108 
Proceedings of the Royal Society of Edinburgh. [Sess. 
The fate of the viviparous forms that remain on Labiatae after the 
appearance of the sexuales is uncertain, but granted mild weather and 
sufficient/ food, there is no reason why they should not continue to 
reproduce viviparously through the winter. Several Aphidince are known 
to be able to do this. For instance, Webster and Phillips (28, p. 93) have 
shown that in the mild winter of the Southern States of America, Toxoptera 
graminum seldom produces sexual forms, but multiplies parthenogeneti- 
cally throughout the year. Borner (1) has shown that the viviparous 
generations of Aphis pruni, Hyalopterus trirhodus, Rliopalosiphum 
lactucce, etc., may live far into, or even survive, the winter ; and Mordwilko 
(20) says the same of Pemphigus affinis. Probably this is rare in our 
climate, for although aphides can stand a good deal of cold, prolonged 
freezing kills them, as is shown by the experiments of Webster and 
Phillips (28, p. 92). 
Thus the persistence of the viviparous line after the production of the 
sexuales is really dependent on accidents of temperature. The whole 
viviparous cycle indeed may be compared biologically to the soma of the 
individual animal. It exists merely as a vehicle of the germ plasm, and 
when the future of the species has been ensured by the production of the 
zygote, it may persist for a time under favourable conditions, but sooner 
or later disappears. 
Natural Enemies. 
More than a hundred and fifty years ago, Bonnet wrote : “ Like as we 
sow grain to provide for our own subsistence, so it appears that nature 
sows plant-lice on all kinds of trees and plants for the nourishment of 
multitudes of different insects.” 
M. ribis has a large share of natural enemies, both predaceous and 
parasitic. To the former class belong the larvae of certain Cecidomyiidae, 
which devour numbers of the aphides from June onwards. Theobald (27) 
points out that Syrphid and Coccinellid larvae seldom attack this species ; 
but I once took two larvae of the lace wing ( Ghrysopa ) from a red blister. 
Individuals of all ages are persecuted by a mite, Anystus cornigera, Koch, 
though there is no evidence that it ever causes the death of its host. In 
July, the fungus Empusa aphidis, introduced into the laboratory on some 
aphides taken from Galeopsis in the field, was very prevalent for a time on 
the generation series on Labiatae. Those on currant suffered very little, 
though kept under identical conditions, and I have seldom found aphides 
on currant killed by this fungus in the open. 
But the principal enemy of M. ribis is a small parasitic Braconid, 
