The differences above are rough guides. There are day flying moths (e. g. the 6 spot burnet), and there are butterflies that rest with their wings open (e. g. the Goliath birdwing).
The drawings below show the eggs of:1 Catocala nupta, the red underwing moth. The eggs are laid either singly or in small groups in crevices on the bark of willow, poplar and plum trees. They hatch the following spring and the caterpillars feed at night hiding on the bark during the day. They are fully grown by June or July; pupate in cocoons spun between leaves or in bark crevices and the adults emerge in August or September.
2 Pieris brassicae, the large white butterfly. In the UK there are two generations a year. The eggs are laid in batches on the underside of the food plant; cabbage, and other related plants. The eggs hatch in about a week, and initially the caterpillars feed together.
3 Catocala fraxini, the Clifden nonpariel. Rare in the UK, but found widely in Eurasia. The eggs are laid on poplar, the larval food plant.
Below is the typical caterpillar body plan usually there are 13 segments including the head. The head segment is the only part encased in hard chitin. Caterpillars do have antennae, but they are so small as to be barely visible.
The foregut is where the food is mixed with the swallowed saliva, and where the salivary enzymes start the digestive process. The midgut is the main site of digestion and absorption.
The hindgut resorbs water from the faeces. Because plant cell walls have a high proportion of indigestible tissues, e.g. cellulose and lignin, and a low nutritive content, the digestive tract occupies a large proportion of the body cavity.
Caterpillars are eating machines. They are little more than a mouth and an anus connected by a bag containing the gut. From hatching from the egg to the resting stage just before pupation a caterpillar can increase its weight by as much as 10,000 times.
A caterpillar has three pairs of true legs at the front of the body and the other legs which are called prolegs towards the rear.
The number of pairs of prolegs varies according to species. Above is a drawing showing the differences between the two types of leg. The proleg is soft, fleshy and surrounded by a circle of hooks.
Eyes. Caterpillars do not have compound eyes; they have a number of simple eyes (ocelli) on the sides of their head. These eyes can probably just detect light, dark and movement.
Above is the typical chrysalis. This is of a cabbage white butterfly.
Before the caterpillar moults into the chrysalis stage it will stop feeding and search for a suitable place, this may be on the food plant or underground or on a wall. This is why you may find a large caterpillar wandering around far from its food plant.
When the adult emerges from the chrysalis and is drying off a drop of red/vermillion fluid is often secreted from the anus. This is called the meconium, and is the stored excretory products of the chrysalis with much of the water removed. Drops of meconium produced in great numbers from butterflies in trees have given rise to legends of "showers of blood".
Above and below are drawings and a photograph showing the overlapping scales of the wings, and the many types of scale. It is the scales which give butterflies and moths the beautiful colours.
The scales are actually modified hairs which are flattened and hollow, and usually have longitudinal ridges. They are attached to the body by a small socket. They can help raise the body temperature when the insect basks in the sun with its wings open. Darker colour absorb more heat than light colours, and so in cold areas many of the butterflies have dark colours.
Nearly all butterflies and moths eat liquid food sucked up through their tongue or proboscis. This is actually two tubes joined together. When not in use the proboscis is curled up like a spring.
Some butterflies have been observed drinking from bird droppings and animal urine. It is believed that they do this to get sodium as nectar, their usual food, can be very low in sodium.
Caddis flies are often mistaken for moths as they are attracted to light in the same way that moths are. The most easily seen differences between them are:-