Page images

different magnitudes, some much larger than the others, and they then prepare for a change that is about to ensue in their mode of life, by emitting from the under side of their bodies numerous little white downy threads, which are fastened, in a radiated manner, around their bodies to the bark, and serve to confine them securely in their places. After becoming thus fixed they remain apparently inanimate; but under these lifeless scales the transformation of the insect is conducted; with this remarkable difference, that in a few days the large ones contrive to break up and throw off, in four or five flakes, their outer scaly coats, and reappear in a very similar form to that which they before had; the smaller ones, on the contrary, continue under their outer skins, which serve instead of cocoons, and from which they seem to shrink and detach themselves, and then become perfect pupæ, the rudiments of wings, antennæ, feet, &c., being discoverable on raising the shells. If we follow the progress of these small lice, which are to produce the males, we shall see, in process of time, a pair of threads and the tips of the wings protruding beneath the shell at its lower elevated part, and through this little fissure the perfect insect at length backs out. After the larger lice have become fixed and have thrown off their outer coats, they enter upon the pupa or chrysalis state, which continues for a longer or shorter period according to the species. But when they have become mature, they do not leave the skins or shells covering their bodies, which continue flexible for a time.

These larger insects are the females, and are destined to remain immovable, and never change their place after they have once become stationary. The male is exceedingly small in comparison to the female, and is provided with only two wings, which are usually very large, and lie flatly on the top of the body. After the insects have paired, the body of the female increases in size, or becomes quite convex, for a time, and ever afterwards remains without alteration; but serves to shelter the eggs which are to give birth to her future offspring. These eggs, when matured, pass under the body of the mother, and the latter by degrees shrinks more and more till nothing is left but the dry outer convex skin, and the insect perishes on the spot. Sometimes the insect's body is not large enough to cover all her eggs, in which case she beds them in a considerable quantity of the down that issues from the under or hinder part of her body. There are several broods of some species in the year; of the bark-louse of the apple-tree at least two are produced in one season. It is probable that the insects of the second or last brood pair in the autumn, after which the males die, but the females survive the winter, and lay their eggs in the following spring.

Young apple-trees, and the extremities of the limbs of older trees are very much subject to the attacks of a small species of bark-louse. The limbs and smooth parts of the trunks are sometimes completely covered with these insects, and present a very singularly wrinkled and rough appearance from the bodies which are crowded closely together. In the winter these insects are torpid, and apparently dead. They measure about one tenth of an inch in length, are of an oblong oval shape, gradually decreasing to a point at one end, and are of a brownish color very near to that of the bark of the tree. These insects resemble in shape one which was described by Réaumur* in 1738, who found it on the elm in France, and Geoffroy named the insect Coccus arborum linearis, while Gmelin called it conchiformis. This, or one much like it, is very abundant upon apple-trees in England, as we learn from Dr. Shawt and Mr. Kirby;$ and Mr. Rennie states that he found it in great plenty on currant bushes. It is highly probable that we have received this insect from Europe, but it is somewhat doubtful whether our apple-tree bark-louse be identical with the species found by Réaumur on the elm; and the doubt seems to be justified by the difference in the trees and in the habits of the insects, our species being gregarious, and that of the elm nearly solitary. It is true that on some of our indigenous forest-trees bark-lice of nearly the same form and appearance have been

* Mémoires, Vol. IV. p. 69, plate 5, figs. 5, 6, 7. + General Zoology, Vol. VI., Part I. p. 196.

Introduction to Entomology, Vol. I. p. 201. $ Insect Transformations, p. 92.

observed; but it is by no means clear that they are of the same species as those on the apple-tree. The first account that we have of the occurrence of bark-lice on apple-trees, in this country, is a communication by Mr. Enoch Perley, of Bridgetown, Maine, written in 1794, and published among the early papers of the Massachusetts Agricultural Society. These insects have now become extremely common, and infest our nurseries and young trees to a very great extent. In the spring the eggs are readily to be seen on raising the little muscle-shaped scales beneath which they are concealed. These eggs are of a white color, and in shape nearly like those of snakes. Every shell contains from thirty to forty of them, imbedded in a small quantity of whitish friable down. They begin to hatch about the 25th of May, and finish about the 10th of June, according to Mr. Perley. The young, on their first appearance, are nearly white, very minute, and nearly oval in form. In about ten days they become stationary, and early in June throw out a quantity of bluish white down, soon after which their transformations are completed, and the females become fertile, and deposit their eggs. These, it seems, are hatched in the course of the summer, and the young come to their growth and provide for a new brood before the ensuing winter.

Among the natural means which are provided to check the increase of these bark-lice, are birds, many of which, especially those of the genera Parus and Regulus, containing the chickadee and our wrens, devour great quantities of these lice. I have also found that these insects are preyed upon by internal parasites, minute ichneumon flies, and the holes (which are as small as if made with a fine needle), through which these little insects come forth, may be seen on the backs of a great many of the lice which have been destroyed by their intestine foes. The best application for the destruction of the lice is a wash made of two parts of soft soap and eight of water, with which is to be mixed lime enough to bring it to the consistence of thick whitewash. This is to be put upon the trunks and limbs


of the trees with a brush, and as high as practicable, so as to cover the whole surface, and fill all the cracks in the bark. The proper time for washing over the trees is in the early part of June, when the insects are young and tender. These insects may also be killed by using in the same way a solution of two pounds of potash in seven quarts of water, or a pickle consisting of a quart of common salt in two gallons of water.

There has been found on the apple and pear tree another kind of bark-louse, which differs from the foregoing in many important particulars, and approaches nearest to a species inhabiting the aspen in Sweden, of which a description has been given by Dalman in the “Transactions of the Royal Academy of Sciences of Stockholm,"* for the year 1825, under the name of Coccus cryptogamus. This species is of the kind in which the body of the female is not large enough to cover her eggs, for the protection whereof another provision is made, consisting, in this species, of a kind of membranous shell, of the color and consistence almost of paper. In the autumn and throughout the winter, these insects are seen in a dormant state, and of two different forms and sizes on the bark of the trees. The larger ones measure less than a tenth of an inch in length, and have the form of a common oyster shell, being broad at the hinder extremity, but tapering towards the other, which is surmounted by a little oval brownish scale. The small ones, which are not much more than half the length of the others, are of a very long oval shape, or almost four sided with the ends rounded; and one extremity is covered by a minute oval dark colored scale. These little shell-like bodies are clustered together in great numbers, are of a white color and membranous texture, and serve as cocoons to shelter the insects while they are undergoing their transformations. The large ones are the pupa-cases or cocoons of the female, beneath which the eggs are laid; and the small ones are the cases of the males, and differ from those of the females not only in size and shape, but also in being of a purer white color, and in having an elevated ridge passing down the middle. The

• Kongl. Vetenskaps Academ. Nya Handlingar.

minute oval dark-colored scales on one of the ends of these white cases are the skins of the lice while they were in the young or larva state, and the white shells are probably formed in the same way as the down which exudes from the bodies of other bark-lice, but which in these assumes a regular shape, varying according to the sex, and becoming membranous after it is formed. Not having seen these insects in a living state, I have not been able to trace their progress, and must therefore refer to Dalman's memoir above mentioned, for such particulars as tend to illustrate the remaining history of this species. The body of the female insect, which is covered and concealed by the outer case above described, is minute, of an oval form, wrinkled at the sides, flattened above, and of a reddish color. By means of her beak, which is constantly thrust into the bark, she imbibes the sap, by which she is nourished; she undergoes no change, and never emerges from her habitation. The male becomes a chrysalis or pupa, and about the middle of July completes its transformations, makes its escape from its case, which it leaves at the hinder extremity, and the wings with which it is provided are reversed over its head during the operation, and are the last to be extricated. The perfect male is nearly as minute as a point, but a powerful magnifier shows its body to be divided into segments, and endued with all the important parts and functions of a living animal. To the unassisted eye, says Dalman, it appears only as a red atom, but it is furnished with a pair of long whitish wings, long antennæ or horns, six legs with their respective joints, and two bristles terminating the tail. This minute insect perforates the middle of the case covering the female, and thus celebrates its nuptials with its invisible partner. The latter subsequently deposits her eggs and dies. In due time the young are hatched and leave the case, under which they were fostered, by a little crevice at its hinder part. These young lice, which I have seen, are very small, of a pale yellowish brown color, and of an oval shape, very flat, and appearing like minute scales. They move about for a while, at length become stationary, increase in size, and in due time the whitish shells are produced, and the included insects pass from the larva to the

« PreviousContinue »