RAVAGERS OF CROPS 239 



that warm weather and a plentiful food supply 

 are conditions which result in the production of 

 parthenogenetic females. The return of autumn, 

 bringing with it a lower temperature and a 

 growing scarcity of food, heralds the birth of 

 winged males and egg-laying (oviparous) females. 

 Sexual intercourse takes place, and the winter 

 eggs, from which the first generation of vivi- 

 parous and parthenogenetic females will emerge, 

 are deposited. 



The offspring of the viviparous females at 

 the time of their birth closely resemble their 

 parents in general appearance ; they do not 

 pass through any regular metamorphosis, but 

 at once begin to feed, cast their skins once 

 or twice, and within a few days of their own 

 entry into the world are ready to give birth to 

 young. As under normal conditions viviparous 

 female Aphides live for four or five weeks, it is 

 possible for the parent insect to find itself sur- 

 rounded, not only by its children, but by its 

 grand-children, great-grandchildren, and great- 

 great-grandchildren. Reaumur estimated that 

 one female Aphis may be the progenitor of no 

 less than 5,904,900,000 individuals during the few 

 weeks comprising the period of her existence. 

 Professor Huxley brings home to us even more 

 clearly the extraordinary fecundity and the 

 astonishing rapidity with which the Aphides 

 increase. Assuming that one thousand Aphides 

 weigh collectively about one grain avoirdupois 

 (and this is rather under- than over-estimating 

 their weight), and that only a very stout man 

 can weigh as much as two million grains, or 



