Studies on Eggs of Apple Plant Lice 15 



legs come next, starting with the prothoracic. The last appendages 

 to free themselves of the egg and exuvium are the metathoracic 

 legs and mouth-parts. A distinct skin is withdrawn from the beak. 

 This in part probably surrounded the maxillee and mandibles. The 

 process of hatching is apparently a difficult one, for many nymphs 

 are unable to free their various appendages, especially the meta- 

 thoracic legs which often remain attached to the white mass of 

 skin at the cephalic end of the open egg. Numerous nymphs are 

 thus killed in the very last stage of hatching. The above fate of 

 many a nymph may be due to the fact that the exuvium hardens 

 on long exposure to air and this makes it very difficult for the 

 nymph to remove the last pair of legs. 



The above detailed observations on the morphological struc- 

 ture of the egg and the behavior of the respective coverings during 

 the hatching period (February 15 to April 5 for A. avence) shows 

 conclusively that the egg is not a hard resistant body and that it 

 goes through a critical change previous to the emergence of the 

 nymph, which means that it is not as resistant during these changes 

 as in the dormant period. It is in the midst of these changes 

 or just previous to any visible change that certain control measures 

 may produce their greatest effect. Some eggs start to split 30 to 

 :>5 days before the first nymphs appear, but the largest percentage of 

 split eggs, 47 per cent or better, occurs at the time the first nymphs 

 emerge (only 29 per cent of A. avenca emerged this past season). 

 In brief, the percentage of eggs showing a split outer shell is pro- 

 gressive and this is an important point when one wishes to obtain 

 the best results with certain contact insecticides. 



Susceptibility of Eggs to ^Moisturb and Temperature 



Moisture 



A few experiments have been conducted with the eggs of 

 A. avence and A. pomi to determine the effect of different per- 

 centages of moisture on the hatching of the egg and the relative 

 permeal)ility of the two outer layers. The results of these few 

 experiments are very suggestive and undoubtedly have an im- 

 portant bearing on the percentage of hatch which will occur under 

 varying out-of-door conditions. They also throw considerable light 

 vm the problem of how certain contact insecticides may prevent 

 hatching. 



During 1917 experiments were conducted with different per- 

 centages of moisture i-n the incubators and moisture control ap- 

 paratus used by Dr. T. J. Headlee in his investigations on grain- 

 infesting insects; the writer wishes to express his appreciation for 

 the privilege of using this efficient apparatus. The four incubators 



