SCHOOL IN THE OPEN.

Star (Christchurch), Issue 17829, 24 April 1926, Page 23

SCHOOL IN THE OPEN.

STUDIES IN THE GREAT OUT-OF-DOORS.

(By

J. J. S. Comes,

8.A., B.Sc.)

The “Star" has arranged with Mr J. J. S. Comes, 8.A., B.Sc., to wr * te a series of illustrated articles which will give teachers and others a fuller appreciation of the Great Out-of-doors. They will deal with various aspects of plant and animal life, as well as with inanimate nature. Questions and material for identification will be welcomed.

my cabbage patch. (CXII.) My cabbage patch, if it does not supply many cabbages, provides me with plenty of interest. When walking there, in the cool of the eve,” I notice many tiny grey moths fitting about.* These are associated with the holes in the cabbage leaves—not because they made them, but because they were once caterpillars which did. and because they will leave behind them large families of such caterpillars. Raise a leaf, and numbers of these larva; drop to the ground on silken threads, up which they will climb again when danger passes. These light green, pointed, brown headed leaf-eaters, when full-grown and fed, spin thin cocoons of silk, and in these, after moulting, “fall asleep” as resting “pupae.” After two weeks’ sleep each emerges from its pupa-skin and cocoon as a tiny grey moth, with wings meeting tent-wise and turned up at the “tail ’ end, and with black markings on its back that give it the name of Diamond-back moth. But these moths and I are not the only inhabitants of the place—there are the green-“fly” (aphides), and preying upon these are the larvae of lacewing “flies,” and of syrphid flies—while parasitising them are tiny chalcid wasps. Let us first consider the life-storv of a typical aphis, that we may understand its stages upon our leaves. Let us compare it with the caterpillars’

story. A butterfly or a moth lays eggs, from which hatch tiny worm-like, wingless caterpillars or “larvae.” As these grow up, they moult their outgrown §kins several times. Between the last (ultimate) and the last-but-one (penultimate) moults the larvae “goes to'sleep” as a “pupa.” But just as it “falls asleep”—that is, upon its penultimate. moult —wings suddenly appear beneath the new skin, the pupa skin.

“ Natura on saltum faeit”—“ Nature takes no leaps,” said old Linnaeus. These wings have been growing, ail along, within the caterpillar skin, though they have only now become visible, through the turning out, so to speak, of the pockets in which they hid. Now a larva like this, changing overnight from a wingless worm to a winged though resting pupa, is said to show “metamorphosis”—sudden change. But some insect eggs hatch to a tiny “nymph” instead of a “larva.” This “nymph” shows already the outward rudiments or buds of wings, and in many cases, except for the rudimentary proportions of these wing-stumps, a fairly close resemblance to its parents. It is, indeed, a miniature, a small edition, a baby, of its parents. As it grows up. through a series of moults, these wing-buds sprout outwardly, larger and' longer, until on the last moult they expand full-grown. So we have (neglecting those insectgroups, such as that of the “Silverfish.” which do not own wings) :-r—----(a) Those of which the “larva;” grow' wings inwardly, and which therefore pass through a metamorphosis and a resting pupa stage. Such are butterflies and moths : bees and wasps, beetles, flics (with one pair of wings), cad-dis-“fiies”; lacewing-“flies.” (b) Those of which the “nymphs” grow wings outwardly, and so pass through no great metamorphosis nor marked resting stage. Such are the Orthoptera (“straight-winged”) group (grasshoppers, locusts, crickets, praying mantis, etc.) ; dragon-“flies”; may“flies"; stone-“flies”; Cicadas (“locusts”), aphis, and scale. Thus aphis is not peculiar in having “baby” or “nymph” little ones. But in these respects it is more unusual; (a) that except at the end of winter, these arc born alive (viviparously) instead of from eggs (oviparously), and (b) that except at. the beginning of winter, they are born as drone bees are born, without the help of a male, that is, by virgin-birth, or parthenogenesis. Let us follow the reproduction of aphis from the following diagram. A

fertilized, egg (shown as a circle*containing a cross) lasts over winter (shown as a vertical black barrier). In spring it hatches to a nymph, which grows up to a wingless female <P), reproducing viviparously and parthenogenetioaUy. She gives birth thus to a couple of dozen fatherless offspring, and in a fortnight these are all fullgrown wingless females like their mother, and reproducing similarly (the second P in the chain). So it goes on all summer, and you can understand the possibility of Professor Forbes’s calculation that, if all the progeny of

one aphis lived, there would emerge from the eggs next spring some three and a half trillion; these, each about li millimetres long, touching head to tail, would form a procession 100 C times around the equator. Or Iluxlev’s estimate, that up to the tenth generation, one aphis would produce the weight of 500 million men. No wonder aphides spread quickly, and fortunate it is that they have many foes, that they are but the grain which Nature sows for other animals. At last, just as winter approaches, males are born for the first time. These are winged and fertilise wingless females, that little oval, yellow eggs ’may be laid in crevices of the stem to carry on the race through the cold. So indeed the race might carry on, but could scarcely extend its dominions. For your aphis and her nymphs are sluggish creatures, since they feed on the leaf-sap by thrusting a tubelilce proboscis through the skin, - and sucking, while their liquid food is pumped to their door. Hence arises the need of a winged generation of females to emigrate, and found new colonies. This happens, is shown in the lower line of the diagram, some time in summer (usually when conditions at home are growing serious) and the winged generation colonises the new plant with wingless females. Let us now examine the cabbage leaves and see if we can make out. these stages. On some leaves the under surface is dotted evenly over with little families of young aphides, a dozen or more per bunch, with a dark winged

mother in the centre of each. These well-spaced groups, on an otherwise clean leaf, have every suggestion of being new colonies—and indeed their mothers are immigrants from “ dirtier ” plants. On the plants which are more “messy” (these are the homelands) we find wingless mothers and young. Also some large green winged males. But the stages arc more plainly seen in the oak-aphides. You will have noticed how dark arc the oaks this Season. This is because there drips from the aphides beneath the leaves, on to the upper surfaces of the leaves below them, a “ honey-dew ” of surplus sweetness from the sap; and in this grows a sooty fungus. Under these leaves you will easily distinguish young and adult wingless females, green-wing-ed females (immigrants, or oreparing to emigrate) and dark-winged males; and probably also these winged generations in their nymph stages, with obvious buds of wings. Then in crevices of the twigs, are many amber eggs just visible to the naked eye. To return to our cabbage plot—this time to study the enemies of aphis. You will find a small alligator-like brown larva of our native lacewing or goldeneye (Micromus tasmaniae). It is about half an inch lone, and a most voracious creature, piercing the aphides with its two curved “ tusks,” which are hollow, and through which it sucks the aphides dry. It might save us from both cabbage and oak aphis, but unfortunately its eggs, apparently, are parasited by a wasp. So the Cawthron Institute has lately introduced a green lace wing of Columbia, as adults, and therefore free of parasites. These are so greedy for aphis that when some 300 larvae were being reared in Nelson it required the services of two men to keep them fed. separately on 40 aphides each per day. ou will notice also that some aphides are broxHi, apparently dead, and that some of these have a hole in the back. This suggests that a little parasite wasp, like the Aphelinus late 1 ” introduced to check woolly aphis on apple-trees, is here laying its eggs in the cabbage aphides. I caught several small chalcid wasps about my cabbages, but canhot say they came from the aphides. Nor were they Aphelinus mali. However. I took some of the brown aphides to the museum, where Mons. Tonnoir (whose help T gratefully acknowledge) dissected a lit tie wasp grub unharmed from one, and put the others aside to yield up their chalcid parasites. Now whether these turn out to be Aphelinus or not, it is obvious that here we have a powerful insect ally which we are not giving a fair chance to aid us. For in autumn we destroy the cabbage leaves, and the aphides bearing the wasp-grubs - but we do not destroy the aphis eggs, already laid in stems and soil. So next spring our chalcid friend starts far behind scratch. (To be continued next Saturday.)