22 COLORATION IN POUSTES. 



are to suppose that either of these species has arisen from P. variahis, 

 or if, on the contrary, the characters of the latter are the result of the 

 mixing of two distinct species, P. pallipes and P. aurifer, it is first of 

 all of interest and importance to find whether there is anj' correlation 

 between the characters of the dorsal surface of the abdomen and the 

 character of the rest of the body. The most important differences 

 between these three species relate to the markings of (i) the abdo- 

 men, (2) the clypeus, (3) the metathorax, and (4) the appendages. 

 Accordingly the following correlations have been worked out: Between 

 (i) the dorsal and ventral sides of the abdomen, (2) size of metameric 

 spot and width of border of segment, (3) size of metameric spot and 

 markings of clypeus, (4) size of metameric spot and markings of meta- 

 thorax, (5) size of metameric spot and markings of the appendages. 



Here again it has not been considered practicable to apply the 

 methods in use for the statistical study of correlation. The surfaces 

 are in most cases so minute and difficult of measurement that it would 

 be almost out of the question to secure and manipulate the data for so 

 large a collection. Moreover, the results aimed at are relative rather 

 than absolute, so for our conclusions we will rely rather upon the evi- 

 dence from the comparison of classes belonging to fixed types than 

 upon mathematical expressions derived by actual measurement and 

 calculation. The method will develop as we proceed. 



CORRELATION BETWEPON DORSAL, AND VENTRAt, SIDES OF ABDOMEN. 



In this determination the males and females are considered sepa- 

 rately, since the type of marking for the under side of the abdomen 

 in the two is quite different. In the former the yellow of the ventral . 

 side preponderates over all other colors. The first segment is wholly 

 yellow or marked with one or two yellow spots of varying size. The 

 second segment has a yellow border greatly dilated medially, and a 

 yellow- ventral mark which varies from a single or a double dot to a 

 large rectangular patch. At the two posterior angles of this rectan- 

 gle there often appears a brown dot to correspond with the metameric 

 spot of the dorsal surface. The borders of the segments following are 

 each greatly dilated medially to form a triangle, the depth of which 

 increases as we pass backward. Plate I shows the stages by which 

 the ventral surface of the males passes from the condition correspond- 

 ing most closely to the modal condition of the dorsal surface to a more 

 or less melanic condition, and fig. 10 the proportion of specimens 

 in the various classes. From this it appears that the modal condition 

 for the males is decidedly more xanthic (^') for the ventral than for 

 the dorsal surface of the abdomen (cf. </', figs. 7 and 10). 



