Insect Categories of Great Britain
The host of creeping, crawling, burrowing, swimming and flying creatures known loosely as insects are so numerous and varied that zoologists divide them, on the basis of relationships, into smaller units to narrow their field and facilitate identification. Nearly all the small creatures to be found in a garden belong to one of three great groups:
Annelida (or ringed worms), which include earthworms, and many of the marine worms.
Mollusca (or ‘soft-bodied ones’), which include slugs and snails, as well as cockles, oysters, winkles and other shellfish.
Arthropoda (the ‘jointed-leg’ creatures), the group to which most of the small garden creatures belong. This group is so vast that it has been divided and sub-divided in order to get it down to really manageable units. It is divided into four main classes:
1. the Crustacea, which include crabs, shrimps, lobsters and the hump-backed little woodlice and slaters found in gardens;
2. the Arachnida, which include spiders, harvestmen and mites, all having four pairs of legs;
3. the Myriapoda, or ‘many-legged’ creatures, which include the centipedes (literally ‘hundred-footed ones’), millipedes, etc., and
4. the Insecta, or true insects.
Earthworms are familiar to most people. They feed on vegetable matter, and draw considerable quantities of grass and leaves below ground. They also break up and aerate theby their movements. They are thus very beneficial in gardens, although their casts can be a nuisance on a lawn. Moisture is essential to them so that in dry weather they burrow deeper. In ground, however, there is a danger of their drowning. Worms are hermaphrodites, that is, each individual is both male and female.
SLUGS AND SNAILS
These feed on vegetation. They prefer damp places—old walls, rotting logs, deep grass—and avoid the heat of the day as much as possible. Because of their protective shells (which are a familiar sight) some species of snail are able to withstand long periods of drought. Many wild plants have developed protective devices, such as hairy leaves and stems, against slugs and snails, but unfortunately most garden plants, particularly the more succulent vegetables, have no such protection and are frequently devastated.
WOODLICE AND SLATERS
Both are about 1/2 to 3/4 in. long and look alike, but an easy way of telling them apart is that the woodlice curl into a ball when touched and the slaters do not. They are grey or brownish creatures with oval, armour-plated bodies. They are frequently found in decaying wood, especially under the bark, and also under stones. They eat stems, leaves and roots of certain plants and can be a pest in frames and.
SPIDERS AND HARVESTMEN
Both have eight legs but there the similarity ends, for in the harvestmen the head, thorax and abdomen are united in a single, undivided body, but in spiders they are separate, with a distinctive narrow waist between the thorax and abdomen.
Spiders feed on flies and other small insects. Many spin webs to catch their prey, the web of the common cross or garden spider being particularly beautiful. Others stalk their prey. Some live in burrows, and one species even lives under water, breathing from a bubble of air trapped in a domed web.
Harvestmen are omnivorous, and have claws for tearing food. They hide by day, and lay their eggs underground.
These are usually brownish in colour, with flattened bodies, long antennae and only one pair of legs to each segment of the body (of which there may be as many as 173). They live in dark, damp places, under stones, in walls, decaying vegetation, etc., and are generally beneficial to gardeners as they live on insect larvae, slugs and snails, though they also eat small worms and smaller centipedes.
Most species of millipedes have tubular bodies and are distinguished from centipedes by having two pairs of legs to each segment of the body. They live in the kind of places centipedes occupy, but as they are vegetarians they can do considerable harm by damaging roots, boring into stems and even into fruit.
A true insect in the adult stage has a body divided into three parts, head, thorax or chest, and abdomen. It has three pairs of legs attached to the thorax, one pair of antennae or feelers, and usually wings, though these may be tucked out of sight beneath hard wing cases, as in the case of the ladybird and other beetles.
Some million different species of insects (in the zoologist’s sense of the word) have been discovered all over the world and named. There are 20,000 in the British Isles alone, and they are divided into 22 still smaller categories, called ‘orders’, which may be quite small, or bewildering large.
There are, for example, only five British species of earwig (order clermaplera) but over 6,000 British species of Hymenoptera—the order to which bees, wasps and ants belong. And even within the species themselves a creature may exist in four or five different forms at different stages of its life: eggs, larva or caterpillar, pupa or chrysalis, and adult— in which final stage the male and the female may be so dissimilar as to seem quite unrelated.