Imágenes de página

affecting Ulmus campestris.* This author, by a series of ingenious experiments, rightly came to the conclusion that the insects hibernate. on the trunk; but he failed to discover in what condition they so hibernate.

M. J. Lichtenstein of Montpellier, France, who has paid much attention to these insects, was led to the belief, announced in various publications t during the year 1877, that the European species inhabiting Elm and Poplar migrated to the roots of grasses and there hibernated. He was doubtless misled by the great general resemblance between all the species of this sub-family in the immature and apterous stages. In a letter dated December 25, 1877, I informed him that I had discovered that the sexed individuals of our Elm species inhabited the bark, to which the female consigned her single winter egg, and that his theory was altogether inconsistent with this fact and with what Derbès had discovered of Pemphigus cornicularius affecting Firs. With this clue my friend has done good service the past season, by correctly tracing the life-history of several species, and showing that there are no such migrations as he assumed, from the trees in question. Indeed, nothing but the most thorough and absolute proof can establish the fact of any such migration. Species of the same genus often so closely resemble eaclı other that they are more readily distinguished by their mode of life, or by the galls they produce, than by structural or describable differences; and this holds particularly true of the immature or apterous stages. This fact, taken in connection with what is here recorded and what is already known of the habits of the sub-family, renders it extremely improbable that any of the species subsist at one time on one plant and habitually change, by migration, to another of a totally different nature. Stranger things happen in nature; but until M. Lichtenstein experimentally proves the accuracy of his conclusions, I must reject his theory.

Led by previous investigations into the habits of the Grape Phylloxera, I discovered, in 1875, that some of our Elm-feeding species of Pemphiginæ produce wingless and mouthless males and females, and that the female lays but one solitary impregnated egg, just as in the case of Phylloxera. Continuing my investigations, especially during the present summer (1878), I have been able to trace the lite-history of those species producing galls on our own Elms, and to show that they all agree in this respect, and that the impregnated egg produced by the female is consigned to the sheltered portions of the trunk of the tree, and there libernates—the issue therefrom being the stem-mother, which founds the gall-inhabiting colony the ensuing spring. Thus the question as to what becomes of the winged insects after they leave the galls is no longer an

Die Lebensgeschichte der auf Uhnus campestris vorkommenden Aphidden-Arten. + Stettiner Ent. Zeit. 1877, p. 489, &c. Ann. d. Sc. Nat. Paris, October, 1871.

I alopt this term as a literal translation of the German "Stamm-mutter," and as meaning the ancestress or progenitor of all succeeding generations until the impregnated egg is produced and another cycle commences.

[ocr errors]

open one. They instinctively seek the bark of the tree, and there give birth to the sexual individuals, either directly or through intervening generations.

It is my purpose in a subsequent paper to go more fully into a consideration of the habits and classification of this interesting family of Plant-lice; but my present object will best be accomplished by giving a full account of the two commoner gall-making species found upon the American Elm, with less complete accounts of some other species. It will be seen by the facts recorded, and by the descriptions, how futile all attempts must be to establish anything like a natural system of classification, whether the number of antennal joints, the character of wingvenation, or the habits be considered; and the lesson I would draw from the study of these minute insects is the same that must be drawn by all naturalists who thoroughly study any one group, viz., that any system of classification will be unsatisfactory, except on the hypothesis that it is purely a matter of convenience. We find extreme variation in the number and proportions of antennal joints in the different stages of the same species. We find a great tendency to variation even in the pterogostic characters; and, finally, there is not even unity of habit in the species of the same genus. The deflexed or horizontal position of the wings has no value in this sub-family, because most of the species carry their wings horizontal while yet in their galls or for some time after issuing from the pupa. That all of the insects of this sub-family possess, however, at least one feature in common with the species of Phylloxera so far known, namely, the mouthless, wingless, and generally degradational nature of the sexual individuals, and the production by the female of a solitary, impregnated egg, there can be little doubt, since I have traced these sexual individuals and this impregnated egg, not only in the cases indicated in this paper, but further in Schizoneura Rileyi (Thos.) and Schizoneura lanigera (IIausm.); while M. J. Lichtenstein, as already stated, has discovered them in several of the European species of this sub-family. Another feature common to the whole sub-family, though by 110 means peculiar to it, is the flocculent exudation from the body and the absence of bright coloration, the winged females having, all of them, a dull, dark ground-color of head and thorax. The front wings have invariably a fold or thickening of the posterior marginal vein in the region of the first discoidal; and the hind wings are correspondingly prodluced on the costal margin and armed with hooklets that catch in the fold in flight. These are features common to all Aphididæ known to me; but their prominence or faintness often has specific value, and the angle on the hind wing may, for descriptive purposes, be called the "hook-angle."


[Curling and gnarling the leaves of the White Elm (Ulmus americana), forming thereby a sort of pseudo-gall. The curl made by a single stem-mother in the spring takes the pretty constant form of a rather wrinkled roll of one side of the young leal; but, according as there is more than one stem-mother, or as several contiguous leaves are affected, the deformation assumes various clistorted shapes, sometimes involving quite large masses of the leaves.)


There is a good deal of irregularity in the time of appearance of the different generations, but the general history of this species, as I have observed it for several years, is herewith given. There is much greater difficulty in fully tracing the life-history of one of these small creatures than might be supposed. They languish in confinement and ill bear handling. To trace their growth and movements in a state of nature requires vigilance and perseverance, and a great deal of time; and I have been fortunate, in my studies of this and the next species, in securing the patient aid of Miss M. E. Murtfeldt of Kirkwood, Mo., a lady to whom I have already had repeated occasion to express my indebtedness.

If, during the winter, we carefully examine the cracks and crevices of an American or White Elm that was badly infested with this leafcurling species the previous summer, we shall be pretty sure to find its impregnated egg-a ininute, dull-yellowishi, ovoid object, about 0.5mm long (Fig. 1, a), either free or still more or less effectually covered with the parent's dry skin, which faintly shows the insections that characterized the living female. The same spring influences that cause the leaf-buds to swell and open, likewise induce the hatching of this winter egg, and the little creature that issues from it instinctively crawls to the inore terminal twigs and branches, and settles upon the first tender leaflet it meets with. It constitutes the stein-mother, or first generation, and, stationing itself on the under surface of the leaf, very soon causes the same to swell and curl by the irritation and punctures of its beak. The curl is usually from the lateral edge, and the more normal form it takes is shown at Fig. 1, c. It is, however, very irregular, and takes on many different forms, according as it is produced by one or several stem-mother's settling on the same leaf, and as it affects a portion of one leaf only or embraces several from the same bud. At first, pale yellowish-red, with dark members, the stem-mother increases in size more or less rapidly, depending to some extent on the development of the leaf. Moving about in her curled house, within which she is destined to live and die, this stem-mother goes through her last moult, and attains maturity about the twelfth day from the time of hatching. This period may be lengthened by unfavorable weather, as an indefinite period of lethargy, both of plant and insect, may ensue, after hatching, if the temperature be too low. The number of molts I have not definitely ascertained, but from analogy there will be three. Having attained maturity, she com


mences peopling her pseudo-gall with young at the rate of about one erery six or seven hours, according to temperature, increasing in bulk and prolificacy from day to day, until by the early part of May, in the latitude of Saint Louis, she has attained her fullest development, and soon perishes. She may attain to nearly four millimetres in length, and, with greatly swollen body, be almost as wide (Fig. 1,.b). Her immediate issue, or the second generation, are like her in many respects, but never grow to be quite so large. The individuals of this generation soon accumulate in great numbers around her, and in their turn commence to bring forth young, some remaining within the original curl, others scattering to found new colonies. Their issue, or the third generation, show certain marked structural differences from the first (see description), and are destined to become winged.

During most of the month of May, we may find, where large clusters of leaves are affected, the few more or less exhausted stem-mothers, and these second and third generations in every stage of development. As the lice increase in numbers, the leaves no longer protect them, but present on both sides multitudes of busy atoms—livid, old and paler youngthose with wings and those getting wings—interspersed with white exuviæ, cottony secretion, and globules of pearly liquid. At the same time, in single curls of more terminal leaves, we may find the second generation of wingless mothers surrounded by smaller colonies, all of which will become winged. The winged females (Fig. 1, d) are short-lived, bringing forth a dozen or more pseudova at average intervals of about half an hour. The glossy pellicle that compresses all the members of their newly-born issue is ruptured very shortly after birth, and is worked off in the course of about ten minutes. These facts are easily ascertained by confining the winged mother, but the exact positions to which the pseudova are naturally carried I have not been able to definitely learn; but we may rest pretty confident that they are consigned both to the leaves and to the twigs. The young lice, forming the fourth generation, are very active, running swiftly in all directions. In color, they are at first of a pale and bright red, but soon acquire a brownish tint. In general appearance, they resemble the young from the stem-mother. The beak is very long, thickened at the end, which always projects beyond the tip of the abdomen, and terminates in a sickle-like point. Experiments made by attaching and confining these young to the trunk of the tree show that they do not flourish thereon, but naturally crawl out to the more tender, terminal leaves, which they immediately begin to curl. They may be found scattered over an infested tree, with their beaks for the most part inserted in the tender leaf-stem or in the midrib on the under side, the leaf in such case already beginning to show the effect of the poisonous puncture. They are, however, able to sustain themselves on the tender bark of the twigs alone, and may be found nearly full-grown, there exposed to view and enveloped in the white cottony matter, which brushes off at the slightest touch. When full-grown, they commence reproducing, and their progeny, under favorable circumstances, becomes exceedingly abundant. The growing points of the tree are affected with larger or smaller colonies, crowding and covering both the surfaces of the leaves, the petioles, and the stem. I have known young Elm trees to be so thoroughly covered with these lice, in the earlier part of June, that not a single leaf was unaffected, and upon giving the tree the slightest jar there would be a perfect shower of the liquid globules excreted by the lice. At this season of the year, when the lice are thus numerous, they may be found during the heat of the day actively crawling over all portions of the tree-a veritable migration, necessitated by the want of sufficient succulent leaves, but evidently premature, and destined to be the death of the individuals participating in it, excessive multiplication here, as in all other cases, obliging the destruction of the excess. While the individuals thus wandering are mostly the younger ones, the migrating instinct seems sometimes to possess individuals of all ages, especially where the tree is badly affected; and that they perish is proved by the mass of dead lice which in such a case may be found around the base of a tree. So far as I have been able to learn by confining specimens of the fifth generation, which is very similar to the fourth, but with shorter promuscis, the fifth reproduces like the fourth without acquiring wings. The individuals of the sixth generation, on the contrary, all acquire wings, the pupa being active, with but a small amount of flocculence, confined to the posterior part of the body The winged lice of this sixth generation abound during the latter part of June and the early part of July. They resemble those of the third generation, except that they are perhaps on the average somewhat smaller and paler, and less prolific. They instinctively congregate on the bark, and consign to the crevices, and sheltered parts thereof, their young, which, as in the fourth generation, are enveloped in a sort of pellicle. These young also resemble the young of the fourth generation in general form, but have very short and stout beaks. Instead of being active, they are quite sluggislı, congregating in clusters in the sheltered portions of the bark, and being essentially bark-feeders. The color soon inclines strongly to orange or salmon, and, after two or three days of sluggish existence, they shed their skin, and become more active, penetrating more deeply into the interstices of the bark, and huddling together in groups of various sizes. They are now of a pale buff, or, more correctly, salmon color, the surface at first smooth and polished, but becoming in some instances slightly pulverulent. Simulating closely the color of the bark, and being quite small, they are not easily detected, unless in great numbers. A careful examination shows that they have entirely lost the beak, and that they consist of both males and females, the females being the larger, and the males showing the genital characters given in the description. They live grouped together for several days with little motion, the female (Fig. 1, e) increasing in size by the enlargement of the single egg contained in her body. Both soon perish,

« AnteriorContinuar »