190 Annals Entomological Society of America [Vol. XV, 



the diagram representing curves for the total number of pores 

 of all five groups. In order to plot these curves at all it was 

 necessary to place the insects in classes with respect to the 

 number of pores, for example, those having 65-G9 pores, 

 70-74, 75-79, etc. The curve for the apple form shows that 

 the total number of pores for an individual is most commonly 

 70-74. In other words, the mode of the apple form is 70-74, 

 while that for the lilac form is 100-104. Only five insects of 

 the 220 examined from lilac had as few as 74 pores, while not 

 a single apple insect was found to have 100 pores, and only 

 three had as many as 90. 



A study has been begun of the pygidium of the second 

 instar. This pygidium resembles, in many ways, that of the 

 adult insect, though of course, it lacks entirely the circum- 

 genital pores. The dorsal and marginal gland openings can 

 be distinctly seen, there being eight marginal gland openings 

 (four on each side) in both the apple and lilac forms. Sufficient 

 material was not available for a careful study of the dorsal 

 gland openings, but all the specimens examined showed more 

 of these openings present in the lilac than in the apple form. 

 In the lilac form they varied from 14-18, while in the apple 

 they ranged from 8-12. It is planned to secure during the 

 coming summer, material for a more careful study of the gland 

 openings of the pygidium of the second instar. 



HOST PLANTS OF THE TWO FORMS. 



Examination of the scales and counts of the circumgenital 

 pores show that insects found on the following host plants 

 belong to the apple form: 



Red dogwood (Cornus alba). 

 Alternate leaved dogwood (Cornus alternifolia). 

 Round leaved dogwood (Cornus rugosa). 

 Mountain maple (Acer spicatum). 



The biological development of the insects on red dogwood, 

 which has been studied, paralleled that of the apple form. 

 The lilac form was found on: 



American ash (Fraxinus americana). 

 European ash (Fraxinus excelsior). 

 Fringe tree (Chionanthus virginica). 

 Golden current (Ribes aureum). 

 Laurel leaved willow (Salix pentandra). 

 Heart leaved willow (Salix cordata). 



