SCHOOL ,IN THE OPEN.

Star (Christchurch), Issue 17853, 22 May 1926, Page 25 (Supplement)

SCHOOL ,IN THE OPEN.

STUDIES IN THE GREAT OUT-OF-DOORS.

(By J. J. S. Comes. EA. 350.)

THE CALL-MAKERS.

CXVI. Probably no hedge-plant abOut Christchurch or its suburbs is more common than the hedge-daisy (Olcarial fosteri). It is sometimes confused with‘ the Matipo, but can be distinguished by" the white, downy under-surface of the‘ leaves. and by the bunches of daisy-l like florets. l Many owners of such Olcaria hedges have been puzzled, and not aiew greatly concerned, to find that here and there appear bunches of tiny leaves amOng the larger ones. These little tassels of dwarf leaves seem to arise in place of bunches of florets, for not only do they grow in the axils of ordinary leaves, but also they divide readily into bunchlets, as if each little floret of a daisy head had transformed its petals to tiny leaves. Moreover. there appear to be transitional stages—yet of this I am not certain.

But if the exact change’ is obscure, the cause at least is obvious enough. For if you split open a little bunchlct of a leafy bunch, even just with the fingers, you find at the heart of each a wee yellow “grub," hardly larger than a. pimhead. This is the larva of a small fly. The parent laid her eggs by means of her sharp egg-placer,

either within the soft floret ovaries or about the growing-point of a branchbud. From these hatched young larvae, which drank of the sap pumped to them by the plant. Apparently the irritation due to the presence of the larvae causes an extra flow of sap to the part, resulting in increased growth there, as shown by the wealth of little leaves, and by the swellings at their base, in each of which dwells a larva. There, in a cell with sap—sweet walls, it grows and awaits its change first into a resting pupa, and then into a winged fly. \Nithered bunches nearby, from which flies have escaped this summer, show that the fly does not have to eat its way out—‘for as if each swelling really representd an ovary, accustomed to open naturally when ripe for the scattering of its seeds, a large opening appears at the top of it. There are a great many plants, in the native bush and by the wayside, with each its special gall-persistent visitor. Many and varied are the freakish designs of the malformations. On the stems of Cat'star (a weed like dande-‘ lion, but with several flowers on a‘ aEtalk) you find, in winter, swellings From these there emerge in summer lit—tle chalcid wasps (Aulax). 0n the bluegum trees many of the twigs are dying back, not only because on the outside, there is a scale-insect (Eriocm cius) piercing the bark with its beakl and sucking the sap, but also because,

The “Star“ has arranged with Mr I J. S. Comes, BA.. B-Sca to write a mics of illustrated articles which will gixfe teachers and others a fuller appreciation of the Chat Outroi-doors. They will deal with vurious aspects of plant and animal life, 3.5 well as with inanimate nature. Questions and material for identificafion will be welcomed. _

living wholly within the twigs. at the‘ dfspth. of‘the Cambium layer. there arei little grubs of a wasp (Rhicnopeltella) encased in hard wooden cells. and living on the plant's life-blood Their presence is shown externally by ridging ‘ and bursting of the bark above them. Then againfin the bush you come “13011 soft: round, woolly gailsL ESPECianY 0“ lielichrysum "glomeratum. A visitor from England might expect to find on our introduced English trees the wealth of galls which characterise them in their homeland. but the trees have come here without their associated insects. These insects, so small that they would pass unnoticed but for the galls they. Cause. growing up as larvae, sucking the plant's sap from their 'cradle walls‘, and leaving the plant only when, full-grown, they fly 0‘“ through their pin-hole archwayS, may be described as “parasites." Is it possible for such parasitic larvae to have parasites praying in turn upon them? I find,that every other golden maggot in these leafy galls on Olearia fosteri has, crawling upon it, and biting at it, a much smaller, silvery waspgrub, invisible as yet to the naked eye. Next summer this white grub will have eaten the golden larva, and will become a pupa, so that there will emerge from the gall a little wasp instead of a.fiy. Even now, in some older galls, we find large wasp grubs and black pupae of a brood which

should have emerged this summer. The wasp grubs are as large as the fly larvae which they have eaten and replaced, but white and more pointed at the tail-end, and with far better developed head and jaws‘

It. would appear that the fly larva has 'growu to some size within its gall before the little waspvmother comes along and with her ovipositor, pierces the wall of the gall to lay her egg against the yellow maggot which is to be the grub‘s food. Probably she is guided, through her sensitive antennae, by some smell from the larva, just as we might discover a strong cheese. though it were wrapped in bags. Yet is it not wonderful that the gall~making fly knows instinctively just where on this special plant to leave her eggs to the care of the bubbling sap. and that the wasp (this parasite upon a parasite) knows just as surely where to lay her eggs,'that her larvae may find other larvae for food? The hedge-9Wner. therefore, who is anxious for- his Olearia hedge. will welrome this was , the enemy of the gallfly. If he wishes to rear for examina» tion, either fly or wasp (neither of which has yet been bred out) it will be necessary to wait until next summer, when they have completed their growth' within the'gall. before picking the latter and keeping it in a covered bottle. (To "be continued next Saturday.)