126 THE INSECT WORLD. 
of 590,490,000, will yield a progeny of 53,142,100,000; at the 
seventh, we shall thus have 4,782,789,000,000; and the eighth 
will give 441,461,010,000,000. This immense number increases 
immeasurably when there are eleven generations in the space of a 
year. Fortunately a great many carnivorous insects wage fierce 
war against the plant-lice, and destroy immense numbers of them. 
Thus they are kept in check, and prevented from multiplying 
inordinately. To show with what prodigious abundance the re- 
production of these little but formidable parasites must go on, we 
will relate a fact which was made known to us by M. Morren, 
Professor in the University of Liége. 
The winter of 1883—34 had been extremely warm and dry ; 
whole months had passed without any rain. A well-known savant, 
Van Mons, had foretold, as early as the 12th of May, that all the 
vegetables would be devoured by plant-lice. On the 28th of 
September, 1834, at the moment when the cholera had began to 
spread its ravages over Belgium, all of a sudden a swarm of plant- 
lice showed themselves between Bruges and Ghent. They were 
to be seen the next day at Ghent, hovering about in troops, in 
such quantities that the daylight was obscured. Standing on the 
ramparts, one could no longer distinguish the walls of the houses 
in the town, so covered were they with plant-lice. The whole 
road from Antwerp to Ghent was rendered black by innumerable 
legions of them. They appeared everywhere quite suddenly. 
People were obliged to protect their eyes with spectacles and their 
faces with handkerchiefs, to keep off the painful and disagreeable 
tickling caused by them. The progress of these insects was inter- 
rupted by mountains, hills, even by undulations of land of very 
slight elevation, but sufficient to have an influence on the wind. 
M. Morren thinks that they came from a great distance, and that 
.they arrived in Belgium by the sea-coast. Whatever be the 
explanation of the phenomenon, it establishes sufficiently the 
prodigious multiplication of these little insects. 
There is another trait, and without doubt the most curious in 
the history of the aphides, to which we have still to call the atten- 
tion of the reader: we mean the relations which exist between 
them and the ants. 
No one can have failed to observe ants frequenting those places 
