56 



COLORATION IN POLISTES. 



anism as we go northward along the Alantic and the Pacific coasts. 

 On the Atlantic side the light red-brown carolinus type passes insen- 

 sibly into the lightest members of the pallipes group, and this group dis- 

 plays an increasing melanism as we proceed, reaching its culmination 

 in the uniform dark-colored pallipes of New England. 



On the Pacific side the tendency is illustrated in the increase in the 

 amount of black in the species aurifcr, which finally in Washington 

 merges into the variety of variatus characterized by narrow yellow 

 borders and small metameric spots. 



This melanic tendency is discussed in detail in the following sections. 



RELATION BETWEEN LATITUDE AND AMOUNT OF DARK PIGMENT IN POLISTES. 



In this determination use was made of three lots of ico specimens 

 each, collected at random at Gotha, Fla. ; Willow Grove, Pa., and 

 Cold Spring Harbor, Long Island, N. Y. This material, which 

 probably represents three or four different species, falls into the fol- 

 lowing classes with respect to the second abdominal segment : 



Class o, segment light red-brown with a yellow margin. 



Class I, segment light red-brown, yellow margin obscure. 



Class 2, segment red-brown with black triangle at base. 



Class 3, segment fuscous (nearly black), red-brown in the form of 

 large oval spots. 



Class 4, segment fuscous (nearly black), red-brown spots becoming 

 obsolete. 



Class 5, segment black, some yellow still present at the margin. 



Class 6, segment entirely black. 



The distribution of the material from the three localities in these 

 classes is as follows : 



The curves on fig. 18 represent the condition graphically. From 

 this it is apparent that there is a steady increase in the proportion of 

 melanic specimens as we pass northward. The change does not affect 

 all the specimens equally, but the mode is shifted one stage toward the 

 extreme of melanism for each collection from Florida northward. 



