IN TOUCH WITH NATURE

Otago Witness, Issue 3861, 13 March 1928, Page 76

IN TOUCH WITH NATURE

By

THE SILVER-FISH.

J Duummond, F.L.S., F.Z.S.

I shall be pleased if some day you § lve ’VU the histoi- y of the silver-fish,” Mrs b. M Dougall writes from Kohe Kobe. She means a wingless insect not more than half an inch long, with jointed feelers, hard teeth, a bristly tail, and scales all over its body that give it a faintly metallic lustre. The presence of the scales accounts for its popular name, and for one of its official names, Lepisma, from the Greek word lepis, a scale. Its other official name, saccharina, shows its fondness for sweet and starchy stuff. It is in somewhat bad repute as a destroyer of books and papers, which it nibbles. It may be seen scurrying from food in a pantry. Otherwise, it is harmless. On the whole, it is an old-fashioned insect, antiquated in form, resembling the grubs of many other insects, and devoid of the wonderful metamorphosis of the moths and the butterflies, the dragonflies, and the beetles, for instance.

It belongs to an order named the Thysanura, that is, the bristle-tails, and it ranks amongst the most primitive of living insects. It is too delicate to have its ancient history written even dimly in the rocks, but its structure speaks of its antiquity. Many specimens have been preserved in Baltic ■ amber of the Oligocene Period, merely some 35,000,000 years ago. A single silver-fish is in a remarkable collection of more than 1000 species of insects entombed in a Pompeii at Florissant, Colorada, in the Miocene Period, perhaps 18,000,000 years ago. In Miocene times there was a small lake at Florissant, five miles long and a mile wide. Hard by, an explosive volcano ejected ash, which, falling into the lake, or being washed into it, formed layers. These covered many insects., , Lava-flows and mud-flows covered, sealed, and compressed the deposits. The shale formed in that way can be split with a knife, and wellpreserved fossils are disclosed.

In the ages that followed a stream carved a valley through the deposits on the old lake and exposed the beds of shale. The fossils show that in Colorada in those days the silver-fish lived with May-flies, wasps, bees, butterflies, at least 600 species of beetles, spiders, millipedes, snails, and birds. The fossil plajits speak of a climate much moister and milder than the present climate. Great redwoods and pines, figs and magnolias, elms, beeches, walnuts, chestnuts, maples, poplars, and oaks beautified the lake’s shores. Ou the floors of the forests were roses, grasses, ferns, mosses, and liverwort.

Sir Francis Darwin described fhe silverfish as like a minute sardine running on invisible wheels. He confesses that, when he was a boy in England, he found diversion in church by watching silver-fish run about amongst prayer books or amongst the baize cushions. When the silver-fish failed him he pulled threads of indiarubber out of his Sunday boots, which had elastic sides, and made small harpstrings, which he played upon while service was held. In later life the silver-fish did not interest him. He became a distinguished professor of botany, and, as might be expected, an Authority on strange musical instruments.

Lepisma saccharina is an uninvited guest in New Zealand from the Old Country. New Zealand’s only silver-fish, Notolepisma zealandica, unlike its immigrant cousin, is not a household insect; it has been found in only one habitat, under the bark of the native beech trees. In rocky or mossy places, especially cn Stephen Island, Cook Strait, there lives another member of the group. Another ally, compodea, pale yellow, has been found in rotten wood on the mount Arthur Tableland, Nelson. The silvery scales of the silver-fish are microscopic. The refraction is caused by very fine lines on its scales. These are used as a test of the lenses of microscopes; the patterns on the scales are so fine that they can be seen only under high magnification. v

Professor A. J. Thomson, Aberdeen, has explained why these tiny creatures, so unattractive outwardly, claim unusual interest. “ They are quaintly, although not very obviously, beautiful,” he states. “They are individual—themselves and no others. People do not know a great deal about them. They are primitive, old-fashioned types, without any trace of wings, with antique mouth-parts, with limbs on their abdomens that no reputable insect has in adult life, and with hardly a trace of the metamorphosis common in ins>ects. They are survivors of very ancient days. Finally, they illustrate survival in spite of handicaps. The fact is, these Lilliputians are highly successful. They are saved by their minuteness, their dislike of the light, their quickness their crumb diet. They have found niches of opportunity all their own. This is an aspect of evolution one is a little apt to forget—the success of the elusive.”

Insects like the silver-fish- which have no metamorphosis, but have the same form from the cradle to the grave, arc the lowest members of their class. The mayflies and the dragon-flies have an incomplete metamorphosis. The flies, the fleas, the beetles, the ants, the bees, the wasps, the antlious, the moths and butterflies, and some other insects have a complete metamorphosis, one of the wonders of the world. In these cases, the grub or caterpillar that crawls out of the egg is as unlike the perfect insect into which it develops as it can be. It lives a different life. Eating voraciously and usually very active, it stores material in its body. Lt becomes quiescent as a chrysalis or a nymph, and ceases to eat. Outwardly, it is at rest. Inwardly, amazing changes are taking place. The organs are reconstructed ; wings bud; and appendages are formed. The husk, of the chrysalis bursting, there emerges the adult insect,' winged, equipped for life in the air, very perfect, and often very beautiful t'o mortal' eyes. When the , chrysalis', phase Is entered upon, internal changes' ‘begin. Most of the organs in the grub phase ' are broken'of

burst asunder. They are partly absorbed by tiny cells, and their debris is used to build up new organs. Structures in tho perfect insect have their foundations in parts of organs in the grub that had npi been, highly specialised. The perfect in* sect’s wings, legs and outer skin ari§a from ingrowths of the skin of the grub. There is no abruptness. All the changes are as gradual as they are wonderful. This is the internal process in the metamorphosis of the higher insects. Their wings at first are small bags filled with fluid. As they expand, the two sides of tlte bag come closer, and finally fuse every* where except along the courses of the veins) The expansion of the wings always ij sufficiently rapid to be watched . Perfect insects breathe air through a complicated system. of air-tubes which ramify all over the body. The air enters through openings along each side of the body.

In a garden at Ellerslie in December, Mr C. E. Foweraker, of the School of Forestry, Canterbury College, saw a young shining cuckoo being fed by a single grey warbler, which seemed to be quite distressed to keep the noisy -young parasite satisfied. Mrs Bunbury and members of her family, while picnicking in native bush at Longacre Wanganui, heard a twitter, ing overhead. Looking up, they were surprised to see a grey warbler busy at its act of charity. In this case the young cuckoo was fully fledged, and was very, handsome as it flew about in the bright sunshine.

A Dunedin correspondent, who hue had a green mantis under observation in a cage, writes for information as to the ■significance of this insect’s bright green coat. The colours of creatures in relation to their environment is a modern study, but already it has an extensive literature of its own. The marvellous adaptation many creatures undoubtedly practise in this way has been divided into classes. There are deceitful colours, warning and signalling colours, and courtship colours. It is believed that the mantis’s uniform green colour helps to protect it from its enemies and to prevent it from being seen by flies and other small insects on which it preys. In other countries, members of the mantis family seem to go much further in this direction than the green mantis that is plentiful in New Zealand. 'ln South Africa there is a mantis that sits amongst pink and white flowers on the heath and takes on their colours. This is a case of deceitful colouration. Many moths and butterflies resort to colour adaptation, their colours and the patterns of the upper surface or their wings making them almost invisible on trees on which they rest by day. Still more remarkable examples are pro.ided by the leaf insects. Their wings are shaped, and even veined, to resemble green or dry leaves, and their bodies and limbs have leaf-like outgrowths. The famous chameleon and some other creatures change colour at will. They do this bv using a complex contrivance, controlled by the nervous system, whose only function seems to be to bring the creatures into colour harmony with their environments.