Ll. LliOYD 



9 



They are almost invariably placed on the undersides of the leaves. 

 As indicated above the incubation period may be very prolonged in 

 cold weather outside. The varying incubation periods were recorded on 

 a series of plants from December to August and the results are summarised 

 in Table III. The longest period observed outside was 117 days and the 

 shortest 13-16 days in August, mean temperature 58° F. The incubation 

 period outside in Augu>st is little more than half that recorded in the 

 greenhouse in December, though the mean temperature in the latter 

 case was two degrees higher and is approximately the same as that in 

 April under glass with a mean temperature of 67°. This seems to show 



¥k 



Egg of A. vuporariorum ( x 350). 



that sun heat is more stimulating than artificial, the sunshine hours in 

 the three months being: December, 27; April, 87; August, 158. 



(2) Scale. The characteristics of the four scale stages are described 

 by HargreavesO). The first larva (Fig. 3a, h) moves about on the surface 

 of the leaf but usually only a sufficient distance for it to grow without 

 coming in contact with others from the same batch of eggs. The move- 

 ment was usually confined to a few hours and once only was one seen 

 to move the day after hatching. On one occasion a larva was seen walking 

 on the stem of a plant, a very small Trifolium pratense, the leaves of 

 which were overcrowded with scales. When cut foliage heavily infested 

 with hatching eggs was placed on the soil around the stems of Urtica 



