VIGNETTES FROM NATURE.

Otago Witness, Issue 3336, 20 February 1918, Page 55

VIGNETTES FROM NATURE.

XXII.—A WAYSIDE POOL IN TARANAKL—(Continued.) By G. M. Thomson, F.L.S. Winter, especially in regions subject to frost, is naturally very destructive to the floating fronds of duckweed. The frost kills many and sends them to the bottom to enrich"the layers of mud there. But others get checked only. During the milder intervals fresh fronds are budded out: but they get smaller and smaller as the light and warmth decline. "These winter-fronds, which are often so smo,ll as to escape tho notice of any but a close observer, are very hardy, and survive a hard frost in considerable numbers, serving, together with the seeds, to perpetuate the race. In spring they emit larger fronds, which multiply with great rapidity, and soon cover the water with a green carpet. I believe that the rapid buddingout of the new fronds is materially aided by their tendency to form strings and chains, which spread loosely and irregularly over the surface of the water. Most floating objects, such . as seeds of waterplants or bits of stick, attract one another at all points, and gather into a dense mass. But duckweed fronds attract one another at certain points only. Hence they cling together' in strings and stars with unoccupied spaces between." If we put a stick into water covered with duckweed the fronds will be found to cling to it in a particular way, which enables them to be transported easily. The wetted surface of the frond is attracted to the stick by the force of capillaritv, while the water-repelling surface, which best resists drying, is turned outwards,, and exposed to the air. Duckweed fronds and, no doubt, the seeds and small fragments of other aquatic plants, cling to the legs of water-birds* and to the elytra of water-beetles and other aquatic insects, and may thus be carried to other and often far distant sheets of water. As Miall says, "the wide distribution of the various species (of Lemna) and the extraordinary speed with which they overspread any water surface to which they may get access, are due, among other things, to the capillary forces which come into play at the surface of any liquid." Our common duckweed (Lemna minor) is met with in all the temperate regions of the earth, while, of the other aquatic plants found in our pond, the common pond-weed (Potamogeton natans) and the little Callitricho. verna have the same wide distribution, and Myriophyllum intermedium occurs in Australia, Malaysia, and away into India. I do not think the seeds of any of these three would adhere to birds' legs or water insects by capillarity; but they might easily be carried by wet mud so adhering. Very small portions of their stems also, carrying ever so small a bud, are probably readily conveyed in this manner. The pond-weeds (Potamogeton) are readily recognised by their beautiful floating leaves, which are often bronze or copper-coloured, and have a beautifullynerved and cross-veined upper surface. I have referred to P. natans, the most wide-spread species, which has oval floating leaves from 2in to A in long; but the most abundant species in New Zealand is P. cheesemanii, which has much smaller floating leaves. Both in these plants and in Myriophyllum an interesting adaptation occurs. In addition to the floating leaves they bear numerous submerged leaves. In P. natans these submerged leaves are reduced to long, narrow leaf-stalks without any blade at all, or are totally wanting; in P. cheesemanii the blades are long (about twice as long as those of the floating leaves), but are very narrow and translucent. In Myriophyllum (which means thousand-leaved) the submerged leaves are cut into innumerable linear, comb-like segments. In these submerged leaves and on the under-surface of the floating leaves there are no stomata or breathing pores. Such are not, indeed, needed. The absorption of dissolved carbon dioxide and the elimination of oxygen take place directly through the surface layer of the cells, and the extreme subdivision of the leaf, or its thin translucent character, tends to expose a maximum of surface for breathing purposes. - Myriophyllum belongs to a small order of Calycifloral plants known as the Halo ragese; many of these are mud-growing, marsh, or aquatic plants, and all are more or less inconspicuous and furnished with imperfect flowers. One of these species, common round the pool, and to be found on every damp clay bank throughout Taranaki, is Gunnera monoica. It forms large patches of dark green foliage, with long creeping .stems, which cling close to the damp surface, and completely hide away its cold, muddy appearance. The leaves are more or less rounded and lobed, as much as an inch in diameter in large specimens, and among them are crowded short panicles of inconspicuous flowers. At this time of year the plants are in fruit, and these are produced in little crowded clusters, mostly under the leaves. These fruit are fleshy drupes, or l-10in in diameter, mostly white, but occasionally red in colour. I do not know what eats those fruits and distributes the solitary seeds. No doxibt, in pre-European days some of the indigenous fruit-eating birds were the active agents in the distribution of the species; but these birds are wanting to-.day, yet the plants are extraordinarily abundant. On tho margins of the pond, on very shallow patches, and on many shallow pools and muddy areas in its vicinity, are bright-green mantles of the übiquitous Callitriche verna. The slender, creeping stems bear numerous pairs of thin and membranous leaves, usually about £ln or less in length. The flowers are usually solitary in "the axils of the leaves, and are greatly reduced, having no trace of either calyx or corolla. The male flower consists of a single stamen, with a bract on each side of its base; the female is a

minuto four-celled ovary, which ripens into a small, dry fruit, -which separates—when quite mature —into four one-seeded nutlets. Naturally such a plant is difficult to place in its true relationships, and though included by Hooker (followed by Cheese man) in the Haloragea?, it has been suggested that its real position is near the Euphorbias. The only conspicuously-flowered native plant in the pond is the' tall willow-herb (Epilobium pal'lidifiorum), which is common in marshy ground throughout New Zealand. Its stem? grow to as much as 3ft in height, and~bear at the top numerous white or rose-coloured blossoms, which are about -|in in diameter. It is a truly handsome species, but will not grow away from water. The animal life in thus little pond is very varied and abundant; but in the absence of text-books it is not possible to mako sure of the species which are met with. I can therefore only deal in a general way with them, and refer to a few of the more familiar forms. Man is a classificatory organism, and does like to label all natural objects . he finds and collects; but, after all, observation is quite independent of mere classification, and the former is more important to the naturalist in the field than the latter. Of aquatic insects the most conspicuous are the dragon-flies, of which at least two species are commonly to be seen in summer hovering over the surface of the pool, but often a long way from the water whence they have emerged. They are beautiful insects as far as colour is concerned, and are somewhat difficult to catch on account of their strong, jerky flight. When another insect, sufficiently big to be worth catching, crosses its track the dragon-fly darts at it, and, if successful in its attack, seizes it-in its powerful jaws, continuing its flight as before. If caught and held between the finger and thumb the dragon-fly curls its abdomen below and forward in a menacing manner, and inexperienced collectors avoid the tail for fear of being stung. But they have no stinging apparatus, and the movement is probably meant to deceive and bluff its captor. There is a great deal of imitative make-believe in the animal kingdom, and many harmless animals possess shapes and habits which lead to their being taken for dangerous creatures. Dragon-flies seem to be savage creatures ; even when held by a captor they will chew up whatever is presented to their jaws, whether it be a stick or a straw, or another insect. I have read of a case in which a collector caught one with his net, but in doing so broke off the long, slender, abdominal portion. The head and thorax, with the legs attached, settled on the ground, and when a large fly was held close to the jaws of the mutilated animal it was seized and was chewed up vigorously. The whole fly was eaten, and fragments of the body were seen to emerge at the back of the -thorax. One wonders how far the sense of pain or even of discomfort enters into Wae economy of these creatures. This particular dragon-fly was placed in a vivarium, and was quite active at the end of a couple of days/ when its movements attracted the attention of a lizard (also a prisoner), which promptly attacked and ate it all but its large and broad wings. The larva? of the dragon-flies live at the bottom of the pool. Those of the larger species are somewhat sluggish in their movements, and lie more or less hidden in the mud; those of the smaller forms belonging to the family Agrionida? are more slender and active. The jaws of one of these larval insects cannot be seen at first, as they are hidden by the labrum or lower lip, which so effectively covers the front of the head as to be commonly called the "mask." This mask, when extended, is seen to be a powerful organ acting like a pair of jaws, and by means of it the animal catches its prey. The big larva? arc too clumsy and slow to forage for their food; they mostly lie perdu and wait for their prey (other larva?, small worms, fishes, and, indeed, any living creature not too large to be disposed of) to come within range. Then the. mask is shot out with surprising agility, the victim is seized and brought quite close to the mouth, where it is rapidly devoured by the aid of the true jaws, which are quite concealed behind the mask when the latter is closed. When the pupa stage is reached the dragon-fly is as voracious as the larva. When it'is about to change to the imago, it creeps up the stem of some aquatic plant right out of the water, and remains quite still, except for a few slight movements of the body. Then, after a short time, the skin, which is now loose on the body, gets dry and splits. Almost immediately the "insect contorts the body till it gets its head and legs free, and at last emerges from its pupal skin, which is left clinging by the claws to tho object on which the pupa rested. Thus the dragonfly, like all the articulate animals which moult in this complete fashion, leaves a coat so perfect in all its details of structure, down even to the very hairs, that it might easily be mistaken for the former occupant. Among the varied forms of insect life in the pond the most familiar are the waterboatmen and the water-beetles. The former is very like a beetle, but does not belong to the group at all. It is a true bug, and belongs to the Heteroptera. It is characterised by a body shaped like an inverted boat, the back being rounded and bluntly keeled, while the ventral surface is fiat. The outer wings are partly horny and partly membranous, and in this respect differ from those of a beetle, which are horny throughout. The inner wings are very delicate and membranous, but are traversed by strong but exceedingly fine nervures. The water-boatman is a strong flyer; but it is very seldom seen on the wing during the day, as it usually makes its flight in the evening, sometimes rising directly from the water, but more commonly climbing up some support before expanding its wings. The legs of the first pair are short, and serve to anchor the body to weeds and other objects at the

bottom of tho pond. This is necessary, for tho insect carries a large store of air on its abdominal surface, and thereforeits specific gravity is low. The second pair of legs are used for holding objects and for pushing away the foliage and stems of aquatic plants. The third panares the most specialised organs of locomotion, for they arc long, strong, and fringed with stiff bristles. They form, therefore, admirable swimming organs, and act liko a pair of "sweeps" or powerful oars. You have only to see ono of these water-boatmen turn on its back and dive with several vigorous sweeps of these largo oars to the bottom of the water to realise the appropriateness of the name popularly given to tho insect. Though this animal lives mostly under tho w r ater it is entirely dependent for its supply %i oxygen on the air, of which it carries a layer among the hairs on the under side of the body. It breathes by stigmata or tubes passing into the body, and cannot in any way absorb dissolved oxygen as gill-bearing animals do. Hence it must frequently come to the surface to renew its layer of air, and that is how these insects are so frequently seen in ponds. The water-boatman is entirely predaceous, and when it catches any animal suitable for food it holds it firmly under the thorax by means of its second pair of legs, and then sucks the juices completely out of it. If you catch a boatman in the hand it can give an uncom monly .sharp nip with its beak; but the bite is harmless. I find this pond contains several other interesting creatures which are worthy of some pasmg notice, but must postpone this to a further paper.