Fig. 286—Gall, or false cone of Chermes abietis. Cambridge. The small figure, to the left, is a section made at the level indicated by the pointing line a, and shows the chambers containing young.
Galls.—Like Phylloxera, many species of Aphidae live partially, others wholly, in galls that are produced by plants as the result of one or more Aphids interfering with a delicate part of the plant when it is in a young and growing state. The usual position of Aphid galls is on a leaf or leaf-stalk. But in the case of the genus Chermes, a bud or some growing part of the spruce-fir is affected in such a way that it gives rise to an object having externally the appearance of a little fir-cone, while inside it consists of chambers in which the Aphids reside. The forms of Aphid-galls are very diverse, but this is probably due to the plant rather than to the Insect, for the same species of Aphis may give rise to different forms of galls. Réaumur thought that each Aphid-gall was due to a single individual that irritated the tissue of the plant, so that the latter grew up at the point of irritation and enclosed the Insect.
A few points as to the anatomy of Aphids should be noticed. It is doubtful whether the antennae have ever really more than six joints, the apparent seventh joint being actually a sort of appendage of the sixth. The rostrum is externally three-jointed, and is remarkable for the great diversity in its length, sometimes it is quite short, at others several times longer than the body (Fig. 285); the setae are often very much longer than the sheath; in cases where this great length of rostrum exists, the individual may often be found with the tip firmly fixed in the bark, and, as it were, tethered by means of the rostrum, the length of which allows, nevertheless, considerable locomotion. Suction is performed by contractions of the pharynx. There has been much difference of opinion as to whether there is a salivary syringe, and Witlaczil failed to find it. Krassilstschik is, however, positive that it exists,[[527]] and that it is analogous to that described by Mayer in Pyrrhocoris, but there are great differences of structure between the two. It is very difficult to determine the number of segments at the extremity of the body; this is terminated dorsally by a median organ placed above the anus, and known as the cauda. Balbiani apparently considers that there are ten abdominal segments and the cauda. The alimentary canal has a small stomach, and an elongate intestine, the terminal division of which is capacious and remarkably long. There are no Malpighian tubes; according to Kowalevsky, their function is discharged by the posterior part of the alimentary canal. There exists, however, a peculiar structure, the pseudovitellus, a sort of cellular, double string; and Witlaczil, in his valuable paper[[528]] on the anatomy of Aphidae, suggests that this organ may in some way replace the missing Malpighian tubes. Another highly peculiar structure is the siphons, frequently called nectaries, honey-tubes, or siphuncles. They are situated on the dorsal aspect of the fifth abdominal segment, but exist only in certain of the sub-families; they are of very different lengths according to the species, and are capable of movement; they open directly into the body cavity, though exceptional openings into the body cavity are extremely rare in Insects. They excrete a waxy matter, which first appears as oil-like globules. It was formerly supposed that they were the means of secreting the sugary matter, called honey-dew, so much prized by ants and some other Insects; but this is now ascertained to be erroneous. This matter comes from the alimentary canal, and is secreted in large quantities by some species, Büsgen having observed that forty-eight drops, each about 1 mm. in diameter, were emitted by a single individual in twenty-four hours.[[529]] Certain gall-dwelling Aphidae—Pemphigus, Chermes (Fig. 285), Schizoneura—possess numerous wax glands; these seem to replace the siphons, and excrete the peculiar, whitish flocculent matter that is so conspicuous in some of these Aphids.
Earlier anatomists failed to find any dorsal vessel, and it is consequently reported in books to be absent. It has been, however, recently detected by Witlaczil, and Mordwilko states that it does not differ from that of other Insects.
We have already alluded to the fact that the mode of reproduction of Aphids leads to an unrivalled increase. This, however, is not due to the prolificness of the individual, which, in point of fact, appears to be considerably below the average in Insects, but rather to the rapidity with which the young begin to reproduce. This has been discussed by Huxley, Buckton, and others. The first-named naturalist calculated that the produce of a single Aphis would, in the course of ten generations, supposing all the individuals to survive, "contain more ponderable substance than five hundred millions of stout men; that is, more than the whole population of China."[[530]] It has since been contended that Professor Huxley's calculation was much below the mark. Although it is somewhat difficult to make a calculation dealing adequately with the actual facts, yet it is clear that the increase of Aphids is such that, drawing as they do their nutriment directly from the plant in its growing state, in the course of two or three years there would be no nutriment available for other animals, except such as might be derived from plants not attacked by Aphids. The numbers of Aphidae would be so great that they could not be expressed by ordinary numerical methods, and their increase would be actually limited only by the relations existing between different kinds of plants, and between plants and Aphids. This result is avoided by the fact that Aphids are themselves the victims of a whole army of Insect enemies. They have the numerous members of a special group (Braconidae, Aphidiides) of minute Hymenoptera to live inside their bodies, and many Aculeate Hymenoptera depend entirely on the Aphidae as the source of food for their own progeny. The Lady-birds—Coccinellidae—live on Aphids and Coccids, and themselves increase to such an extent as to be in many years a conspicuous part of the Insect world. Crowds of the larvae of Hemerobiids and Syrphids are constantly engaged in spearing and sucking the Aphides. Hence the old naturalist Bonnet said that, just as we sow grain for our benefit, Nature has sown Aphids for the benefit of multitudes of different Insects. He might have added that these different Insects are for the benefit of man, it being clear that without them the population of the world must rapidly decrease.
Ants treat Aphidae more intelligently than most other Insects do, for they do not destroy the helpless creatures, but utilise their products in the way man does those of the cows he keeps. The relations between ants and Aphids is itself an extensive chapter in Natural History; many facts have been brought to light showing that the ants manage the Aphids in a prudent or intelligent manner, distributing them when too numerous in one place, keeping guard over them, even building shelters for them, and in some cases keeping them in direct association, by retaining the Aphids in their own dwellings. The further investigation of these points goes, the more it tends to raise the actions of the ants to the level we call in ourselves intelligent. It would even appear that the ants are acquainted with the migrations of the Aphids from one species of plant to another, Webster informing us that as the Aphis-population on an apple tree multiplied the ants in attendance anticipated their migration to wheat and grass by carrying them to those plants.[[531]] We have nearly 200 species of Aphidae in Britain,[[532]] and there may perhaps be 800 known altogether. To what extent they may occur in the tropics is undetermined. There are said to be no native species in New Zealand.
Fig. 287—Instars of Aleurodes immaculate. Europe. (After Heeger.) A, Nymph, from above; B, nymph, under surface; C, imago.
Fam. 8. Aleurodidae.—Minute Insects, with four mealy wings, seven-jointed antennae, two-jointed feet, terminated by two claws and a third process. These minute Insects are at present a source of considerable perplexity, owing to the curious nature of their metamorphosis, and the contradictory accounts given of them. In the earlier stages they are scale-like and quiescent, being fixed to the under side of a leaf. The French authors Signoret and Girard state that the young are hatched having visible appendages and segmentation, but that after they are attached to the leaf the organs gradually suffer atrophy. Maskell states the opposite, saying that the organs in the earliest stages are not usually recognisable, but become faintly visible with the growth of the Insect. Heeger states that the larva undergoes three ecdyses, and he gives the figures we reproduce; if he be correct it would appear that the nymph undergoes a great development. Réaumur, on account apparently of their great metamorphosis, treated the species known to him as being Lepidopterous, though he correctly pointed out their distinctions. At present we can only conclude that the Aleurodidae undergo a metamorphosis of a kind peculiar to themselves, and requiring renewed investigation. The family has been monographed by Signoret, and more recently by Maskell, who has increased the number of species to about sixty.[[533]] We have three or four in Britain, one of which, A. brassicae, is extremely abundant on various kinds of cabbage in certain years.
