Aphids which feed on plants in the Asclepiadaceae family, for instance, store cardenolide substances which have psychoactive properties, causing the spiders which eat them to spin disrupted webs. Many such poisonous insects are brightly coloured as if to advertise their toxic nature, with, for instance, the vivid pigments of carotenoid substances which are obtained from the host plant during feeding. A large number of plants, including the well-known rubber plant, simply gum up the mouthparts of their predators. When these plants are damaged they produce latex, which oxidises on exposure to air to form a thick glue. This clever ploy is also used by some insects on leaves which contain poisonous chemicals.
Some plants protect themselves using cunning tricks with molucules.
Many plants in the legume family, for instance, produce amino acids which, though different, are structurally similar to the amino acids found in proteins. More than 1200 legumes, for example, are known to contain canavanine, a non-protein amino acid that is related to the protein amino acid, arginine. The bruchid beetle, however, uses the seed of one such plant, Dioclea megacarpa , which contains almost thirteen per cent canavanine, as a nursery for its offspring. The larvae feed on the seeds with freely, because they manufacture a protein-building enzyme which can tell the difference between canavanine and arginine. The beetle even manages to metabolise the canavanine for use as food.