Studies on Eggs of Apple Plant Lice 11 



So far as is known, no egg hatched under out-of-door con- 

 ditions without splitting its outer semi-transparent layer at least 

 48 hours before the nymph emerged. The above 47 per cent of split 

 eggs observed on March 25 probably increased to 53-55 per cent be- 

 fore all the eggs hatched, at least other checks showed a total of 53 to 

 55 per cent of eggs with split outer coverings. This 53-55 per cent 

 of eggs with split outer coverings shows that this percentage of eggs 

 was normal, alive and preparing to hatch, but during the past 

 season an average of 29 per cent of all the eggs actually hatched. 

 In other words, 24 to 26 per cent of the eggs after spHtting their 

 outer covering were unable to complete the process of hatching. 



Eggs of A. avence (200 collected from J. L. Lippincott Co.) 

 were brought into the laboratory on February 26, 1918, and kept in 

 a moist chamber (70° F.). The percentage of split eggs increased 

 with great rapidity because of the increase in temperature over 

 out-of-door conditions: on February 26, ]0 per cent split; Feb- 

 ruary 27, 23 per cent split; March 1, 25 per cent split, and 1 per 

 cent hatched ; ]\Iarch 6, 33 per cent split and 10 per cent hatched ; 

 March 11, 20 per cent split and 29 per cent hatched; March 13, 

 9.5 per cent split and 41 per cent hatched ; and March 15, 7 per cent 

 split and 43.5 per cent hatched. There was no increase in hatched 

 or split eggs after ^larch 15. The minimum period of time between 

 the splitting of the outer covering and the emergence of the nymph 

 under indoor conditions was not observed ; however, so far as known 

 all eggs split their outer shell at least a few hours before emergence. 



In these indoor experiments only 28 per cent of the eggs showed 

 a split outer covering when the first nymphs emerged. This means 

 that only 55 per cent of the live eggs which were preparing to hatch 

 had split their outer shell before the first nymphs appeared. This 

 is quite different from the 95 per cent among the live eggs kept 

 out-of-doors. Temperature, therefore, has a marked influence on 

 the rapidity with which the nymph will emerge after the outer shell 

 is broken. In the above indoor experiment it was also observed 

 that 50.5 per cent of all the eggs split their outer shells and that 

 43.5 per cent of all the eggs hatched. In other words only 7 per 

 cent of the eggs with split outer coverings failed to hatch, while 

 24 to 26 per cent of the eggs with split outer shells failed to hatch 

 when kept out-of-doors. The explanation for this decided difference 

 is probably due to differences in moisture (evaporating factors) as 

 explained further on. 



The splitting of the outer layer is probably due to the pressure 

 exerted along the meso-dorsal line by the developing embrj^o or 

 nymph. It is also possible that certain secretions may help to 

 dissolve or weaken the brittle outer layer. A careful histological 

 study may throw some light on this question. All eggs with a split. 



