In another experiment, he obtained even to the ninth successive generation, when the death of the aphis last produced unfortunately interrupted his progress. Thus it was clearly demonstrated, that aphides are herma phrodites, capable of themselves to perpetuate their race.
This, however, is not a privilege common to all aphides, without exception; for certain species copu late like the larger animals, and seem possessed of the same generative organs. These are large brown aphides, living in numbers on the oak, particularly attaching themselves to branches beginning to blacken. Their size is fully equivalent to that of a connuon fly, and they have only tubercles behind instead of the cornicula, which do not discharge the mellifluous liquid. They exhale a peculiar odour, which, at first sight, is one of their most remarkable characters. Mr Bonnet observed the copulation of a small winged aphis with several apterous females, and having confined one of the fe males, she produced 12 eggs. The same species also multiplies without copulation, and he proved by incon testible experiments, that at one time of the-year they were viviparous, and at another oviparous.
But in pursuing the history of the animal, we find it necessary to render it less complicated, by an abstract in fewer words, of the observations of Dr Richardson on those of the rose. The first aphides appearing in spring, are hatched from eggs that have withstood the cold of winter. These eggs are at first green, and then blacker; they contain a mucilaginous fluid of a uniform consistence. Two successive generations are produced in spring, five more in summer, and three in autumn, making ten in the whole. Though all the preceding generations contained females, in this tenth one are se veral males. However, the aphides producing them, have previously produced females ; and Dr Richardson affirms, that all the males are winged. Some of the third generation of females, though apterous at first, acquire wings also, after casting the last skin. A cer tain fullness in the breast indicates those that will have them, and from being contracted in extremely narrow compass, they are now unfolded, erect and longer than the body, in a surprising manner. The number of wing ed aphides seems to depend less on their original struc ture than on the quantity and quality of nutriment with which they are supplied. But all the females of the tenth generation are apterous. Copulation now takes place, and in a clay or two eggs are produced. The
general colour of the insects throughout the preceding part of the year is green. The females of the last ge neration are yellow ; the males at first of a reddish brown, and, after casting the last skin, of a bright yellow', with black wings.
Some naturalists seem to think that the temperature of the weather, under certain exceptions, regulates the propagation of the aphis by eggs or living young. Mr Curtis found the aphis salicis, a large aphis, constantly viviparous from the 24th of September to the Gth of De cember, the thermometer sometimes at 29°. On 22d November, he found a number of eggs from a common small green aphis on the auricula in the open air, while the same species, on a geranium within doors, produced young. In mild winters, he observed numbers of the females viviparous on the primula in January. He thence infers, that all aphides are not viviparous and oviparous during the same season, but that some may be wholly viviparous, and that those which are both viviparous and oviparous do not lay eggs in autumn, nor through winter, unless a certain degree of cold takes place. In most species, he says, both males and females acquire wings, but some of both sexes are always apterous ; and also, that of the same species, some females • are apterous, and some winged.
Observations, partly similar, had previously been made by M. de Geer. From the experiments on the aphis ros.r, made with the view of elucidating whether the same aphides which produce young in summer will lay eggs in autumn, or whether those laying eggs have never produced young, he concludes that the oviparous aphides of autumn have never produced young, and are destined to lay eggs only ; and that viviparous aphides never lay eggs, but the young last produced by them in the late season, are those which are oviparous, and pre serve the species by means of their eggs, which are ca pable of resisting the cold of winter. Though his mean ing is obscurely expressed, it is evident, that he here speaks of particular generations in the succession.— Among these aphides, he found many apterous females which were viviparous. Winged ones are unquestiona bly always so, and never lay eggs, but their young will produce eggs. He professes himself, likewise, inclined to believe, that the last winged aphides are produced by large apterous viviparous females; but that apterous oviparous females and males spring from large winged mothers.