Parasite's web of death

By BBC News Online's Jonathan Amos

The extraordinary behaviour of a parasitic wasp that forces its spider host to weave a special web on which it can hang a cocoon has been described by an American researcher.

On the evening of the day that it will kill the spider, the larva induces the spider to spin this unusual web

Dr William Eberhard

The larva of the wasp Hymenopimecis sp. will suck on the "blood" of the spider and eventually eat it - but not before it has injected a behaviour-bending chemical that makes the spider construct a special scaffold.

Only this design, which is quite different from the spider's normal fly-trap, has the strength to support the pupating wasp.

Dr William Eberhard, of the Smithsonian Tropical Research Institute, US, and the University of Costa Rica, told BBC News Online that the manipulation of the spider was probably the most complex alteration of behaviour ever attributed to an insect parasitoid.

Small abdominal holes

The spider, Plesiometa argyra, is doomed from the moment it is stung in the mouth by the adult female wasp. This paralyses the spider and allows the wasp to lay an egg on the arachnid's abdomen.

The altered design provides a more durable support for the cocoon

When the spider recovers it goes about its daily business of web weaving and feeding, unaware that it has become a meal for the developing larva now hatched and clinging to its body.

The larva will make small holes in the spider's abdomen through which it can suck the creature's haemolymph, a task made easier by the apparent introduction of an anti-coagulant that prevents the circulatory fluid from clotting too quickly.

When this blood does eventually clot, it makes a large scab that acts as a "saddle" for the larva to hang on to the spider and reach for its next meal.

Midnight feast

"Finally, on the evening of the day that it will kill the spider, the larva induces the spider to spin this unusual web," Dr Eberhard said. "This is basically the ideal web from the wasp-larva point of view because it needs to hang up its cocoon on a very solid and durable support.

"The design is unique - the spider will not build anything similar during its normal life.

"When the wasp somehow senses that the construction is finished, it will kill and start to eat the spider. This happens more or less at midnight and lasts until about midday.

"It will then drop the spider's empty body to the ground and sit in this special web until the next evening when it begins to build a cocoon."

Biochemical manipulation

Dr Eberhard believes there are at least three types of biochemical manipulation taking place. And because the larva lives on the outside of the spider, it is clear these chemicals must be injected.

The effects of an anti-coagulant and a death-inducing poison are relatively obvious. Just how the web-spinning behaviour is altered is not so easy to explain.

"The larva somehow biochemically manipulates the spider's nervous system causing it to perform one small piece of a subroutine, which is normally only a part of orb construction, while repressing all the other routines."

Dr Eberhard removed larvae from spiders at different stages on what should have been the arachnids' final evening of life to see how this would affect web construction. These spiders started to build the special cocoon platforms and continued to do so for several days before reverting to normal behaviour.

Dr Eberhard says this shows whatever biochemical is introduced into the spiders' bodies is fast-acting and long-lasting.

The research is detailed in the journal Nature.