8 Control of the Greenhouse White Fbf 



on 37 days and on 23 of these they were close together. This repetition 

 appears to be unnecessary as in one case when coitus occurred on 

 January 29 and the male died on February 3 the still isolated female 

 continued to oviposit until March 19 and the effect of the fertilisation 

 was evident to the end in the sex of her offspring, the eggs laid from 

 January 30 to March 2 producing 82 females and 21 males and those 

 laid from March 2-19 producing 41 females and 12 males. 



(4) Parthenogenesis. Morrill first noted parthenogenesis in Aleyro- 

 didae and in conjunction with Back (2) found that unfertilised eggs of 

 Ast. vaj)orarioru7n, Al. citri and Al. nubifera always gave rise to male 

 offspring. Hargreaves(3), working in England, found just the reverse, 

 and bred two generations of females of Ast. vaporariorum in the absence 

 of males and states "out of the hundreds of flies that I examined I did 

 not encounter a single male." Williams (4) found several colonies of the 

 insect in England consisting entirely of females or in which this sex 

 largely predominated. In his experiments in breeding he obtained from 

 mated females small families in which the sexes were equal but none of 

 the offspring of his virgin females reached maturity. This author and 

 later Schrader(5) discuss the genetics of the insect and the suggestion 

 is made that the parthenogenetic female producers in England may have 

 arisen by mutation from the American parthenogenetic male producers 

 of America and that the occurrence of some males in England may be 

 due to fresh importations. 



In the greenhouses in the Lea Valley the sexes occur in approximately 

 equal proportions, counts giving in July out of 305 insects a male per- 

 centage of 52-8 and in October out of 118 a male percentage of 46-6. 

 The breeding experiments show that the strain agrees with the American 

 race in this important point, as the data in Table II show. Five virgin 

 females produced offspring totalling 267 all of which were males. Seven 

 mated females produced offspring also totalling 267 and of these 91 

 (34 per cent.) were male and 176 female. 



5. DEVELOPMENT. 



(1) Egg. The eggs (Fig. 2) (Plate I, fig. 1) are laid in circles on smooth 

 leaves, but on hairy leaves like those of the tomato they are scattered 

 in groups. A firm attachment to the leaf is gained by means of a short 

 stalk which rests in a cut made by the female. Like all the other stages 

 of the insect they are covered with wax. At first they are greenish 

 yellow in colour but darken in two or three days in warm weather and 

 during the greater part of the incubation period they are quite black. 



