Insects are the most diverse and widespread of the animal groups. They are invertebrates and classed as arthropods. The adult forms are characterized by six legs and three distinct parts to the body (head, thorax and abdomen). Some animals often referred to as ‘insects’ do not in fact belong to this group. The life cycle consists of separate phases, for example egg, larva (caterpillar), pupa (chrysalis) and imago (perfect insect). Insect pests either chew plant tissue (such as flea beetle, leaf-cutter bee, leaf miners) or suck the sap (such as aphids, froghopper, thrips). Their activities result in a general weakening of the whole plant, and often distortion and discoloration of the leaves and stunting of growth. Sap-sucking pests can transmit viruses from plant to plant. But there are some insects that prey on the pests (such as ladybirds, ground beetles). Some modern insecticides are designed not to harm these or other useful or decorative insects such as bees and butterflies.