A Certain Small Insect

Dominion, Volume 28, Issue 169, 13 April 1935, Page 20

A Certain Small Insect

(By J. Drummond, F.L.S., N.Z.S., for “The Dominion.”)

CINTEMPTIBLE in size and mischievous in disposition, the common iruit-fly is disliked. Yet it has helped to reveal laws that control variation and heredity, which profoundly influence life. its grubs love rotten-ripe and fermenting fruit and almost any decaying vege table matter. They do not reject growing fruit. Eating glutton ■jusly and thriving prosperously, they need only three weeks to develop into perfect fruit-flies. One generation follows another quickly. With a few glass rubes and a few pieces: of banana, many fruit-flies can be reared in a laboratory quickly and easily. This has led to the discovery that they produce many modifications or variations, occurring in the colour and shape of the eyes, the colour of the body, the size, shape, and veins of the wings, and the shape of the ■cgs Four hundred of these modifications from the normal type have been mled amongst millions of pedigreed fruit-flies reared, watched, and recorded it captivity They have been studied for another and widely different reason. They •ire a good subject for investigating coloured bodies in minute cells of proto plasm, which is the essential essence of all living things, the physical basis of life. As the coloured bodies carry factors that influence heredity, they are highly important in studying hereditary characters. Al) this has made intensive observation'of the fruit-fly a great work. Amongst modifications developed by fruit-flies are white eyes and pink ■yes. instead of red eyes, yellow wings instead of grey wings and vestiges of wings instead of long wings. The modifications may have no value to the in dividunis that produce them. That does not make the modifications less in wresting to biologists. The main point is that they are attributed to changes >n the co cured bodies, and that, in transmission from generation to generation Hiey obev strictly the law of heredity discovered sixty-eight years ago by the \bbot Mendel amongst garden peas in the monastery garden at Brunn. Austria. He crossed tall and Short plants, ami peas with seeds of different shapes and colotii He found that inheritance of those characters followed a simple rule. His results were published in the proceedings of a natural history society in Brunn. It was obscure, and his work was not generally recognised

until thirty-four years later. Fruit-flies have disclosed 400 Mendelian characters, but they have only four pairs of coloured bodies in a germ-cell. The explanation is that each coloured body carries many hereditary factors, and that the modifications are inherited in groups. A theory has been advanced that modifications in fruit-flies are controlled and directed by a chemical stimulus. It is not known why they arise. Associated with these investigations is the problem of the inheritance of, disuse. The kiwi, the weka. the kakapo. and the moa, flightless New Zea laud birds, are examples of a theory that, after long continuance through gen erations the disuse of a limb becomes fixed. The disuse is inherited ; the limb ceases to function; it may even disappear Kiwis have small wings, mere ap pendanges Their embryos supply evidence that their remote ancestors bad wings they could fly with In the absence of enemies, and in the presence of abundant food, they disused their wings, which degenerated and now are use less Wekas and the kakapo are not in such a bad plight. but the moas com pletely lost their wings. Most species of moas were not only flightless, but wingless. Captain F. W. Hutton accounted for this by inheritance of the habit of disuse. To inquiries as to this, the fruit-fly replied negatively. Long-winged fruit flies were kept in narrow vials, in which they could not use their wings Forty three generations were reared in the vials The wings ->f tin- first loi and of their descendants were measured. There was no difference in Ihe length ami strength of the wings. Disuse had not been inherited. Sixty-nine generations of fruit-flics were reared in darkness. The eyes of the sixty-ninth generation had not decreased in size They were as efficient as the eyes of tbe first generation which were accustomed to the light. Experiments on mice gave ll.e same negative results Scores of cases, on the oilier Im nd. are advanced to support rhe theory that disuse is inherited. Many biologists discredit it Xew Zea.ami’s flightless birds seem to favour it. Fuses in lhis class are cliff erent from modifications product'd by the coloured bodies in germ-cells They are the direct effect of environment, an external influence, on tin- body, or on organs.