Australia from Redback envenomation, all prior to 1956 when an antivenom was introduced. However, up to several hundred people are treated each year with antivenom for Redback Spider bites, mostly during the warmer months. The young and frail are most at risk if bitten.

A common backyard resident

The Redback Spider, Latrodectus hasselti, from family Theridiidae, is found in all Australian states and territories in a variety of environments. It is more at home in urban and rural habitats than forest or bushland. Redbacks have taken up residence in my shed, on my verandah, and amongst rocks, dense shrubs and broadleaf vegetable vines, and piles of left-over building materials.

Large Wolf Spiders feature high on my backyard Redbacks' menu

Every backyard, bar the most fastidiously maintained, is likely to support a Redback or two. My backyard is over-run with them this summer and I have had to cull many from my grandchildren's play spots.

Breeding and behaviour

Redback silk is extremely strong and shiny and I have learned to recognise it amongst potted plants and outdoor furniture.

As with most spiders, the male is much smaller than the female. In the picture below, the entire Redback family is present. The female is generally unmistakable with it's glossy black abdomen with splash of red on top and underneath. Occasionally the red on top of the abdomen can be replaced with a diminished or indistinguishable orange on a body which can appear more brown than black, which can lead to mis-identification.

The male is the tiny brown and white spider immediately right of the female in the image below. This photographed male, however, is unlikely to be the father of the hatchlings in the far right of the image.

Redback family: unmistakable black and red female with tiny brown and white male, together with spiderlings. The egg sac is top left.

A male Redback Spider leads a short and hazardous life. He does not build a web, but lurks on the female's web and performs an elaborate courtship routine in order to establish if the female is receptive. I have observed this strumming of the web and constant advances and retreats by the male. He dashes to the perimeter of the web if mother does not accept his advances. There's no sense becoming a meal before passing on genes.

Mating in Redback Spiders is characterised by sexual cannibalism. The male makes the ultimate sacrifice, offering his body to the female as a meal either during or after copulation. The female will construct spherical egg sacs to contain her eggs.

The male Redback (3 to 4 millimetres in body length) top left with spiderlings

The female often constructs a tightly woven thimble-like haven under cover at the top of the web which serves as a retreat during daylight hours or when disturbed (illustrated in first picture). A haphazard network of trip-lines extend to the ground to ensnare invertebrates and even small lizards.

Struggling prey will alert the female, who then wraps the meal in silk to immobilise it before safely biting it.

The female Redback, Latrodectus hasselti

Common prey I have seen trapped in Redback webs are Wolf Spiders, beetles, slaters and garden snails, along with the occasional centipede. When she has sucked out the juicy contents, she will drop the empty 'shells' under her web. Upon close inspection, I have seen the dismembered fangs of Wolf Spiders, Lycosa godeffroyi, alongside the discarded hollow bodies.

A large beetle becomes a Redback's meal

I'm not suggesting you get up close and personal with Redbacks, or invite them to camp under your favourite patio chair. But rather, I'm just sharing my observations of nature, where everything has a place in the complex web of life.


Esperance Blog said...

Hi Gaye, very interesting, but I don't think being over-run by redbacks is my idea of ideal co-habitation. I found a gecko entangled in one of their webs and no doubt its next meal, so their snare lines are very strong and highly effective.

Impressed with your blog layout, I must pinch the concept for mine, hope you don't mind.


Evan said...

Great post Gaye,

I do like red backs. They are beautiful spiders, and are shy enough to stay away from me. We have one just outside the door (have to make sure not to put shoes there), and she has about 3 egg sacks. Can't say I am looking forward to the hatching, but they will most likely disperse somewhere hidden, and I won't care.

There are cooler spiders out there. I especially like the orb spiders, and the trap doors. Especially when in the bush, and you see a clay wall. Hundreds of holes just dissappear!

Great photos too, the first one is brilliant.


Gaye from the Hunter said...

hello Jack,

I try to maintain a balance around me. There have been far too many Redbacks, so I have dispatched those that lurk in space that we use often, and from around skink habitat. I also watched a skink get strung up in a Redback web but it freed itself after a very short struggle as I was going to its aid.

I don't mind at all if you use the same blog layout as I have - go for it.


Gaye from the Hunter said...

hi Evan,

Thank you. Redbacks are rather secretive in comparison to the orb spiders, but, yes they are still an interesting beastie.

I'm not very familiar with Trapdoors - perhaps that is a good thing. I have seen some of their lairs and they can be amazing.