86 REPRODUCTION 



"summer eggs" of two sizes, and "winter eggs." The 

 summer eggs are thin-shelled and are not fertilized. The 

 larger ones develop at once and produce females. These 

 females may in rapid succession produce several 

 generations of parthenogenetic females. Later in the 

 season, these females lay the thick-shelled winter eggs, 

 which must be fertilized in order to develop. About this 

 time the males appear, hatched, seemingly, from the 

 smaller summer eggs. After fertilization, the winter eggs 

 lie dormant through the winter and develop, with the 

 return of warmth, into the females with which we began. 

 It has been found that external conditions, such as 

 temperature, state of the water, food, etc., have a part in 

 determining the kinds of eggs produced and the length 

 of the parthenogenetic generations. 



5. The Aphids, or Plant Lice. A condition similar in 

 many respects to the last is found in the aphids, a group 

 of small sucking insects that are parasitic on the tender 

 twigs, roots, and leaves of many plants. In the autumn 

 there are females and males, and the eggs are fertilized. 

 These eggs are attached to the twigs of the host plants 

 in crevices where they may be partly protected. In the 

 spring they hatch out into females, which in their turn lay 

 eggs that hatch without fertilization. All the offspring at 

 this time are females with this power of laying 

 parthenogenetic eggs. This may continue for ten or 

 twelve generations, giving rise to an unthinkable number 

 of offspring which may be a great tax on the host plant. 

 Finally, in the autumn, a generation is formed that 

 includes both males and females. These mate and the 

 fertilized eggs go over the winter, as described at the 

 beginning, until the following spring. In some aphid 

 mothers, the parthenogenetic eggs develop before they 

 leave the body and the young are thus brought forth alive 

 (vivipary). 



6. Parthenogenesis and Non-Sexual Reproduction. At 



