Fig. 407.—Larvæ of the Hornbill emerging from eggs.
The larvæ of most Hymenoptera are footless grubs, furnished with a soft head, and exhibiting but little, if any, advance upon those of Diptera ([Plate VI]., No. 141). In the saw-fly, however, the larva, instead of being as above described, a mere footless maggot, presents the closest resemblance to the caterpillar of the Lepidoptera; it is provided with a distinct head, with six thoracic legs, and in most cases from twelve to sixteen pro-legs are appended to the abdominal segments.
One other conspicuous object represented in [Plate VI]., No. 128, is the maple Aphis, also known as the leaf-insect, averaging in size about the one-fiftieth of an inch in length. Although recognised and described under the name of the leaf-insect, nothing was known of its origin and history, with the exception of what the Rev. J. Thornton published in 1852, and to whom we owe its re-discovery on the leaves of the maple. Subsequently it attracted the attention of the Dutch naturalist, Van der Hoeven, who regarded it as the larval form of a species of Aphis, and named it Periphyllus. It has more recently engaged the attention of Dr. Balbiani and M. Siguoret, whose united investigations will be found in “Comptes Rendus,” 1867. These observers assigned it definitely to Aphis. A brown species is also met with during a great part of the year feeding upon the young shoots of the maple. The female produces two kinds of young, as do all the genus Aphis, one normal the other abnormal; the first are alone capable of reproducing their species, while the latter retain their original form, which is not changed throughout their existence. They increase so slowly in size that it may appear doubtful whether they eat, the mouth being rudimentary; they undergo no change; do not acquire wings, and their antennæ always retain the five joints peculiar to all young Aphides before the first moult. Neither are they all of the same colour, some being of a bright green, as represented in [Plate VI]., while others are of a darker, or brownish-green colour. The brown-green embryos differ from the adult female only in those characters analogous to all other species, and this chiefly with regard to the minute hairs, which are long and simple. In the green embryos, in the place of setæ, the body is surrounded by transparent lamellæ, oblong in shape. These scales not only cover the body, but also the anterior portion of the head, the first joint of the antennæ, and the outer edge of the tibiæ of the first pair of legs. The dorsal surface in these insects is covered with a mosaic of hexagonal plates, very closely resembling the plates of the carapace of the tortoise. In this particular my artist has fallen into a slight error. Another peculiarity is that the body is much flattened out, and looks so much like a scale on the surface of the leaf that it requires considerable practice, as well as quickness of sight, to detect the young maple Aphis. One of the lamellæ is seen highly magnified at c, and a tenent-hair at b. The antennæ, tapering off towards the apex, are serrate on both edges, and terminate in a fine lancet (shown at a), with which it penetrates the leaf of the plant. Beneath the insertions of the antennæ is a complex form of sucking mouth, and on either side of the head are two brilliant scarlet-coloured eyes.
Aphides, as is well known, live upon the juices of plants, which they suck, and when they occur in great numbers cause considerable damage to the gardener and farmer. Many plants are liable to be attacked by swarms of these insects, when their leaves curl up, they grow sickly, and their produce is either greatly reduced or utterly ruined. One striking instance is presented in the devastation caused by the hop-fly (Aphis humuli).
Fig. 408.—Aphrophora spumaria, Cuckoo-spit.
a. The frothy substance; b. The pupa.
The Aphrophora bifasciata, common frog-hopper, is a well-known garden pest. The antennæ of this insect are placed between the eyes, and the scutellum is not covered; the eyes, never more than two in number, are occasionally wanting. These pests are furnished with long hind legs, that enable them to perform most extraordinary leaping feats. The best-known British species is the cuckoo-spit, froth-fly ([Fig. 408]). The names cuckoo-spit and froth-fly both allude to the peculiar habit of the insect, while in the larva state, of enveloping itself in a kind of frothy secretion, somewhat resembling saliva.
Arachnidæ.—In this class of insects, spiders, scorpions, and mites are included, all of which belong to a sub-class of Arthropoda, and are appropriately placed between the Crustacea on the one hand and the Insecta on the other. The highest Crustaceans have ten feet, the Arachnidæ eight, and insects six. The Arachnidæ are wingless, have no antennæ, and breathe by means of tracheal tubes, or pulmonary sacs, these performing the function of lungs. As a rule they have several simple eyes, have no proper metamorphosis, and they are essentially predaceous, the females being larger than the males. Most of the Arachnidæ live on insects, and may therefore be regarded in the light of a friend to the florist and gardener.