Walter Ritchie 337 



between August 3rd, 1919, and August 12th, 1919, but up till now, 

 December 11th, 1919, none of the eggs have hatched. 



From experiments and from my observations made on eggs and 

 larvae in their natural habitat, the length of the life cycle of S. carcharias, 

 in Scotland, is about four years, a very considerable part of this time 

 being passed in the over-wintering egg-stage. In warmer conditions 

 than in Scotland, the period of the life cycle is shorter; for example, 

 continental writers state that in Central Europe the typical length of 

 the life cycle is three years. 



As an illustration of the influence of temperature on the length of 

 the life cycle, I may say, that from some pieces of stem cut down in the 

 open on January 25th, 1919, containing fully-grown larvae ready to 

 pupate, and kept under laboratory conditions, adults emerged during 

 the first days of May, 1919, while in corresponding material left in the 

 open, the larvae only reached the pupal stage on May 23rd, 1919, the 

 adult stage on July 8th, 1919, and emergence followed on July 20th, 1919. 



Host Trees. 



In the areas in Aberdeenshire, where my observations were made, 

 $. carcharias is attacking one species of poplar, namely, Populus tremula 

 Linn. 



Adults, kept in the laboratory and offered the leaves of various 

 species of poplar fed willingly on all of them. Further, in summer, 1919, 

 females in captivity readily laid their eggs on pieces of stem of the 

 Black Italian Poplar (P. monilifera Ait.). 



In the following list, the poplar species attacked by S. carcharias 

 are collated from the works of the continental authorities named below. 

 Ratzeburg(iO) records S. carcharias on Black Poplar; Altum(ii) states 

 that it attacks Canadian, Black and Trembling Poplars, and also 

 willows; Schiodte (12) says that the larvae are found on Populus moni- 

 lifera, P. ontariensis, P. tremula and on willows. Kaltenbach(i3) names 

 as host trees, P. nigra, P. dilatata and P. tremula; Tachenberg(i4) names 

 Black Poplar, Trembling Poplar, Italian and German Poplars, also 

 willows. Judeich and Nitsche(i5) state that the species is found on all 

 poplars but most commonly on the aspen (P. tremula Linn.) ; Nusslin(iti) 

 confirms this statement and adds willows as host trees. A more recent 

 record is a Spanish one, S. carcharias having been found on P. nigra in 

 the province of Gerona(i7). 



22—3 



