It is 10 years since Grace Hall last saw a katipo spider at Port Waikato.

It's about the same time since she saw one at Muriwai Beach, where there used to be "a good population".

Ms Hall, the spider specialist at Landcare Research in Auckland, says the nearest place she knows where she can still find a katipo is at Foxton Beach, four-fifths of the way down the North Island. And even there they are now "quite hard to find".

In fact, others have found them closer to home. Auckland Museum insect specialist John Early says they are "thick as anything" on Great Barrier Island, and a recent Conservation Department survey has found small numbers at Matakana Island, Papamoa, Kaituna and Maketu on the Bay of Plenty coast.

But the experts are in little doubt that there are almost certainly fewer katipo than there are of that other great national icon, the kiwi.

Two blows have hit our poisonous spider. First, humans have built on, ridden beach buggies over, planted and tamed much of the sand in which katipo live. Second, foreign spiders have intruded into what sand remains.

Steatoda capensis, a South African immigrant, was reported in the 1990s to have displaced the katipo along the west coast of the North Island from Wellington to Wanganui, including Foxton.

But Brian Patrick of Otago Museum, who looked for katipo at 127 places for the Conservation Department in 2002, often found the African species coexisting with the katipo in the same dunes and even under the same piece of driftwood. He concluded that Steatoda might not actively displace the katipo, but might recolonise places faster after katipo are driven from them by storms or other events.

Both species are members of the same worldwide family of cobweb-making spiders called Theridiidae.

Like others of this kind, the katipo lives for only a year, just time to produce the next year's generation.

They are tiny. The female's abdomen is about the size of a pea, black with the katipo's distinctive red stripe on its back. The male is only a sixth as big, with the size and whiter colouring of juvenile females. (They are too small to bite humans.)

The females lay their eggs in November or December in an "eggsac", enclosed by a layer or two of silk.

When the young spiders, called "spiderlings", emerge, they have to shed their hard outer body walls four or five times for males, and six to eight times for females, to make room for their bodies to grow.

Once they reach maturity, their instincts teach them to build webs to catch their insect and beetle meals. All spiders use poison to kill captured prey, but the female katipo is one of the few that can bite creatures as big and tough as us.

By August or September, it is time to mate. The little males go looking for the females' webs, and try to convince their giant partners that they are there to make love, not to be eaten.

It's a perilous affair. Ray and Lyn Forster, in their magnificent Spiders of New Zealand and their Worldwide Kin (1999), say the female is often aggressive at first and chases her little mate away.

"After the male performs a courtship consisting of bobbing, plucking and tweaking actions on the web, interspersed with periods of cautious advances and being chased by the female, mutual agreement is reached."

Once the deed is done, using a kind of antenna on the male's head called a "palp", the male has completed his role in life and soon dies. In some species, but not the katipo, he is actually eaten by the female.

The female is then ready to lay her eggs and start the cycle over again.