The chemistry of the aphid colouring matters
Some fifteen years ago, for reasons into which I shall not enter here, we commenced in Garnbridge a study of the colouring matters present in the haemolymph of dark-coloured insects belanging to the family Aphididae. Our preliminary sturlies were made on the common bean aphid (Aphis jabae) which is widely distributed in Europe where its most common summer host is the cultivated broad bean Viciajaba, but they were later extended to cover some twenty species of dark-coloured aphids from which the same or closely related colouring matters were obtained. We do not as yet know the exact distribution of these colouring matters-to which the general name aphins has been given-among the Hemiptera but they are apparently not confined to the Aphididae but occur also in at least one member of the Phylloxeridae (Adelges strobii). Investigation of these colouring matters quickly revealed a complex and interesting situation. In the blood of the living insect there is present a golden yellow glucosidic pigment, protoaphin, which shows a reddish-purple colour in alkaline solution. On the death of the insect or even upon injury protoaphin is rapidly converted by a specific enzyme system present in all these insects into a yellow fat-soluble substance xanthoaphin which shows a strong green fluorescence; this change is accompanied by loss of glucose. Xanthoaphin itself is unstable and passes on standing, or very rapidly in presence of acids or alkalis, into the similarly unstable orange chrysoaphin. In similar fashion the latter undergoes conversion to the red, strongly fluorescent erythroaphin, the relatively stable end product of the aphin series. This series of changes from protoaphin to erythroaphin we found in all the species of dark aphids we examined, but the colouring matters fell into two series, which differ stereochemically from each other and which we describe as the aphins:fb and the aphins-sl. The descriptions -fb and -sl derive from the name of the particular aphid species from which the pigments were first isolated, i.e. the aphins:fb from the bean aphid Aphisfabae and the aphins-sl from the willow aphid Tuberolachnus salignus. There exist, then, a protoaphin:f"b, xanthoaphin:fb etc., and a Stereoisomerie series protoaphin-sl, xanthoaphin-sl etc. The relationships between these series are set out in the following table: Protoaphin:fb {, Xanthoaphin:fb {,