ART. VII.-Researches on the Development of Viviparous Aphides; by WALDO I. BURNETT, M.D., Boston.

EVERY naturalist is aware of the remarkable phenomena connected with the viviparous reproduction of Aphides or plant-lice, for their singularity has led them to be recounted in works other than those of natural science, and, from the days of the earlier observers, they have been the theme of a kind of wonder-story in zoology and physiology.

I need not here go over the historical relations of this subject. The queer experiments and the amusing writings of the old Entomologists are well known. The brief history of the general conditions of the development of these insects is as follows: In the early autumn the colonies of plant-lice are composed of both male and female individuals; these pair, the males then die, and the females soon begin to deposit their eggs, after which they die also. Early in the ensuing spring, as soon as the sap begins to flow, these eggs are hatched, and the young lice immediately begin to pump up sap from the tender leaves and shoots, increase rapidly in size, and in a short time come to maturity. In this state it is found that the whole brood, without a single exception, consists solely of females, or rather and more properly, of individuals which are capable of reproducing their kind. This reproduction takes place by a viviparous generation, there being formed in the individuals in question, young lice which, when capable of entering upon individual life, escape from their progenitor and form a new and greatly increased colony. This second generation pursues the same course as the first, the individuals of which it is composed being like those of the first, sexless, or at least without any trace of the male sex throughout. These same conditions are then repeated, and so on almost indefinitely, experiments having shown that this power of reproduction under such circumstances may be exercised, according to Bonnet,* at least through nine generations, while Duvaut obtained thus, eleven generations in seven months, his experiments being curtailed at this stage, not by a failure of the reproductive power, but by the approach of winter which killed his specimens; and Kyber‡ even observed that a colony of Aphis dianthi which had been brought into a constantly heated room, continued to propagate for four years, in this manner, without the intervention of males, and even in this instance it remains to be proved how much longer these phenomena might have been continued.

The singularity of these results led to much incredulity as to their authenticity, and on this account the experiments were often

*Bonnet, Traité d'Insectologie, ou observations sur les Pucerons, 1745. + Duvau, Mém. du Mus. d'Hist. Nat., xiii, p. 126.

Kyber, Germar's Magaz. d. Entomol., 1812.

and carefully repeated; and there can now be no doubt that the virgin Aphis reproduces her kind-a phenomenon which may be continued almost indefinitely, ending finally in the appearance of individuals of distinct male and female sex, which lay the foundation of new colonies in the manner just described.*

The question arises, what interpretation is to be put upon these almost anomalous phenomena ? Many explanations have been offered by various naturalists and physiologists, but most of them have been as unsatisfactory as they have been forced, and were admissible only by the acceptance in physiology of quite new features.

As the criticism I intend to offer upon some of these opinions, will be the better understood after the detail of my own researches, I will reserve their future notice until the concluding part of this paper.

My observations were made upon one of the largest species of Aphis with which I am acquainted, the Aphis Carya of Harris. While in Georgia, this last spring, it was my good fortune that myriads of these destroyers appeared on a hickory which grew near the house in which I lived. The number of broods on this tree did not exceed three, for with the third series their numbers were so great that their source of subsistence failed and they gradually disappeared from starvation. The individuals of each brood were, throughout, of the producing kind, no males having been found upon the closest search; they were all, moreover, winged; and those few which were seen without these appendages appeared to have lost them by accident. I mention this fact especially, since it has been supposed by naturalists that the females were always wingless, and therefore that the winged individuals, or the males, appeared only in the autumn.‡

The first brood, upon their appearance from their winter hidingplaces, were of mature size, and I found in them the developing germs of the second brood quite far advanced. On this account it was the embryology of the third series or brood alone, that I was able to trace in these observations.

* For details of experiments by which Bonnet's original results were verified, sce Réaumur, Mémoires, iii, Mém. 9 and 11, and vi, Mém. 13. Also,

Dutrochet, Mémoires, ii, p. 442. See also the more modern writers, and especially Kirby and Spence, Introduction to Entomology, iv. p. 161.

+ Harris, A treatise on some of the Insects of New England which are injurious to Vegetation. 2nd ed. 1852. p. 208. As Dr. Harris says, it is probably Lachnus of Illiger, (Cinara of Curtis.)

See Westwood, An introduction to the modern Classification of Insects, &c. London, 1839. ii, p. 438-but especially

Owen, Parthenogenesis, &c., p. 23, note, and p. 59, note, where he says, "Many of the virgin viviparous Aphides acquire wings, but never perfect the generative organs!"

A few days after the appearance of these insects, the individuals of second brood (B), still within their parents (A), had reached two-thirds of their mature size. At this time the arches of the segments of the embryo had begun to close on the back, and the various external appendages of the insect to appear prominently; the alimentary canal had been more or less completely formed, although distinct abdominal organs of any kind belonging to the digestive system were not very prominent. At this period, and while the individuals of generation B, are not only in the abdomen of their parent A, but are also enclosed, each, in its primitive egg-like capsule, at this time, I repeat, appear the first traces of the germs of the third brood (C).

These first traces consist of small egg-like bodies arranged, two, three, or four in a row, and attached in the abdomen at the locality where the ovaries are situated in the oviparous forms of these animals.

These egg-like bodies consisted either of single nucleated cells, of of an inch in diameter, or, a small number of such cells enclosed in a simple sac. These are the germs of the third generation; they increase with the development of the embryo in which they have been formed, and this increase of size takes place not by a segmentation of the primitive cells, but by the endogenous formation of new cells. After this increase has gone on for a certain time, these egg-like bodies appear like little oval bags of cells-all these component cells being of the same size and shape, there being no cell which is larger and more prominent than the rest, and which could be comparable to a germinative vesicle. While these germs are thus constituted, the formation of new ones is continually taking place. This occurs by a kind of constriction-process of the first germs, one of their ends being pinched off, as it were, and in this way what was a single sac, is changed into two which are attached in a moniliform manner. The new germ thus formed, may consist of even a single cell only as I have often seen, but it (the germ) soon attains a more uniform size by the endogenous formation of new cells within the sac by which it is enclosed. In this way the germs are multiplied to a considerable number, the nutritive material for their growth being apparently a fatty liquid with which they are bathed, contained in the abdomen, and which is thence derived from the abdomen of the first parent.

300

When these germs have reached the size of of an inch in diameter, there appears on each, near one end, a yellowish, vitellus-looking mass or spot, which is composed of large, yellowish cells, which in size and general aspect, are different from those constituting the germ proper. This yellow mass increases pari passu with the germ, and at last lies like a cloud over and concealing one of its poles. I would also insist on the point that it

does not extend itself gradually over the whole germ-mass, and is therefore quite unlike a true germinative vesicle or a proligerous disc. When the egg-like germs have attained the size of r of an inch, there distinctly appears the sketching or marking out of the future animal. This sketching consists at first of delicately-marked retreatings of the cells here and there, but which soon become more prominent from furrows, and at last the whole form of the embryo stands boldly out. As the whole idea and form of the insect is thus moulded out of a mass of cells, it is evident that the separate parts which then appear, such as the arches of the segments, the extremities and the oval apparatus, consist at first of only rows of simple cells. This point is here beautifully prominent, and nowhere have I observed finer illustrations of the cell-constitution of developing forms.

The development thus proceeding, each part of the dermo-skeleton becomes more and more distinct, and the increase of size of the whole is attained by the constant development of new cells. During this time, the yellow vitellus-looking mass, situated at one of the poles of the embryo, has not changed its place; it has increased somewhat in size, but otherwise appears the same. When the development has proceeded somewhat farther, and the embryo is pretty well formed, the arches of the segments, which have hitherto remained gapingly open, appear to close together on the back, thereby enclosing this vitellus-looking mass within the abdominal cavity.

It is this same vitelloid mass thus enclosed that furnishes the nutritive material for the development of new germs which would be those of the fourth brood or D; this development of germs. here commences with the closing up of the abdominal cavity, and the same processes which we have just described are again repeated.

The details of the development subsequent to this point, are like those of the development of ordinary insects or of the Articulata in general; and although this ovoid germ has at no time. the structural peculiarities of a true ovum-such as a real vitellus, a germinative vesicle and germinative dot; yet, if we allow a little latitude in our comparison and regard the vitellus-looking mass as the mucous, and the germ-mass proper as the serous fold of the germinating tissue, as in true eggs-if, I repeat, we can admit this comparison of parts, then the analogy of development between these germs and true eggs of insects, may be traced in considerable detail.

This comparison I have been inclined to admit at least in part, from the striking resemblance of these developing forms at certain stages, with the embryological forms of spiders as they have been figured by Herold* and as I have myself traced them. Herold, De Generatione Aranearum in ovo. Marbourg, 1824. SECOND SERIES, Vol. XVII, No. 49.-Jan., 1854.

9

When, in spiders, the serous fold of the germinating tissue has extended so as to cover two-thirds of the developing form, leaving the vitelline mass on the dorsal surface near one of the poles, the whole embryo quite resembles that of a developing Aphis just before the arches of the segments close up on the back.

With this view of the relative parts of the germ, the following would be the details of the development of the different systems, and in the noticing of which I shall follow Kölliker.*

1. The germinating tissue consists of two parts; a serous and mucous fold.

2. The abdominal plates arise from the serous fold, sprout out towards the vitelloid mass, pass over it and unite on the dorsal surface of the future animal; on the opposite side are formed plates which do not unite, but are formed into the hind legs.

3. The wings are the lateral limbs.

4. The first traces of the abdominal column appear in the chain of abdominal muscles, situated between the nerves and the intestinal canal.

5. The nervous system in all its parts arises from the serous fold, as well also as the organs of sense.

6. The mucous fold or the vitellus-looking mass, serves no purpose in the formation until the closing in of the visceral plates.

7. Thus enclosed in the abdominal cavity, it is not transformed directly into the intestinal canal, but simply furnishes the material from which the component cells of said canal and its hepatic deverticula are formed. It also furnishes the material from which the new germs are formed, as already shown.

8. The heart is formed on the dorsal aspect between the mucous and serous folds. In this way the details of development closely correspond with those of the embryology of the other Articulata which I have studied; and the subject is all the more interesting as the germ-masses, from which such development occurs, in no way and at no time structurally resemble true eggs.

When the embryo is ready to burst from its developing capsule and make its escape from the abdomen of its parent, it is about th of an inch in length, or more than eight times the size of the germ at the time when the first traces of development were seen. From this it is evident that, even admitting that these germmasses are true eggs, the conditions of development are quite different from those of the truly viviparous animals; such as for instance in Musca, Anthomyia, Sarcophaga, Tachina, Dexia, Miltogramma, and others among Dipterous insects ;† or in the vivipa

* Kölliker, Observationes de prima Insectorum genesi adjecta articulatorum evolutionis cum Vertebratorum comparatione. Diss. Inaug. Scr. Alb. Kölliker. Turin,

1812.

A work replete with facts and interesting suggestions.

+ See Siebold in Frorieps neue Notiz. iii, p. 337, and in Wiegmann's Arch. 1838, i, p. 197.-also his Observat. quaed. Entom., &c., p. 18.