122 THE INSECT WORLD. 



In 1866 M. Balbiani asserted that the plant-lice are herma- 

 phrodite, or of both sexes at the same time, which would explain the 

 facts observed by Charles Bonnet. But the anatomical proofs 

 appealed to by Balbiani in support of this idea are far from establish- 

 ing the existence of this arrangement of sexes among them. The 

 observations of Charles Bonnet produced profound astonishment 

 among naturalists, and, in this respect, 1743 may be considered as a 

 memorable year. 



The simple statement of the few experiments which he made, and 

 which we have cited, has sufficed to show how rapid is the multipli- 

 cation of aphides. A single female produced generally 90 young 

 ones ; at the second generation these 90 produce 8, 100 ; these give 

 a third generation, which amounts to 729,000 insects ; these, in their 

 turn, become 65,610,000 ; the fifth generation, consisting 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 reproduction 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 Liege. 



The winter of 1833 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 i2th 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 ot 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 interrupted 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 



