There are too many kinds of endangered invertebrate to breed them all in zoos: there could be more than 30 million species in all, of which half are probably in trouble. But it is reasonable, at least, to select a shortlist: those that can be bred in captivity, as London is breeding red-kneed tarantulas; those that could be returned to the wild, as London intends to return the wart-biter (a giant cricket) to its habitat in southern England; and those that test the bounds of biodiversity, such as the giant robber crab, a hermit crab that dines out on coconuts and, at 4kg, is the largest of all land invertebrates.
London Zoo's Invertebrate Conservation Centre is a brick shack - a poor relation among the distinguished array of listed buildings. But it plays a large, and in some cases leading, part in five national and international breeding programmes. There are too few such endeavours, but, as London Zoo's curator of invertebrates, Paul Pearce-Kelly, comments: 'If the National Gallery was on fire, it would be better to snatch one Rembrandt from the flames than none at all.'
There are advantages in breeding invertebrates in captivity - as revealed in the latest International Zoo Yearbook, the zoo-person's bible, edited by Peter Olney for the Zoological Society of London. Captive populations are unlikely to thrive unless they contain at least several hundred individuals: fewer than that and they are liable to become inbred. Zoos cannot maintain populations of several hundred rhinos or tigers except by continent-wide co-operation; but, robber crabs aside, most invertebrates are small, and viable groups can be kept in tiny spaces. Tama Zoo's 'Insect Ecological Land' near Tokyo, with its shell-like roofs - a spectacular variation on the theme of Sydney Opera House - covers 2,480 square metres. The valuable, heated, off-exhibit breeding rooms at London total just 16 square metres.
But there are problems with keeping invertebrates, too. Many are extremely pernickety. Minutiae are important - temperature, light, humidity and diet, and a great deal more besides. Upside-down jellyfish, Cassiopeia, derive most of their nourishment from symbiotic algae living in their tissues: a device favoured by corals and giant clams, but rare among jellyfish. The algae need light to photosynthesise. Daylight at London Zoo is supplemented by 250W metal halide lights. Without these, the Cassiopeia would starve.
Every invertebrate centre these days seems to have a colony of leaf-cutter ants, which hack out fragments of leaf (privet, lilac, rose, hibiscus and banana at London Zoo) and carry them home aloft, like sails - a regatta of ants. The leaf fragments become compost on which the fungus grows on which the ants feed. Their fungus factories fill the space available, which zoos provide in the form of glass vessels, vying to provide the most bizarre sculptural glass forms. Temperature is critical. Above 28C, the queen, who alone lays the eggs, becomes sterile.
Dung beetles like a high cage, at least a metre tall, to allow them to fly. They will eat many varieties of dung, provided it comes from a herbivore; dogs and cats need not apply. But dung beetles also provide a food store for their young in the form of a 'brood ball' - one ball per egg - and the beetles at London will build brood balls only out of elephant dung. Hartebeeste, moose or caribou dung will not do. The beetles also need a substrate at least 30cm deep in which to dig a chamber to bury the ball. The parents share the work with the male digging the chamber.
The technicalities compound. Medicinal leeches at London are housed in aquariums with tight-fitting, ventilated lids because they are adept escape artists. They are fed on fresh human blood, but only at two-monthly intervals. If the adults are given too much at a time they remain torpid (and boring) for many weeks afterwards; if the young ones receive too much, they suffer 'osmotic shock'. Heliconius butterflies are given fresh nectar from buddleia and lantana, plus fructose solution, and pollen from hibiscus. The whole intricate husbandry is underpinned by recondite technologies: pollen tablets to supplement the Heliconius diet; 'Liquifry Marine' to supplement the sugars supplied by the Cassiopeia's resident algae; a beer-chiller to cool the fevered brows of the medicinal leeches.
Most famous, and perhaps important, of all London Zoo's invertebrate endeavours, however, is the programme for Polynesian tree-snails, Partulas. More than 100 species live on the high volcanic islands of the Pacific. All must now be considered endangered in the wild because of the introduction to their islands of a carnivorous snail from Florida, Euglandina. All seven species of Partula from the island of Moorea, near Tahiti in the Society Islands, are now extinct in the wild.
But six of the seven Moorean Partulas survive in captivity in 18 co-operating centres worldwide. Another 15 or so related types have now been brought into captivity. There are plans, once the Euglandina are dealt with, to return the Partulas to the wild. Paul Pearce-Kelly co-ordinates the entire world breeding programme.
Yet if London Zoo closes, all this work could come to an end. It is not easy to see where the invertebrate creatures themselves would be housed. Whipsnade, for example, so often bruited as the 'proper' home for London's inhabitants, is primarily suited to big animals that like open spaces. Forest creatures prefer intricacy, security and a delicately balanced microclimate. They must be kept out of the cross-wind, and gain nothing from the view over Bedfordshire. Even less feasible is to re-create instantly the nerve centres of international breeding programmes, which are completely dependent on particular teams of people.
Why bother, in view of the effort that goes into destroying invertebrates worldwide? There are economic reasons: some endangered invertebrates, such as the medicinal leech and butterflies that sell to tourists, have cash value. There are aesthetic reasons: butterflies, beetles and corals are among the world's natural beauties. Biology in general benefits from studies of invertebrates: the Partulas illustrate the evolutionary principles that Charles Darwin first revealed in Galapagos finches more clearly than the finches did.
Other breeding programmes benefit, too; the breeding protocol for Partulas devised by Georgina Mace of London's Institute of Zoology and the computer programme to co-ordinate the breeding devised by Ajay Burlingham-Johnson of London Zoo could in principle be applied just as well to antelopes or hunting dogs.
But the real answer, in the end, is that it is right to conserve these animals. Robber crabs and medicinal leeches are biodiversity. Conservation cannot work, long term, until we come to regard all other species as our fellow creatures.
'1990 International Zoo Yearbook, volume 30' (edited P J S Olney and Pat Ellis, published by the Zoological Society of London, pounds 49).
The writer is a founder member of the London Zoo Survival Group. His latest book, 'Last Animals at the Zoo', was shortlisted for the 1992 Rhone-Poulenc Science Book of the Year Award.