1854] 



DEVELCPEMENT OF THE VIVIPAROUS APHIDES. 



137 



line old naturalists who have long passed away, owe the basis of 

 their high eminence to Entomological studies of this kind, into 

 which they were seductively drawn in their earlier days. 



Every naturalist is aware of the remarkable phenomena con- 

 nected with 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 earlier observers, they 

 have been a kind of wonder-stories in Zoology and Physiology. 



I need not here go over the historical relations of this 

 subject. The q : ieer experiments and the amusing writings of the 

 old Entomologists are well known. The brief history of the 

 general conditions of the developement of those insects are as 

 follows : In the autumn the colonies of plant-lice are composed 

 of both male and female individuals ; they pair, the males then 

 die and the females deposit their eggs, after which fiey 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 indivi- 

 duals which are capable of reproducing their own 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 indhidual 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 male sex throughout. 



These same conditions are here repeated, and so almost 

 indefinitely. Experiments having shown that this power of 

 reproduction under such circumstances may be exercised, accord- 

 ing to Bonnet, at least through nine generations, while Duvan 

 obtained thus eleven generations in seven months, his experiments 

 being here curtailed, not by a failure of reproductive power, but, 

 by the approach of winter, which killed his specimens; and 

 Keyber even observed that a colony of Aphis dianthi, which was 

 brought into a constantly heated room, continued to propagate 

 lor four years in this manner without the intervention of male3, 

 and e ,r en 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 and carefully repeated ; and there can now be no doubt that 

 the virgin Aphis reproduces her kind — phenomena which are 

 continued almost indefinitely, ending finally in the appearance of 

 individuals of distinct male and female sex, which lay the foun- 

 dation of new colonies in the manner just described. 



The question arises, what interpretation is to be put upon 

 this almost anomalous phenomenon? Many speculations 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 re- 

 searches, I will reserve this further notice until the concluding 

 part of this paper. j 



My observations were made upon one of the largest species 

 of Aphis with which I am acquainted, the Aphis Caryce of 

 Harris. 



While in Georgia this last spring, it was my good fortune 

 that myriads of these destroyers appeared on*a hickory which 

 M 



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 

 source of subsistence failed and they gradually disappeared from 

 starvation. The individuals of each brood were throughout of 

 the producing kind, no males were to be found, upon the closest 

 search; they were all, moreover, winged, and those few which 

 were seen without those 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 autumn. 



_ The first brood, upon their appearance f-om their winter 

 hiding place, were of mature size, and I found in them the 

 developing forms 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. In a few 

 days after the appearance of these insects, the second brood (B), 

 still within its parent (A), had reached two-thirds its natural 

 size. At this time the arches of the segments 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 promi- 

 nent. At this period, and while the individuals of generation B 

 are not only inside 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 to the locality where 

 are situated the ovaries in the oviparous forms of these animals. 

 These egg-like bodies were either single nucleated cells of 1-3000 

 of an inch in diameter, or a small number of such cells inclosed in 

 a single sac. These are the germs of the third generation ; they 

 increase with the developement of the embryo, in which they 

 have been formed, and this increase of size takes place not by 

 an augmentation of the primitive cells, but by the endogenous 

 formation of new cells. After this increase has gone on for a 

 certain time, they appear like little oval bags of cells— all these 

 component cells being of the same size and "shape, there being no 

 one wh'chis longer and more prominent than the rest, and which 

 could be comparable to a germination vesicle. While 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 thus 

 what was a single sac becomes two, which are attached in A 

 moniliform manner. This new germ, thus formed, may consist 

 of only a single cell, as I have often seen, but it soon attains a 

 more uniform size by the endogenous formatiou of new cells within 

 the sac by which it is inclosed. In this way the germs are mul- 

 tiplied to a considerable number, the nutritive material for their 

 growth being apparently a sort of liquid with which they are 

 bathed, contained in the abdomen, and which is here derived 

 from the abdomen of the first parent. When these germs have 

 reached the size of l-300th of an inch in diameter, there appeal's 

 on each, near one side, a yellowish, vitellus-looking mass of spots, 

 which in size and general aspect are different from those consti- 

 tuting the germ proper. This yellowish mass increases pari 

 passu with the germ, and at last lies like a cloud over and con- 

 caals one of its poles. 



I would also insist on the point that it does no* extend itself 

 gradually over the whole given mass, and is therefore quite unlike 

 a true germinative vesicle or a proligerous disc. When the 

 egg-like germs have attained the size of l-50th of an inch, then 

 appears distinctly the sketching or marking out of the future 

 animal. This sketching consists at first of delicately marked 



