Ommochromes and Ommins

This discussion of pigments does not exhaust the list of coloured substances from insects. Some mention should be made of the ommochrome pigments from insect eyes. They function as screening pigments to cut out stray light. They can also be found in insect integument. They are divided into the ommatins of lower molecular mass, labile to alkali, and ommins, of higher molecular mass and stable to alkali. They are extracted from the ommatidia of the compound eyes. The ommatins are formed from the...

Arthropods And Insects

The arthropods were the first organisms to emerge from the sea, and insects were the first invertebrates to fly. The arthropods consist of Crustacea (crabs, lobsters, shrimp, barnacles and woodlice), Chelicerata (spiders, ticks, mites, scorpions and others), Hexapoda or Insecta, and Myriapoda (millipedes, centipedes and other minor groups). These classes separated a long time ago, so they have developed quite differently, but it is interesting to discover parallel developments. Spiders and...

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Figure 8.5 Aromatic compounds from chorismic acid. Mechanism a in the top left structure leads to anthranilic acid and tryptophan, while mechanism b leads to p-aminobenzoic acid. The route from chorismic acid to tryptophan is found in plants and micro-organisms only. Glomerine is a millipede defensive compound, dimethylquinazoline is found in Triatoma bugs. Indole and skatole are used in various insects as pheromones Figure 8.5 Aromatic compounds from chorismic acid. Mechanism a in the top left...

Alkaloids

Alkaloids were formerly defined as basic, nitrogen-containing plant substances, but examples have also been found in fungi, marine organisms, amphibians, insects and even mammals. There are over 10,000 plant alkaloids known, very varied in structure, from simple ones like coniine to complex structures like strychnine and reserpine (Figure 9.1). The biosynthesis of alkaloids in plants has received a lot of study. Less is known about the biosynthesis of the much smaller number of insect...

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Figure 10.18 Dihydromatricaria acid is found in cantharid beetles feeding on Compositae which contain polyacetylenic acids. Lucibufagins are similar to cardenolides, sequestered by fireflies. Romallenone is present in the defensive foam of a grasshopper The grasshopper Romalea microptera discharges a foam from glands when disturbed. The principal constituent of the foam is romallenone (Figure 10.18) an allenic sesquiterpene. This compound is thought to be salicyl alcohol salicylaldehyde Figure...

Phenols

Phenols and quinones are both found widely among insects, but so far have been found mainly in Coleoptera, Orthoptera (crickets and locusts), Isoptera (termites) and Dictyoptera (cockroaches). Phenols provide protection upwards against predators and downwards against microorganisms. They can be formed through a variety of biosynthetic routes. Some phenols have already been encountered among acetogenins (Chapter 4). They can also be formed from phenylpyruvic acid as in the formation of tyrosine...

Cyanogenic Glucosides

Many plants store cyanogenic glucosides, which on removal of the glucose can decompose to release hydrogen cyanide. As HCN is a powerful toxicant to all haem groups containing complexed iron (present in both plants and insects), it is remarkable how such compounds can be safely sequestered by plants or insects. More than 40 cyanogenic glucosides are known in plants, mandelonitrile glucoside, prunasin (Figure 10.7), and its epimer sambunigrin, are typical examples. A glucosidase cleaves the...

Chirality

The great majority of compounds in nature are chiral. A substance is chiral when it and its mirror images are not identical (the mirror image of the molecule cannot be superimposed upon it). Chirality in natural products is a direct consequence of being produced by enzymes, which are formed from chiral amino-acids and are themselves chiral. The two mirror-image forms, called enantiomers, are different substances. Although most of their chemical properties are identical, they can have very...

Termites

Not many trail pheromones of termites have been identified, but one group is worth mentioning here. In the group Macrotermitinae, for some species Z -3-dodecan-l-ol is the trail pheromone, while for another Z,Zy -3,6-dodecadien-l-ol is. In both cases the sternal gland is the source. It is suggested that these alcohols are formed from oleic acid and linoleic acids, respectively, by loss of three acetate units and reduction to the alcohol Figure 3.31 . Figure 3.30 Outline of the biosynthesis of...

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Figure 3.39 Further examples of insect macrolides, from the ant Linepithema humile, and cell-lining material from the glands of Colletes bees contain mixtures of 18-octadecanolide, 20-eicosanolide and 18-octadec-9-enolide Figure 3.39 as well as co-hydroxy-fatty acids which gives the clue to their biosynthesis and hydrocarbons. These solitary bees line their brood cells with a waxy polymer made from these components. There are many more pheromone structures derived from fatty acids, but they all...