December, '17] PETERSON: aphid egg susceptibility 557 



By careful dissection one can distinguish three layers about the 

 embryo: an outer, semi-transparent, brittle (soft and glutinous when 

 the egg is deposited) layer, an inner, pigmented (glossy black), elastic 

 layer, and an innermost layer which is thin, transparent, and surrounds 

 the young nymph as it emerges. At the time of hatching the outer 

 brittle layer usually splits along the dorso-mesal line before the pig- 

 mented elastic layer is ruptured by the nymph. The time interval 

 between the spHtting of the outer layer and the severing of the inner 

 pigmented layer apparently varies with the temperature. This may 

 be two days under greenhouse conditions or eight days when the 

 temperature registers 30° to 40° F. All details in respect to the be- 

 havior of the layers about the egg during the hatching period have not 

 been observed, nevertheless, it is believed that the egg goes thru a 

 critical change a few days (possibly several weeks) before the njanph 

 emerges and that one important step during the hatching period is 

 apparently the splitting of the outer layer a short time before the 

 inner pigmented layer is ruptured. 



In brief the observations on the morphological structure of the egg 

 and the behavior of the respective coverings during the hatching period 

 shows that the egg is not a hard resistant body and that it goes thru a 

 critical change previous to the emergence of the nymph which undoubt- 

 edly means that it is not as resistant during these changes as in the 

 dormant period. The susceptibility of the egg and its lowered resist- 

 ance near the time of hatching are further substantiated by various 

 experiments with differences in moisture and in the use of certain con- 

 tact insecticides and other chemicals. 



Various experiments with the eggs of all three species under constant 

 temperature and different controlled moisture contents show that the 

 outer brittle layer is somewhat impervious to water under ordinary 

 a-tmospheric conditions and thus it acts as a protective layer to con- 

 serve the moisture content of the embryo. Under extreme dry condi- 

 tions, the outer layer is not carable of indefinitely retarding evapora- 

 tion, consequently the essential water content of the embryo is lost 

 and the egg shrivels. The inner pigmented layer is membranous 

 and does not conserve the water content of the embryo. This is con- 

 clusively demonstrated in one experiment where the outer layer was 

 removed from a number of normal eggs and they shrivelled completely 

 in twentj^-four hours under ordinary atmospheric conditions. The 

 innermost, thin, transparent skin about the embryo and young nymph 

 as it emerges does not help to conserve the water content of the em- 

 bryo, so far as known. 



The percentage of hatch of the eggs of A. avenoe and A. pomi, under 

 a constant temperature of 80° F., varies with the moisture content of 



