Investigating variety
A twig is not a good place to live in the winter. It is blown around by gales, covered in snow and frost and washed by rain. The twig survives because it has a coating of bark. This material covers the entire shoot of the tree, its trunk and branches and it keeps the inside warm enough for survival. On the twig are buds. They are covered in small tough scales. These scales are for protection just like the bark. They protect the delicate leaves and flowers that are developing inside. When the spring comes, the buds burst and the twig becomes a desirable home for many animals.

One of the first animals to arrive is the aphid. A common aphid is pale green and is known as the greenfly. You can sometimes see groups of greenfly waiting round the buds as they burst. Each greenfly has a face a little like an elephant but instead of a flexible trunk it has a rigid tube with a sharp point. When it climbs onto a leaf the aphid sinks the end of this tube through the skin of the leaf and feeds on plant juices.

Aphids are unusual insects: many of the females do not lay eggs. The eggs hatch inside them and they give birth to their young. A female aphid may produce twenty five young every day. In a short time the leaves could be covered with aphids but this rarely happens. The reason for the smaller number of aphids is due to the presence of other animals on the leaf. They eat the aphids and keep the numbers down.

One of the main feeders on aphids is the ladybird. This beetle lays its eggs on leaves where aphids are living. When the eggs hatch the larvae feed on aphids. After three weeks of gorging themselves on aphids, the larvae turn into pupae. When the pupae hatch the adult ladybirds emerge and they feed on aphids too. Other feeders include spiders and leaf bugs.

While a population of aphids is breeding and being attacked, other leaf feeders may be having more success. Many moths lay their eggs on tree leaves and when their caterpillars hatch they feed by nibbling away at the edges of the leaves. Although the caterpillars may be exposed they have dull green and brown colours which help to camouflage them. However, there are sharp-eyed birds, such as the bluetit, which visit tree leaves and the colours of a caterpillar may not be enough to save it from becoming a meal. If a caterpillar survives the attentions of visiting birds it may roll up in a leaf and pupate or lower itself from the tree on a long thread and pupate in the ground.

Some insects do not attack the leaf from the outside. They have young which attack from inside. These insects are called leaf miners and you can see their wavy tracks on the surface of the leaf. The tracks mark the position of the miner's tunnels. Some flies, moths and weevils have larvae, which make tunnels in leaves.

When you turn a leaf over you may find balls or discs. These are called galls and are made by some kinds of wasps, gnats and mites. When a gall wasp, for example, lays an egg on a leaf, the leaf responds by growing material round it. This material forms a ball or disc shape and is called a gall. When the egg hatches the wasp larva feeds on the material inside the gall and then pupates. Later, the adult emerges from the pupa, burrows out of the gall and flies away.

By the time the leaves are falling from the twig they can have been home to a variety of life. Next year that life will be back as soon as the buds burst open again.

What kind of an animal is a weevil?
A weevil is an insect. It is a kind of small beetle. Like all beetles, its first pair of wings is hard and tough and folded across its back. These are called wing-cases and protect the second pair of wings, which the insect uses for flight. Weevils are usually under one and a half centimetres long but some species in tropical countries are up to almost eight centimetres long. Weevils have a long snout. In many species it curves downwards. Weevils tend to walk slowly.

Is a ladybird a beetle?
Yes, it is. The first pair of wings is formed into wing-cases and is coloured red and black. Each species of ladybird has a certain number of spots. The four most common ladybird species in Britain are the four spot, seven spot, ten spot and twenty two spot ladybirds.

Why are ladybirds brightly coloured?
The colours and patterns on a ladybird's back warn other animals that the ladybird is unpleasant to eat. Young predators quickly learn to recognise this after perhaps tasting one and then will leave others alone. Animals such as the caterpillar of the small white butterfly, which predators can find pleasant to eat, protect themselves by having colours which help them hide in their habitat. These animals are said to be camouflaged.

Are the same animals found on different plants?
Some animals, such as greenfly, can be found on many different kinds of plant. Other animals, such as moth caterpillars, may only be found on a few plants or just on one kind of plant. When you look at the leaves of different plants expect to find different animals and different shaped leaf miners.

What would happen to the ladybird larvae if they ate all the greenfly?
If the ladybird larvae were full-grown, they could pupate. When the ladybirds hatched from the pupa they could fly away and find some more greenfly on other plants. If the larvae were not full-grown they might die. If there were insects called scale insects on the leaves the larvae could eat them and survive but if there were no other insects the larvae would die. They could not eat the leaves because they are not adapted for feeding on them.

What is a scale insect?
These insects are related to greenfly but some of them do not look like insects at all. They are the size of greenfly or smaller and some have no eyes or legs. They all have flattened bodies and are covered in a wax substance that is mealy or like cotton. Scale insects may be found in large numbers on some leaves.