1922] Griswold: Oyster -shell Scale 187 



It will be noticed that while transfers were successful from 

 apple to other trees, in no case could insects of the lilac form 

 be made to live for any length of time on apple or pear. 

 Although the young larvae crawled on the apple and pear 

 twigs, in every instance they died while still in the first instar. 

 The scales made by the apple form on the new host plants 

 were always of the uniform brown so constant on the apple. 



DIFFERENCES IN MORPHOLOGICAL CHARACTERS. 



No differences have as yet been found in the pygidial 

 fringe of the apple and lilac forms. Several writers, however, 

 have called attention to the variation in the number of circum- 

 genital pores of the Oyster-shell Scale from different host plants. 

 For example, Cockerell (1895) mentions finding specimens on 

 dogwood in California that had fewer pores than are usually 

 noted on apple. Frank and Kruger (1900) give sample counts 

 of these pores from apple, plum, thorn, poplar, and willow. 

 Glenn (1920) also counted the pores from various host plants. 

 These observations suggested the desirabiHty of making 

 extensive counts of these pores from insects on apple and lilac. 

 Knowing that a study of the pores would be of little value unless 

 made from a long series of specimens, approximately 550 

 insects were mounted and their pores counted, 277 from apple 

 and 272 from lilac. Instead of putting the data in tabular 

 form, giving maximum, minimum, and average counts, varia- 

 bility curves were plotted such as are commonly used by workers 

 in genetics to show fluctuating variations. The information 

 desired was not the average number of pores in a group, but 

 the number of pores found to occur most commonly. 



The accompanying diagrams show these curves. The 

 abscissae indicate the number of pores found in the various 

 groups examined, while the ordinates indicate the proportion 

 of groups in which the varying numbers of pores occurred. 

 To make the curve for the posterior lateral groups of the apple 

 form, pores were counted in 496 groups. These pores were 

 found to vary in number from 6-24 in a group. Only one 

 group (0.20% of the total number of groups examined) had as 

 few as 6 pores and only one group had as many as 24 pores. 

 On the other hand, 72 groups (14.51%) had 11 pores, 100 

 groups (20.16%o) had 12 pores, and 80 groups (16.12%,) had 



