The very hungry caterpillar is quite fussy about its food
ANOTHER LIFE:AT REST IT LOOKS, for those who remember such things, rather like a small, dark cheroot, of the kind once – perhaps still – clamped between the shiny teeth of Mexican bandits. Close up, however, its skin has the sheen of a soft velour, patterned with moth colours like a 1920s sofa. And when alarmed, in a quite absurd simile of its own, it pulls in its snout and puffs up its eye spots to look as much like a menacing snake as a caterpillar can.
This autumn seems to be good for spotting the larva of Deilephila elpenor, the elephant hawkmoth, judging by the number of reader reports, some full of the awe of a first encounter. This is, indeed, one of Ireland’s largest native caterpillars, growing up to 7cm, and much more often seen than the hawkmoth that begets it. That’s a pity, as the moth is a beautiful arrowhead of bright pink and gold wings, seeking nectar in flowers by night and able to tell their colour even by starlight.
Not that the bright, long-tubed honeysuckle that fits the hawkmoth’s tongue especially well would take much finding. But the really important plants of its life are chosen to feed its caterpillars, not itself, and neither rosebay willowherb nor fuchsia exactly shines in the dark. Soon after a ghostly midnight copulation, which can last up to two hours, the female starts laying her glossy green eggs, continuing for several nights until she has deposited about 100, one by one.
The bond that most butterflies and moths have with particular plants to feed their larval stage is part of the relationship between plants and insects that powered much of the diversity of the natural world. Buy a field guide to caterpillars (Collins does the standard one, by Carter and Hargreaves) and you find colour plates bright with the caterpillars that feed on particular groups of trees and plants.
The cabbage whites seem obvious, but the food-plant index runs long, from alder to zea (maize), each with its own lepidopteral consumers.
Some species make a wide choice among quite separate groups of plants, but the bonds can be so exclusive that the very survival of a butterfly species depends on the existence of the plant and conservation of its habitat. In Ireland, for example, only the devil’s bit scabious of wet grassland feeds the black caterpillars of the endangered marsh fritillary butterfly.
Such close affinities have encouraged the idea of co-evolution between these insects and their food plants, with some even proposing that butterflies and moths reached the peak of their development at the same time as the flowering plants. But study still goes on as to why, exactly, such partnerships were formed and still endure. Taken to America, Europe’s cabbage whites insist on laying eggs on sulphurous European brassicas instead of switching to native species.
What is it about rosebay willowherb, for example, that so appeals to the elephant hawkmoth? This plant can sweep a lovely pink haze across a hillside of clear-felled forestry. I have admired it in the summer of the Greenland wilderness. It is the postinferno “fireweed” of Canada, and, having flourished on the bomb sites of London, it is now that city’s county flower.