48 COLORATION IN POUSTES. 



some other alkali, the reverse series of color changes sets in, the color 

 passing through increasingly darker shades until the original blackish 

 hue is again reached. 



Boiling with nitric acid any of the species in which the pigment is 

 lighter in shade than it is in P. variatus (such as P. flavus, where the pat- 

 tern is in varying shades of cinnamon brown and yellow, or P. lineatus, 

 which is prevailingly red-brown) causes the same succession of colors, 

 beginning in every case at the stage corresponding with the adult 

 coloration. Thus the pale brown chitin of flavus passes through yel- 

 low and pale yellow before becoming transparent, and the red-brown 

 lineatits through reddish orange, orange, yellow, and pale yellow. 



Conversely, if the light cinnamon-brown skeleton of P. ftavus be 

 boiled in potassic hydrate, the pigmented areas approach the dull- 

 brown coloration characteristic for the more melanic species ; but the 

 color is not so dark, since the pigment in P. flavus is less concentrated. 

 Similarly the light red-brown areas of P. Hneatus may be changed 

 to dark red-brown and finally to black by boiling with ammonium 

 sulphide. 



Tests were also made on the nature of the yellow hypodermal pig- 

 ment. If this layer of pigment be scraped off and boiled with dilute 

 potash it becomes dull brown in color. If dried skeletons of wasps 

 whose color pattern is varied with yellow be boiled with (NHJ 28 and 

 then carefully dried again, the areas which were originally yellow will 

 be found to have changed to a deep red-brown. 



The foregoing reactions would indicate that the various colors dis- 

 played by Polistes are related to one another by slight differences in 

 chemical composition ; that the darker ones are derived from the 

 lighter by a process of reduction, and the lighter from the darker by a 

 process of oxidation. 



Further, the ontogenetic stages would indicate some such relation. 

 In P. variatus I have sometimes observed a uniform pale yellow con- 

 dition before the darker pigment began to appear in its characteristic 

 areas, and P. rubiginosus passes through a yellow, orange, and reddish- 

 orange stage before assuming its final red-brown color. 



The manner in which the pigment is deposited in the chitinous lay- 

 ers bears out this idea. I have as yet made no attempt to derive the 

 yellow hypodermal pigment from the pupal haemolymph ; but it is 

 undoubtedly so derived, though probably indirectly, through the inter- 

 vention of the hypodermal cells. Moreover, it has been shown by Pap- 

 penheim (19) that in all series of color compounds yellow has always 

 the simplest chemical composition, and that the color deepens through 

 orange to brown and black, or through green to red and violet with 



