They are parasites living on certain cactus plants. Four centuries ago, the Mexican Indians were seen using little brushes to gather the insects from their prickly host plants. Today, however, cochineal can be found, not just in Mexico. People have replanted the special cacti in many hot parts of the world, for instance, South Africa, the Far East and Australia, and there they breed their own insects. Dyeing is often a question of experiment and luck.
Different mordants produce variations in colours.
Also the time of the year when the plant was gathered has an effect. The type of ground and weather in an area also affects plant colours. Because the weavers had to choose their colours from plants in their gardens and fields, tartans made in the same area were very often in the same colours. The diagram below shows the colour variations for heather flowers and dock leaves. You can colour the boxes for yourselves.