Davidson — Biological Studies of Aphis rumicis L. 319 



winged migrants over a great number of intermediate hosts is not an 

 essential part of the life-history, but must be regarded as an adaptation to a 

 search for more suitable food conditions. 



One cannot consider, therefore, that Aphis rumicis is a true migratory 

 species on which the influence of the intermediate host is an essential part 

 of the life cycle. If suitable conditions are available, the life cycle can be 

 passed through on either the primary or the secondary host. 



These conclusions, however, are largely based on the behaviour of Aphids 

 in captivity. It is obvious, from the nature of the primary host and the 

 temporary life of the secondary hosts, that the migration from the one to 

 the other at suitable seasons is decidedly of benefit to the Aphids under 

 natural conditions. It does not preclude, however, two very important 

 possibilities in the life cycle of Aphis rninicis in nature, namely, («) the 

 occurrence of agamic females, which may continue under mild winter 

 conditions, and carry on agamic reproduction in the following season ; 

 (b) the occurrence of sexual forms and winter eggs on plants other than 

 Euonymiis europaeus. These two possibilities may explain how, although 

 one known winter host {Euonymus europaeus) of this species has a very 

 localized distribution, the distribution of Aphis rumicis is a very wide 

 one. 



Unfavourable seasonal conditions over the period when sexuparae are 

 being produced would result in a heavy mortality of the agamic forms, a 

 reduction in the number of fertilized eggs laid, and a smaller outbreak of 

 Aphids the following season. 



It has been shown that on the approach of autumn, the influence of 

 some important factors results in the development of sexual forms irre- 

 spective of the number of the generation. There would appear to be 

 at work some inherent tendency associated with adaptation to seasonal 

 conditions. As autumn advances, an increased number of sexual forms 

 appear. 



S (? appeared on Euonymus Ej on 10. 8. 20, and (? ^ and sexual $ ° 

 were present on Euonymus Eg early in Octobei'. The sexual forms appeared 

 also on Beans Bj early in October. From September onwards an increasing 

 number of sexual forms developed. In Beans Bjg one agamic female continued 

 parthenogenetic viviparous reproduction, giving rise to cJ cJ , winged sexuparae 

 and two a. v. ° + . 



In my experiments the <J <J and oviparous + ? were produced by different 

 mothers, the winged sexuparae producing the oviparous females, and the a. v, 

 ° + (apterous sexuparae) producing the c? 6 ■ 



The closing of the life cycle by the development of sexual forms 



figiBNT. Pi;OC. K.D.S., VOL. XVI, NO. }ixy, 2 If 



