There are two groups of the order Homoptera that have shown great originality in producing pigments. These are the superfamily Coccoidea

Figure 8.15 An in vitro reaction which is a possible model for the oxidative coupling of phenols to give melanin and other polymers

(scale insects and mealy bugs) and Aphidoidea (the aphids or plant lice). Both groups feed on phloem sap of plants. The Coccoidea make anthro-quinones, and the Aphidoidea make complex naphthoquinones.

All pigments of the scale insects are polyketide anthraquinones. The artist's colour, Venetian red, is produced by Kermococcus ilicius feeding on an oak, Quercus coccifera. The pigment is kermesic acid (Figure 8.16). It is probably the oldest used insect pigment. Cochineal, the food colouring, is obtained from dried females of Dactylopius coccus (formerly Coccus cacti), a bug feeding on Opuntia cactus (prickly pear). The pigment, carminic acid, has the same structure as kermesic acid but with a C-glucoside attached. It gives a deep red colour in water. These pigments are present as the potassium salts in vivo. Lac insects (Chapter 6) also produce pigments, consisting of more polar laccaic acids and less polar anthroquinones. The most commonly encountered examples are given in Figure 8.16. Shellac is the resin after the pigments have been removed. The additional benzene ring in laccaic acids is from a tyrosine molecule linked to the naphthoquinone by oxidative dehydrogenation. The anthroquinones can comprise up to 50% of the female body weight, but most of what is produced is secreted externally in the lac. Emodin is an example of a very widely distributed anthraquinone, from the Australian scale insect Eriococcus species, to the roots of rhubarb. It comes from an octaketide folded differently from the others illustrated here. Incidentally, the cochineal bug D. coccus, also produces a long-chain ester wax, which is a biosynthetic curiosity, 15-oxotetratriacontanyl 13-oxodotriacontanoate (Figure 8.16).

0 0

Post a comment