Today there are few butterflies flying in Jerusalem. No buckthorn, no brimstone butterfly. In the 40 years since the Six Day War in 1967 Jerusalem has quadrupled in size and the number of residents has nearly trebled, but the local butterfly population has dramatically diminished. This loss is particularly alarming as, apart from their beauty, butterflies are indicators of the bio-health and biodiversity of an area. The absence of butterflies in Jerusalem has been emphasized this spring by the official launch of "The Butterfly Project," a campaign to collect 1.5 million handcrafted paper butterflies in memory of the 1.5 Jewish children killed in the Holocaust. Inspired by the poem "The Butterfly" by Pavel Friedmann and its words, "The last, the very last, so richly, brightly, dazzlingly yellow..." the memorial butterfly campaign has caught the sympathy of people worldwide. Already over 10,000 brightly colored images have arrived by post in the offices of the project's organizers, the Israel Religious Action Center. Later, these silhouettes will be displayed in Holocaust museums throughout the world. Surely, though, this campaign should be accompanied by efforts to bring back living butterflies to Jerusalem and other urban centres? Although Israel has 150 species of butterflies, their numbers are decreasing. BY GROWING the right plants people can make a significant difference to both butterflies and biodiversity. Female butterflies lay eggs on a limited number of plant species, often only one, so if a foodplant disappears in an area, so will the butterfly. Conversely, by reviving the plants, one revives the butterflies which rely on them. Completely different plants are needed for the infant and adult stages of the butterfly's life cycle: nectar-rich flowers for the adults, and a specific breeding plant nearby, known as a "foodplant," on which the female lays her eggs. As there is no universal plant providing food for all butterflies, each is dependent on specific local plants. And each has unique haunts and habitats, reproducing only when conditions are right. Although adult butterflies can sip nectar from a wide variety of flowers, the number of plants on which they lay their eggs is incredibly restricted. While humans can exist as vagabonds without a home, the female butterfly needs a special breeding base in the shape of its ancestral family plant. It is almost as if each insect has its family coat of arms stamped on its chosen site. TWO EXAMPLES of butterflies which could be encouraged to return to Jerusalem are the Brimstone and the Swallowtail. Both once common, they are now so rare. Planting the broad-leaved buckthorn Rhamnus alaternus would bring back the yellow Brimstone butterfly; and planting African rue Ruta chalpensis would bring back the Swallowtail. In Lismore, Australia, schoolchildren promoted the foodplant of the Richmond Birdwing butterfly - and brought it back from near extinction. Unlike ants and bees, which take great care of their young, butterflies do not rear their offspring. However, they take much trouble to find a suitable food source for their future caterpillars. Whereas man can adapt to varying types of diet and shelter, butterflies cannot. Butterflies, like thousands of insect species, rely solely on one or two particular plants either for nourishment or for egg-laying, and if these are absent, the insects perish, along with some of their predators. A butterfly spends most of its life in immature stages - as an egg, larva (caterpillar) or pupa (chrysalis). It is often forgotten that each caterpillar is just a butterfly waiting for wings. When the caterpillars hatch from the eggs they feed off the leaves they were born on. They are fussy eaters and die unless they have plants which provide them with certain substances. In contrast to its parents, which spend their lives flitting from flower to flower drinking and reproducing, caterpillars, chew, munch and devour food. Some are so greedy and eat their leaves so quickly that one can hear their mandibles crunching as they ingest the leaves of their specific host plant. Host plants may be grasses, foliage or buds of shrubs and trees. Some may be beautiful; others are ones that most gardeners want to hide, such as stinging nettles. Perennial native grasses, such as Piptatherum miliaceum, Andropogon distachyos and Hordeum bulbosum, bring several butterflies belonging to the Satyride family (brown-black butterflies with eye-like markings on their wings) that can add an architectural quality to most gardens. REVIVAL ALSO relies on eliminating chemicals, such as pesticides, from an area, reintroducing local native plants, and on growing enough bushy plants to give butterflies shelter. Sometimes it is also necessary to plant corridors of specific plants at random places to create staging posts and routes to encourage butterflies back into new areas. If schools and the municipality cooperated, a network of specific butterfly foodplants could be created throughout gardens, school yards, parks and open spaces. Then maps could be made of where the buckthorn and African rue are planted. These - and surrounding areas - would then be monitored for eggs, caterpillars and adult butterflies by members of the Israeli Lepidopterist Society. Jill, Duchess of Hamilton is a vice-president of Butterfly Conservation in England, and a member of the Israeli Lepidopterist Society.