THE TURNIP-APHIS 155 



when these small flies were everywhere to be found, getting 

 into people's eyes and mouths, and covering the trees and 

 fields from London to Yorkshire. 



Late in autumn, but at no other season, male apMds are 

 -found in small numbers. They are always winged, smaller 

 than the winged females, and rather different in colour ; they 

 perish immediately after pairing. Shortly after the males 

 appear, a new kind of wingless female may be observed, 

 which is distinguished by a sabre-shaped ovipositor projecting 

 from the end of the abdomen. This is the oviparous female, 

 of which only one generation is formed in a season. It lays 

 fertilised eggs, which remain undeveloped till the following 

 summer, when they give rise to a new succession of viviparous 

 generations. The individuals of a season are therefore of four 

 kinds — viz. 



(i) Viviparous wingless females, 



(2) Viviparous winged females, 



(3) Oviparous wingless females, and 



(4) Winged males. 



The viviparous females really produce eggs in ovaries, like 

 other insects, but the eggs, instead of being laid before develop- 

 ment, are hatched in the egg-tubes of the parent. Like the 

 males of hive-bees and the males of saw-flies, the summer 

 generations of aphis are produced parthenogenetically — that is, 

 without the concurrence of a male. The young are from the 

 first almost identical with the full-grown aphis except in size 

 and colour. A single female will often produce two a day, 

 and sometimes more, for many days together. The young 

 become capable of reproduction in from four to eight days, 

 according to the temperature of the air; a hot, dry season 

 being eminently favourable to their rapid propagation. 



Many circumstances in the life of an aphid are apparently 

 due to the extraordinary abundance of its food at a particular 

 part of the year. Then everything is made to contribute to 

 the rapid production of vast numbers of individuals. The 

 slow processes which are necessary to the production of 

 fertilised eggs are dropped; and young may be produced 

 viviparously by aphids only a few days old, which do not 

 cease feeding even at the instant of birth. For some reason 

 that we cannot guess at, the eggs which have to endure the 

 long cold of winter must be fertilised, as in Hydra, Daphnia, 



