COLORATION IN POLISTES. 35 



few cases are they as wide as in the modal condition of variatus. In 

 many of the more melanic specimens the metathorax is entirely black. 

 Clypeus. — This, as always, is uniformly yellow in the male. In the 

 female it offers all the variations from a uniform reddish ferruginous, 

 through ferruginous with a central black spot of varying size, to an 

 entirely black clypeus. In only 4 specimens does yellow appear, and 

 here at the apical angle. Where the amount of ferruginous in the rest 

 of the body is large, the clypeus is prevailingly ferruginous ; where 

 small, the clypeus is prevailingly black. 



The similarity between the color markings of these two species does 

 not necessarily indicate that they should accordingly be considered one 

 species. This is apparent from the following considerations : Toward 

 the south we find the varieties of P. pallipes just described merging 

 into a lighter series of forms, which, however, differ markedly from 

 , the lighter forms of variatus found in Wisconsin. The fact of this 

 convergence of two distinct species toward a common form along two 

 different lines has an important bearing on the question of the influ- 

 ence of climatic conditions on the species and the various lines of 

 migration which they have taken. This will be considered in detail 

 in the following subdivisions. 



PoivISTES VARIATUS AND POLISTES AURIFER. 



Related to P. variatus through its xanthic trend is the species P. 

 aurifer, which is most sharply differentiated from the former in Cali- 

 fornia, but whose representatives in Colorado are easily confused with 

 those of P. variatus (see PI. Ill) . We have already seen how the latter 

 species might pass into P. auri/er, and it would be interesting to learn 

 just what degree of correspondence exists between the nest variations 

 of the two species. Unfortunately no large series of variations for P. 

 auri/er have been available. Through the kindness of Dr. Heath, of 

 Iceland Stanford Junior University, I am in possession of one brood of 

 P. auri/er irora. San Jose and a number of broods of Polybia flavitarsis. 

 The last named is a species of social wasp occasionally found in south- 

 eastern and southwestern United States, which in its type of marking 

 closely resembles P. atiri/er. 



Nest variations in P. auri/er. — This series of 20 specimens of nest 

 variations of auri/er is interesting by comparison with representatives 

 of the species occurring to the north and south of San Jose. They 

 are materially lighter than the former and darker than the latter. 

 They also show some slight variations among themselves. 



The lateral spot of the second abdominal segment is always large, 

 quadrangular, with the axis in the main axis of the body and its pos- 

 terior edge broadly confluent with segmental border (PI. Ill, fig. 33). 



