ACADEMY OF SCIENCES. 123 



I have entered on the investigations with any hope of success had I not known 

 that the efforts of these gentlemen seem to have been directed to the destruc- 

 tion of the insect without having acquired sufficient knowledge of its natural 

 history. So far as known, the insect is one which goes through a series of 

 generations without changing its form, during which many thousands of in- 

 sects can be produced from a single impregnated ovum (75,000,000 have been 

 calculated), but after a certain number of these parthenogenic generations of 

 the power of non-sexual reproduction ceases, and the development of a new 

 form becomes necessary for the continuation of the species. This alternation 

 of generations takes place in many of the lower tribes of animals; the differ- 

 ent generations of the same animal being in some instances so dissimilar as to 

 have been mistaken for different species. In the phylloxera the forms usually 

 met with, or, at least, that had been described when I commenced my inves- 

 tigations, were two non-sexual forms, the nymphs and nurses, the former be- 

 ing a small insect with legs which allow a certain degree of locomotion, the 

 latter being a form in which the legs are so slightly developed as to be hardly 

 visible, so that the insect can move but a very short distance from the spot 

 where the ovum is deposited. Besides these two non-sexual forms, whose life 

 is entirely subterranean, there is a winged form in which the two sexes are 

 developed, and which passes the greater part of its existence above ground. 

 Within the last few months it has been discovered that this winged insect de- 

 posits its eggs on the leaves and bark of the vine, and from these eggs it is 

 probable that a new generation of nurses and nymphs arises which, at least 

 for many generations, propagate themselves on the roots of the vine without 

 any males being produced. There were two important questions relating to 

 this winged form of the insect which had not been decided — namely, whether 

 they deposit any eggs on the root or bark of the vine under ground, and the 

 form of the insect that is first produced from the impregnated ovum. These 

 questions have an important bearing on the means to be taken for the destruc- 

 tion of the insect, but unfortunately neither of them had received a satisfacto- 

 ry answer. In the spring of this year I presented some specimens at the Mi- 

 croscopical Society, of a form of the insect that had not been described. In 

 my frequent examination of the roots of diseased vines during the wiuter, the 

 insect was only met with under the form of nurses, which remained in a dor- 

 mant state from the beginning of November to April. The first sign of a re- 

 newal of activity in the insect was the appearance of a form much resembling 

 the nymph but rather larger, with a sort of gelatinous body, and so transpar- 

 ent that from ten to twenty ova could readily be distinguished in the abdo- 

 men. It was much more active than the nymphs, running about the roots 

 with agility. My own opinion is, that it was a form of insect directly devel- 

 oped from the impregnated ovum, but whether it had been hatched above the 

 ground and had traveled down on the root, or whether any impregnated eggs 

 had been deposited beneath the surface from which it had been produced, is 

 not known. The same form of insect was described by Mr. Balbiani at a 

 meeting of Academic des Sciences at Paris, about two weeks after I had ex- 

 hibited it at the Microscopical Society. On seeing this form of the insect, the 

 idea at once struck me that this was the phase of its existence in which it could 



