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Vegetable Caterpillars

By J. T. Salmon, Zoology Department, Victoria University College

Those members of our community who are prone at times to wander and explore in the native bush may have come across objects which when dug up from the forest floor resembled that shown in the accompanying illustration. Such objects are popularly known as ‘Vegetable Caterpillars’; and much argument has ensued amongst learned gentlemen as to whether they are animals or plants. True, at the commencement of its existence the vegetable caterpillar is a free-living animal; but in the course of its life the caterpillar becomes attacked by a species of those lowly forms of plant-life known as fungi; and there can be no doubt that the Vegetable Caterpillar as found is essentially a plant.

The fungi are a large group of more or less simply organized plant forms characterized by the absence of chlorophyll — the green colouring matter of flowering plants, and by the fact that they reproduce by means of spores. In their mode of life they are either parasitic, depending upon the living tissues of plants and animals for their food, or they may be saprophytic, obtaining their nourishment from decaying organic matter. The edible mushroom is a fungus, so also are toadstools, rusts, smuts, and the moulds that appear on stale bread, jam, and other foods. However, the similarity between such a well-known fungus as the mushroom, and the vegetable caterpillar, at first sight may not be very apparent. Let us examine a mushroom. It consists of three main portions, a root-like structure referred to as the mycelium, a stalk, and an upper expanded portion which carries the spores. A spore is not a true seed; but it is capable of germination. This commences by the spore expanding somewhat and putting forth a small tube which soon commences to branch in all directions, forming a network of tubes or hyphae collectively referred to as the mycelium. The mycelium permeates throughout the nutrient matter on which the fungus is growing. Soon a number of hyphae fuse and grow upwards to form a stalk which later expands into the characteristic button-shaped head of the mushroom. This expanded portion contains on its lower-gilled page 2 surface the fruiting or spore-bearing area of the plant. Now, to return to the vegetable caterpillar: The larger part filling the caterpillar skin is the mycelium, the long, slender portion is the stalk formed from a number of hyphae, while the upper portion of the stalk is covered with the spore-bearing tissue corresponding to the gills of the mushroom head.

There is in New Zealand a group of moths belonging to the genus Porina. The caterpillars of these moths live in the soil, feeding upon the roots of grasses and young plants. Sometimes it happens that a spore of a fungus known as Cordiceps robertsii settles upon the caterpillar and commences to germinate. The position most favourable for germination appears to be in the head or between the head and second segment of the caterpillar. Delicate hyphae grow into the body of the caterpillar in its subterranean habitat, consuming its nutrient substance and ultimately killing it. The body of the caterpillar becomes completely replaced by the mycelium of the fungus, and only the skin remains; but it keeps its original shape. The fungus then sends up a stalk to a little above the surface of the ground. If examined, the upper portion will be found to be slightly thickened and of a velvety texture. It produces the spores, which are blown off and distributed by the wind, thus affording opportunity for the whole life cycle to be repeated.

The caterpillars of the moths Porina signata and Porina enysii, probably supply the greater proportion of the vegetable caterpillars; but other Porina larva no doubt are parasitized by Cordiceps robertsii also. A larva of the allied Puriri moth (H. virescens) has been reported as attacked and converted to a vegetable caterpillar by this fungus.

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There is in New Zealand another species of Cordiceps, Cordiceps sinclairii, which attacks the subterranean nymphs of cicadas in much the same way as the Porina caterpillars are attacked. In the case of sinclairii, however, the stalk, which usually grows out from between the head and first segment, is branched. There may be just a few branches or there may be a large number, in which case the whole structure is not unlike the antlers of a deer. Hyphae may issue from other parts of the insect's body as well as, for instance, from the leg joints or from the edges of the body segments.

After fruiting has occurred, the fungus dies; but the original shape of the insect's body has been preserved. In texture it is tough and woody; in fact, it may be snapped across, when it breaks cleanly like the dead twig of a tree.

The Vegetable Caterpillar is found throughout New Zealand, being particularly plentiful in the Rotorua District throughout all types of forest. The genus Cordiceps is not confined to New Zealand, however, as a large number of specimens are known from many other countries, about 160 in all; but the largest and finest specimens are found in Australia and New Zealand.