COLORATION IN POI^ISTES. 49 



an increase in molecular complexity. It might therefore easily be the 

 case that the chitin elaborated in the neighborhood of certain develop- 

 ing structures is influenced by the metabolism of these structures, so 

 as to vary slightly in its state of oxidation or reduction. 



So far, however, Ave have been able to learn nothing as to the nature 

 of the color compounds of this genus or of their possible relations to 

 known organic color compounds. A clue to this is, however, furnished 

 by the well-known glacial acetic acid test. 



If a mixture of one part sulphuric acid and four parts glacial acetic 

 acid be added to a fatty substance containing any of the yellow azo- 

 compounds, the whole heated to boiling, and well shaken, the acid 

 mixture will develop a beautiful deep wine color. This is a reliable 

 test for the minutest traces of the yellow azo-colors. When a little of 

 the hypodermal yellow of Polistes was treated in the way just described, 

 the characteristic color was well developed. The same was true when 

 the pupa of P. gencrosus in its orange-yellow stage was similarly treated. 

 Both colors also gave the sulphuric acid test for azo-colors ; that is, 

 the solution of the pigment in HCl or H2SO4 acquired a rosy tinge upon 

 the addition of NH^OH, and this deepened when further diluted with 

 water. 



The azo-color compounds are a group belonging to the benzene series 

 and exhibiting various shades of yellow, orange, brown, red, scarlet, 

 and indigo. Benzene azo-benzene, CgHsNjCsH^, is regarded as the pro- 

 totype of the group. Compounds of the type are produced by the 

 action of mild reducing agents, such as alcoholic potash, on the cor- 

 responding nitro-bodies. 



2CeH,N0, + 4H, = QH, N.QH, + 4H,0. 



From this by various other processes of nitration and reduction the 

 other members of the series may be derived. 



Of the basic primary azo-compounds, amido azo-benzene, chrysoidin, 

 and phenylen brown show a gradation in shade from yellow through 

 orange to brown. Among the acid azo-colors a regular gradation of 

 shade is also shown. With the increase in molecular weight, the lowest 

 members of the series are orange, the highest scarlet of an increasing 

 shade of blueness. Primary azo-colors, as a rule, dissolve in strong 

 sulphuric acid with a red or orange color, while secondary azo-colors, 

 as a rule, give solutions a violet, blue, or green color. 



We will now examine more closely the series represented by aniline 

 yellow, chrysoidin, and phenylen brown. These are among the best 

 and earliest known azo-compounds. 



The first is known chemically as amido azo-benzene and is repre- 

 4 



