1922] 



Griswold: Ovster-shell Scale 



185 



DIFFERENCES IN THE SCALES. 



The new scales formed on apple are of a uniform brown, 

 while the new ones on lilac are traversed by distinct stripes or 

 bands. These bands, three or four in number, are light brown 

 in color and vary in width from those that are quite wide to 

 those that are mere strips. The old scales on apple are of a 

 very dark uniform brown, sometimes of a slight grayish tint. 

 The old scales found on lilac, on the other hand, are of a very 

 pale gray, sometimes almost white, and the bands can be 

 distinctly seen. These differences are uniform and constant 

 in all specimens. 



DIFFERENCES IN THE BIOLOGICAL DEVELOPMENT. 



During 1919, 1920, and 1921, careful observations were 

 made on the biological development of the insects on lilac. 

 In 1920 and 1921 the biological development of the insects on 

 apple was also studied. The differences in the development of 

 the apple and lilac forms are most clearly shown by means of a 

 table. 



It will be seen from the table that the apple form develops 

 from two to four weeks ahead of the lilac form. The cold 

 summer of 1920 seems to have retarded the development of 

 the insects on lilac more than of those on apple, for the lilac 

 insects were nearly four weeks behind those on apple in 

 beginning to lay their eggs. In 1921, however, there was a 

 difference of about two weeks in the development throughout 

 the entire season. 



DEVELOPMENT ON DIFFERENT HOST PLANTS. 



In connection with the study, of the biological development, 

 some transfer experiments were carried on in an effort to learn 

 if the apple and lilac forms are interchangeable as far as host 



