SIX BILLION DESCENDANTS.

Otago Witness, Issue 3581, 31 October 1922, Page 12

SIX BILLION DESCENDANTS.

A GREEN FLY FAMILY. Most, of no. writes Canon Theodore Wood, in the Morning Post, have rnaivellod al the extraordinary in ulti plica Tory powers of the aphis, or “green fly. 17 Where there are half .1 dozen individuals of this insect to-day there will be as many thousands in a few days’ time, and 10 or 20 times as many again a few days later; and every farmer knows only too well that certain crop#? can lie utterly destroyed by them in the course of two or three weeks. The reason for this is that, aphides are nob subject to the ordinary laws of insect reproduction. There is some little variation, no doubt, as regards particular species, but.

as a rule, there are many generations in the course of the year - , and only the members of the last of these lay eggs. All through the spring and summer these insects increase by a kind of “budding” process, the young being born alive, and skipping the egg-stage altogether. More rcnia.rkabie still, the little ones can scarcely be described as larvae. Neither do they turn, at a definite stage in their development, into chrysalides. There is simply continuous and rapid growth from birth to maturity ; and as soon as this is reached the “budding” process begins again. The consequence is that one generation follows another with astonishing rapidity; and this is still further increased by the fact that, until tlie final generation of the season, males are altogether unknown. Female aphides can propagate their own sex, even when living in virginity. They do inis until early in the autumn. Males then appear, and eggs are laid, which hatch m the following spring, when the same round of parthenogeiic succession begins again. —-Weighing Down the Human Race. — 111 captivity no fewer than 20 successive generations hate been bred in this way without the appearance of a single male. Under natural conditions there are probably from 10 to 15. And these follow one another with such rapidity that, according to Reaumur's calculation, a single aphis, if ° allowed to increase w ith any of the ordinary checks up.ciii its multiplication, might find itself the progenitor of nearly 5000 million descendants before it died ! Professor Huxley put the matter in a, still mere striking way. Assuming that 1000 aphides collectively weigh one gram avoirdupois, and noting that only a very stout man indeed would weigh as much as 2,000,000 grains, or between 20st and 21st, he tells us that the tenth generation of an aphis which had been permitted to multiply unchecked would be equal sn point of; ponderable matter to more than 500 millions of stout men, each of them sufficiently corpulent to turn the scale at a lit fere over 2SOllx You might put the human race, as it exists to-day, on one side of the balance, and the aphides on the other, and it, would be the side of the aphides which would go down ! No such increase, of course, ever takes place in a slate of Nature. Cold weather checks their development; ram washes them in thousands, from the plants on which they are feeding. And they have a perfect host of enemies in tile form of predacious insects