IN TOUCH WITH NATURE

Otago Witness, Issue 4063, 26 January 1932, Page 10

IN TOUCH WITH NATURE

By

A USEFUL FOOT.

J. Drummond, F.L.S., F.Z.S.

Perhaps the strangest foot in the world is possessed by the great slug Limaxmaximus. Having come uninvitedly to New Zealand, it makes slimy footsteps on garden paths and other level surfaces in the Dominion to evidence its nocturnal walks abroad. It is less plentiful in New Zealand than another “ homie,” the common slug, Agriolimax laevis, but it is much larger, measuring about five inches from the tips of its horns to the opposite end of its body. Doubtless, it is equally addicted to the green leaf diet, with troublesome results to gardeners.

The whole of the great slug’s pale whitish under-surface ip its foot. By using this, the slug moves forward gracefully, rhythmically, smoothly, not jerkily like a human being, wave following wave on the sole with a soft, silent, and beautiful tread. The sole stretches along the under-surface from end to end, leaving a spot of sticky slime at the point of contact with the object over which the slug moves. Above the sole is the thick muscular foot. This bears the superstructure of the body. About 27 waves carry a slug forward one inch, each wave giving the body a forward displacement of .036 inches. On window glass, slugs, timed with a stop watch, travelled one inch at a speed that represented from 17J feet to 254 feet per hour. On a level concrete walk, where the conditions were not so good, a slug, apparently observing the usual speed, covered a straight course for 14 inches at the rate of 14 feet an hour.

These records were published in the Scientific Monthly by Dr H. A. Allard, a member of the United States Department of Agriculture. He has found the slug, which is shunned and despised by most people, to be a very interesting creature, worthy of intensive study. He discovered the secret of its singular pedestrianism when he induced an individual to walk upon a piece of clear window glass. With a hand lens he studied it in that position. Moving freely over the smooth glass it gave him every opportunity to observe it closely. He saw that the white, expanding sole of its foot was almost as wide as its body, but along the middle of the sole there was a narrow band. As the slug moved its outstretched body forward, light waves and dark waves passed rapidly along the central band from the tail end to the head end. As long as the slug moved there was a steady progression of waves towards the head. It is evident that, when a slug wishes to advance, the locomotive band is thrown into successions of many waves. Over these, the superstructure of the body moves as smoothly as if it was on rollers. The locomotion seems to be on the roller principle, or, as Dr Allard suggests, it is like a caterpillar tractor, except that, in the case of the slug, there is no return of the tread.

On an ordinary level surface a slug leaves spots of slime in regular lines. These separate footprints, instead of uninterrupted lines, show that in the tread the foot is slightly arched at intervals. touching the surface at only points of contact. On a very smooth surface, such as glass, the whole sole is pressed, but the practice is to arch the foot at intervals. Even when a slug walks up a vertical concrete wall, its track is spaced by spots of slime. Dr Allard cannot state what part the slime plays in locomotion. The system of locomotion is more efficient than appears at first glance. It enables a slug to walk or roll along any kind of surface, vertical or horizontal, even to move in a completely reversed position under a ceiling or a well-cover. More than this, a slug can walk serenely up the fine stem of a plant. It does this, Dr Allard explains, by contracting its foot on each side of the stem, which it closely invests and by using the locomotive band, the real tread, with nicely spaced undulations, to carry the body up the narrow cylindical surface.

A slug seems to have an armour of slime effective against some enemies at least. Dr Allard has seen large ants run up to a great slug, but run away quickly as soon as they felt the slime on their feet and antenna?. While studying slugs, he saw tiny flying insects and gnats alight on them and stick helplessly in the slime until they died.

Great slugs, probably, were established in New Zealand when their eggs were brought from the Old Country unintentionally on plants on which the eggs had been laid. In the same way, doubtless, the species spread to the United States. It is much more plentiful there than in New Zealand. It wanders through damp, shady places where there is ample vegetation and Igaf mould. Dr Allard has watched members of the species eating bread, potatoes, and other table wastes in dishes for cats or dogs. Their diet does not end there. They are omnivorous. In the Old Country they have been known to enter a dairy and feed on raw beef.

In captivity, they are cannibals. Three great slugs were kept together in a box. One was found with its tail and two-thirds of its body eaten. The crime was not caused by hunger, as all the captives had been supplied with their favourite plant food. A great slug, kept for 33 days with a young slug of a smaller species, attacked it frequently, and gnawed many small pieces of its skin. Other captive great slugs preferred fungi as food. Five common garden slugs were seen in a group, each eating a may-fly. This was in a wide field, where green food was plentiful. About 36 beetles were placed in a bottle with 10 snails and fresh leaves and grass. When the bottle was opened two weeks later, all the beetles had been eaten, and all the snails were fat and healthy. The common garden slug, on being stopped by a finger from attacking another slug, tried to bite the finger fiercely, rasping the skin with its scraper. Another slug, carried in the hand, almost rasped the skin away, leaving the place tender and sore, like a slight burn, for several days.

Two slugs) in captivity laid many eggs. From October 10 to November 17, one

laid 396. The other, from October 13 to November 30, laid 477. The eggs weighed 624 to the ounce. One slug, alter laying, consumed half a raw potato and then took a bath, remaining submerged for more than half an hour. In damp, warm places, young slugs hatch, out of their eggs in about 60 days. In cold, dry places the time is extended to about 75 days. For several days before hatching a young slug may be seen moving in its shell. A crack is made on the surface of the shell. This gradually enlarges, until the baby slug escapes from its prison. In many cases it creeps back as if afraid of the world. When able to crawl freely it buries itself in the ground for several days. It has no food' then, but is almost double in size when it conies forth. Mr E. M'Diarmid, of Thames street, Oamaru, has forwarded a specimen of a well-known native beetle, Prionoplus reticularis. The species is the largest species of beetle in New Zealand, and, 'perhaps, the handsomest. It is very plentiful at the present time, as the season is a favourable one. In the adult form represented by the specimen, these beetles fly about at night and often enter houses. In the grub, or larval, form they look like large, fat caterpillars, about three inches long. The grubs, called hu-hu by the. Maoris, bore into fallen forest timber, native and. introduced. The adult insect sometimes is called the hu-hu beetle. This specimen was seen by Mr M’Diarmid struggling out of a hole. It had just emerged from its pupal, or chrysalis stage, which was spent in the hole.

The brown chafer, or brown beetle, Odontria zealandica, which in its grub stage is called the grass grub, seems to be having an exceptionally good innings in several parts of New Zealand this season. In the adult form, as an ordinary brownish beetle about half an inch long, square-headed and robust, it feeds on foliage. Mr J. Cooper, Roto-o-rangi. Cambridge, reported in the middle of November that at dusk for at least five nights it had swarmed in hundreds in his orchard and garden, eating all the leaves from his fruit trees and attacking beans, peas, rhubarb, lettuces, cauliflowers, earl}- potatoes, and onions. The beetles were so numerous that he could not check them. They were very troublesome in all parts of the Waikato district.

. The species is a pest in several directions. As a white curled grub in the soil, it destroys lawns, largely damages the roots of pasture plants, attacks other plants and,, particularly in Canterbury, gives undesirable attention to the roots of young forest trees in nurseries, favouring the insignia pine. Females lay their eggs in the ground. Mr D. Miller, biologist at the Cawthron Institute, states that the life cycle from the egg to the adult form takes 12 months.