HOST-SELECTION 295 



on that plant, or even a greater preference for it, than their 

 parents or controls of the same generation derived from privet- 

 fed stock. In a later paper (Sladden, 1935) this conclusion is 

 confirmed and the experiments show that the offspring of each 

 subsequent generation accepted ivy more readily than did their 

 parents, and even exhibited an increased preference for it. 



Harrison (1927) published the results of certain field experiments 

 with the gall-forming saw-fly Pontania salicis. He removed a 

 strain of this insect which is locally adapted to Salix phylicifolia 

 to a district where various species of Salix grew, and the insects 

 were allowed to oviposit unhindered in the open. They showed 

 a marked preference to adopt Salix andersoniana to the compara- 

 tive neglect of the very closely allied original host S. phylicifolia, 

 a feature that was particularly evident during two successive 

 years. In other experiments he found that a strain of the same 

 saw-fly from Salix andersoniana brought to another locality 

 where only Salix rubra was available became adapted to that 

 host. Later, when plants of S. andersoniana were established in 

 that locality in proximity to S. rubra the former species of willow 

 remained entirely free from the galls of this saw-fly. He further 

 mentions that the latter, in its reaction to a changed food-plant, 

 shows corresponding colour differences. Mention needs also to 

 be made of Pictet's experiments (1911) with larvae of Lasiocampa 

 quercus, which feed upon deciduous trees and bushes. When 

 placed upon Pinus many of them died and those which survived 

 fed by biting into the parenchyma at the extremities of the pine 

 needles. In the second generation the insects became adapted 

 to the new diet to the extent that they either starved when 

 offered leaves of deciduous trees, or they only attacked the apices 

 of the leaves, which they hollowed out in a manner similar to that 

 performed by larvae of the previous generation. 



It will be gathered from these experiments that it is possible 

 for an insect to become rapidly adapted to a new food-plant to an 

 extent that it may partially or completely reject its original host. 

 It is reasonable, therefore, to conclude that similar behaviour may 

 occur in a state of Nature, particularly if the normal food-plant be 

 lacking. Harrison's experiments, already alluded to, indicate that 



