46 



the nympli as it emerges. Under out-of-door conditions the outer 

 layer of the egg usually splits along the dorso-mesal line a number of 

 days (2 to 30 or more for A. avenae) before the nymph emerges. So 

 far as observed under greenhouse conditions, the eggs of all three 

 species split their outer covering at least a few hours before the pig- 

 mented layer is severed. In 1918 the first eggs of A. avenae with 

 split outer coverings were seen on 15th February, and when the first 

 nymphs emerged, on 21st March, approximately 95 per cent, of the 

 normal living eggs (45-50 per cent, of the eggs were dead) had split 

 their outer semi-transparent covering. 



These observations on the morphology and behaviour of the egg 

 coverings show 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. It is in the midst of this critical period that the egg 

 is most susceptible to evaporating factors and certain contact 

 insecticides. 



The outer semi-transparent layer of the egg is somewhat impervious 

 to water ; consequently, the water content of the embryo does not 

 undergo very much evaporation in moist weather, or in other words, 

 when low evaporating factors exist, such as high humidity, low 

 temperature and probably small wind velocity. The outer layer, 

 however, is not entirely impervious, for extreme drought will cause the 

 vast majority of the eggs to shrivel and fail to hatch. In other words, 

 low humidity, high temperature and probably air velocity undoubtedly 

 bring about a greater evaporation of the water content of the embryo, 

 and thus destroy the hving form. 



,iThe inner pigmented (black) layer of the egg is not an efficient 

 protector against evaporation. Numerous and varied experiments 

 in the laboratory and observations made on the percentage of hatched 

 eggs of A. avenae during the past two totally different seasons, 1917 

 and 1918, show conclusively the pervious nature of this layer. 



The eggs are most susceptible to evaporating factors and contact 

 insecticides during the latter part of March, or in other words, when, 

 the greatest number show a spht outer layer, and this occurs when 

 the first nymphs start to emerge. 



Experiments conducted in the laboratory under controlled percen- 

 tages of moisture and also experiments where similar eggs of 

 A. are^ae were kept out-of-doors during the critical period 15th February 

 to 31st March, and the especially important period 15th March to 31st 

 March, which was wet in 1917 and dry in 1918, show quite conclusively 

 that the percentage of hatched eggs is much higher in a low evaporating 

 environment than in a high evaporating medium. 



Contact insecticides probably prevent in several ways the eggs 

 from hatching. Substances such as lime-sulphur tend to harden the 

 outer layer so that the nymphs cannot emerge ; the same hardening 

 may occur through desiccation. Crude carbolic acid and cresols 

 soften and disintegrate the outer impervious layer and thus expose 

 the inner layer to evaporating factors. It is probable however that 

 the toxic effect upon the embryo is of greater importance. No 

 technique has at present been discovered for determining the pene- 

 trative ability of the various chemicals used. The best insecticides 

 and the suitable dates for their application are discussed. 

 [R.A.E., A, viii, 30]. 



