765 



been shown by a series of experiments that, when the egg of the galh- 

 cola is laid, the type of larva to which it will give birth is already 

 determined ; external conditions such as the season, kind of vine, and 

 others not yet known, exercise, on the other hand, an influence on the 

 mother, in the sense of changing the relation between the number of 

 eggs that will develop in one particular way or the other. It may be 

 stated as a general fact that neogallicolae-gallicolae are born in greater 

 numbers when the vine is in a state of active growth, while neogalli- 

 colae-radicolae are almost exclusively produced when the season is 

 advanced and, in general, when the vine is no longer producing any 

 new leaves. By transporting artificially neogallicolae-gallicolae 

 from one plant to another, offspring may be obtained in which the 

 proportion between neogallicolae-gallicolae and neogallicolae-radicolae 

 differs from that encountered in the sister neogallicolae-gallicolae 

 remaining on the original vine. A neogallicola-gallicola never develops 

 wings ; therefore the winged forms, which observers have occasionally 

 found in galls do not represent the winged form of a gallicola, but are 

 merely winged radicolae developed in exceptional circumstances in 

 an environment not habitual to them. The parallel drawn by Frances- 

 chini between the root and leaf form, based on the supposition that 

 both could produce wdnged forms, and consequently sexual forms and 

 a winter-egg, is therefore inadmissible. Neither, as Del Guercio has 

 asserted, do the gallicolae produce alate forms after the first generation, 

 nor do those producing radicolae live on the roots of every sort of vine. 

 Artificial methods have been used both by the author and Borner, by 

 means of which galls may be produced in any locality, where, in spite 

 of many years infection by Phjlloxera, they have not previously existed, 

 and this, without introducing the insects from outside. It suffices, 

 for example, to develop from nymphs a quantity of the winged insects 

 in a hothouse where receptive American vines, with sufficiently large 

 stocks, are growing. There is therefore no ground for attributing 

 the capacity for forming a gall to certain varieties only of Phylloxera. 



The product of the winter-egg on European vines is almost 

 always destined to perish. The winter-egg is the only egg laid 

 by the sexual forms, and the sexual individuals can only be 

 produced by the winged insects ; it follows that the winged insects 

 also, contrary to the belief of most authors, may be considered non- 

 existent as regards the spread of Phjlloxem on European vines. The 

 winged insects are, however, dangerous as regards spreading when it is 

 possible for them to lay their eggs on American vines which are also 

 capable of bearing galls. Recollecting, therefore, that the winged 

 insects are either almost entirely male-producing or entirely female- 

 producing, a single casual visitor to a vine, even if this latter be recep- 

 tive, cannot be capable of infecting it. The sterility of the alate forms 

 is probably governed by conditions of environment which may exist 

 in some vears and not in others. Their absolute numbers must be 

 large, though their percentage be small. The view that the alate 

 forms are of two kinds is erroneous ; all are sexuparous. Individuals 

 resembling nymphs (ninfali) occur which closely resemble the apterous 

 forms, but are nevertheless distinct from them ; their eggs develop 

 like those of apterous radicolae and do not give rise to sexual 

 individuals ; there is thus no possibility of a sexual generation 

 independent of the winged insects. These " ninfali " constitute a 



