The excretory role of pteridines in insects.

In 1895, Hopkins described the white and yellow wing pigments of Pieridae as ' . . .excretory substances which function in ornament...' (Hopkins, 1895). ^ those days, it was generally recognized that purines, especially uric acid, were excretory products. Hopkins thought that the white substance was uric acid and that the yellow one was a derivative. In 1925 Wigglesworth still believed the wing pigments of Pieridae to be uric acid and uric acid derivatives. He based a quantitative study on this assumption, and compared storage and excretion of uric acid in Pieris and Vanessa. He agreed with Hopkins's opinion that the white pigment was an excretory substance used in ornament (Wigglesorth, 1925). The whole basis for this opinion crumbled in the next year when Schopf and Wieland published their discovery that the white wing pigment was not uric acid but leucopterin, a substance closely resembling uric acid, but belonging to a different chemical group (Schopf & Wieland, 1926). In 1933 the same workers compared leucopterin and uric acid levels in Pieris. They found no evidence that the two substances are metabolically connected. There is leucopterin and no uric acid in the wings; there is uric acid and no leucopterin in the excreta (Wieland, Metzger, Schopf & Biilow, 1933). Becker took up the 'storage excretion' problem next. He did not consider pteridines excretory products (Becker, 1937a) and published a special paper on the subject (Becker, 19376), basing his rejection of the storage excretion theory on the examination of the meconium of a series of insects all known to contain high amounts of pteridine in the adult. He failed to find any pteridines in the meconium, although he did find uric acid. It is strange that Becker's work was generally accepted without much criticism as he does not seem to have looked at the meconium of those insects which do not contain pteridines in the adult stage. If the presence of pteridines in the adult was a case of storage excretion, then one would expect true excretion in insects lacking pteridines. Since Becker's 1937 publications, it has generally been assumed that pteridines are not excretory products, although it has also been recognized that the wing pigments of Pieridae contain large amounts of highly oxidized, immobilized nitrogen. Furthermore, pteridines are found in trace quantities in any excreta which have been examined closely. Koschara's work in 1936 is the first instance of a pteridine being recognized as an excretory substance. He reported the isolation of the pteridine uropterin (later to be recognized as xanthopterin) from human urine (Koschara, 1936), and described it as a minor excretion of a highly specialized substance, rather than as