88 



J. DAVIDSON. 



During the autumn, when the intermediate hosts are mostly unfavourable for tlie 

 Aphids, the winter hosts, such as Euonymus, offer the most favourable food con- 

 ditions and the greatest chances of survival. From the nature of Rtimex and other 

 intermediate hosts under winter conditions and the difficulty in ensuring food for the 

 young larvae immediately they hatch out from the eggs in spring, these cases, I think, 

 must only be considered as casual winter hosts. The sexual forms may develop in 

 several generations. In fact, in one series of experiments under favourable conditions 

 of food and temperature, sexual forms appeared in five succeeding generations. 

 Weather conditions and the dying down of the intermediate hosts are very important 

 factors in limiting the length of period over which sexual forms are produced. These 

 questions will be discussed more fully in a later section dealing with the appearance 

 of the sexual forms. 



The life-history of Aphis rumicis may be illustrated by the following diagram :— 



^K-. 



'^, 



\<^ 



<1> 



\ 



-v 



V/ 



cv 



© = Oi/'a. §■ = Fundatrices. % —Apterous viviparous females. O ^ Winged viviparous females. 

 ® = Migrantes. O = Alate sexuparae (Remigrantes). = Sexual males. Q = Oviparous females. 



Fig. 6. Diagram illustrating the life-cyle of Aphis rumicis. The line 

 AB divides the circle into two halves, the winter host being represented 

 on the left and the summer hosts on the right half ; the sectors indicate 

 the months, and the dotted concentric lines a varying number of generations 

 occurring in July and August. 



