326 SCIENCE FROM AN EASY CHAIR 



growth of galls on the leaves, and also on the rootlets 

 of many plants, and often the leaves become rolled 

 up into bag-like bodies filled with aphides. Many trees 

 and smaller plants are killed by these attacks, but it is 

 probable that "where the plants have not been rendered 

 delicate by nursing and cultivation, and where the aphides 

 are not a strange foreign kind, introduced by man's care- 

 lessness or by some rarely (if ever) occurring wind or 

 flood, the aphides do not actually destroy any plants by 

 their visitation, excepting the weaklings, and that their 

 numbers are kept within bounds by their natural enemies 

 the lady-birds and other such carnivorous insects. 



We must now notice the most interesting of all the 

 wonderful things which have been discovered about these 

 tiny insects, which are even smaller than fleas. Any one 

 who has a rose-garden and chooses to spend some hours 

 a day in studying the "green-fly" can follow out the 

 facts. They reproduce themselves that is to say, pro- 

 pagate with astounding rapidity. The great Linnaeus, 

 a hundred and fifty years ago, came to the conclusion, 

 from his observation of one kind or species, that in 

 one year a single aphis would produce a quintillion of 

 descendants ! Without insisting upon the exact numbers 

 in different kinds of aphides, we may say that that is a 

 fair indication of the rate at which they produce young. 

 No sooner does a mother aphis produce some thirty or 

 forty young, than in a few hours or days, according to 

 the warmth of the season and the abundance of food, 

 these young have grown to full size and themselves each 

 produce the same number of young, and so on through 

 the summer, and even into the autumn. Nineteen genera- 

 tions in sixteen weeks have been counted in some kinds 

 of the plant-lice. Hence it is no wonder that these little 

 creatures increase exceedingly and cover the leaves and 

 shoots on which they feed ; no wonder that they furnish 



