86 J. DAVIDSON. 



II. Life-History of Aphis rumicis. 



The following account of the life-history of this species is based upon extensive 

 breeding experiments and observations in the field. Experiments were carried on 

 during 1913 and the early part of 1914, and continued during 1920. 



The Aphids were reared from eggs on Euonymus, and transferred to broad beans 

 as the summer host, the winged remigrantes or sexuparae of the later generations 

 being transferred back to Euonymus. The plants were grown in pots in a large open 

 glasshouse and kept covered with muslin bags, and observations on the different 

 generations of the Aphids were periodically recorded. It is hardly practicable, for 

 reasons of economy^ to publish the observations made on the long series of plants 

 infected, but the records of certain plants will be given in later sections of these 

 biological studies. It is from the data derived from these observations, together with 

 contemporary observations in the field, that the life-cycle has been worked out. 



The ova hatch out in spring (March and April) on the winter host, Eiwnymtis 

 euyopaeus* giving rise to the larvae of the fundatrices. In 8-10 days after birth, in 

 favourable temperature conditions, the fundatrices become adult and begin to pro- 

 duce parthenogenetic viviparous young, which become adult about 10 days after 

 birth. This is the lirst viviparous generation, and it may consist of a mixed progeny 

 of both alate and apterous viviparous females, or of the latter only. The a. v. $ $ 

 produce on Euonymus the second viviparous generation, which may consist of w.v. $ $ 

 and a.v. $ $, the former being generally in the majority. The a. v. $ $ produce a 

 further generation, consisting either entirely of w.v. $ $, or of a mixed progeny of 

 w.v. $ $ and a.v. $ $, the w.v. $ $ being in the majority. 



There is a tendency for the a.v. ? ? of these early generations on Etionywus to 

 produce w.v. ? ? in the majority. These w.v. $ $ are the winged migrants from the 

 winter host to the intermediate or summer hosts, such as beans, poppies, etc. It is 

 owing to this tendency that the a.v. $ $ on Euonymus eventually die out, and as the 

 season advances the tree becomes free from the aphis. Some of the w.v. $ $ may in 

 some cases remain a short time on Euonymus and produce young on it, but owing to 

 the innate desire of the winged forms to migrate (a marked feature of the winged forms 

 when the Aphids are reared in captivity), they soon leave the Euonymus and fly 

 to the intermediate hosts. 



The generations of individuals from the fundatrices on Euonymus are the fundatri- 

 geniae, the a.v. ? $ being the fundatrigeniae apterae and the w.v. $ $ the fundatri- 

 geniae alatae. These latter are the winged migrants (migrantes), which fly to the 

 intermediate hosts, being the mothers of the succeeding viviparous generations. 

 The generations of individuals from the migrantes on the intermediate hosts are 

 called the alienicolae, the a.v. $ $ being the alienicolae apterae, and the w.v. $ $ 

 the alienicolae alatae. 



The intermediate hosts are numerous, but of the cultivated plants beans are specially 

 favourable, and afford the best stimulus to rapid reproduction. The migrantes give 

 rise to the first viviparous generation on the intermediate host, the individuals of 

 which are a.v. $ $. These produce a further generation, which may consist of a.v. $ $ 

 or a mixed progeny of w.v. $ $ and a.v. $ $. The former may remain a little time on 

 the same plant and produce young, or may fly to other plants, either of the same kind 

 or other hosts, and produce a further generation. My experiments show that the 

 tendency is for w.v. $ $ to produce a.v. $ ?, and for a.v. 2 $ to produce either 

 a.v. $ $ or a mixed progeny with a varying percentage of w.v. $ $. This would 



* The spindle-tree is undoubtedly a winter host of Aphis rumicis, but considering the local 

 distribution of Euonymus in Britain, it is highly probable that there are other winter hosts. 

 Gaumont (1913) found all stages on E. japonicus. Mordwilko (1907) found Aphis euonynii in 

 spring on Viburnum opulus at Bjclovesh. It was also found on Vihurnnin opuhts by Kaltenbach 

 {Aphis euonymi) and by Passerini {Aphis papaveris). It is evident that further research i*^ 

 greatly needed in order to ascertain the common winter hosts of this abundant Aphid. 



