Arthropod development

Most arthropods lay eggs, but there are a few that produce live young. One of the disadvantages of having an external skeleton is that growth cannot proceed gradually, and so during their life arthropods have to moult a number of times, becoming a little larger each time. Among the insects, development is associated with the process known as metamorphosis, which can take place in two different ways. In some insects, such as bed bugs and cockroaches, the newly hatched young look like miniature versions of the adults. They go through a number of stages, separated by a series of moults. This is known as incomplete metamorphosis. Other insects, such as flies, beetles and butterflies, have complete metamorphosis, in which the young stages show no resemblance at all to the adults. Here a special immobile stage, the pupa, occurs between the larva and the adult. When the adult insect emerges from the pupal stage it ceases to grow, so there is no truth in the idea that, for example, small flies can become large flies.

Arthropod structure

As the name implies arthropod limbs are divided into joints; the body too is segmented. The most striking feature, however, is that, in contrast to the vertebrates, arthropods have an external skeleton. Their skin is modified to form a firm exoskeleton which serves as a protection and also for the attachment of muscles.

The crustaceans are primarily aquatic animals and they breathe by means of gills. The body is divided into head, thorax and abdomen. The head has three pairs of jaw-like mouthparts and two pairs of antennae. The only crustaceans that concern us here are the woodlice or slaters, known zoologically as the Isopoda.

Spiders have a body consisting of two parts which are almost the same size. The front part is formed from the modified head and thorax, and carries the eyes, mouthparts and four pairs of limbs. The rear part or abdomen has no limbs, apart from the spinnerets which are actually modified limbs.

The body of a scorpion has a front part or cephalothorax with four pairs of limbs and an abdomen without limbs. The last abdominal segment forms a kind of tail which ends in a sting. They have two pairs of mouthparts of which one pair is in the forms of pincers.

False scorpions are broadly speaking built as the true scorpions, but they lack the tail and sting.

Mites also have four pairs of legs, but the head, thorax and abdomen are fused to form a body showing no apparent segmentation.

Harvestmen also ••have an apparently unsegmented body, but are much larger than the mites, and the four pairs of legs are extremely long in relation to the body.

Millipedes and centipedes have a distinct head but the thorax and abdomen are united. The body consists of numerous segments, and each of these has one pair of legs, or in the true millipedes two pairs.

Insects are the dominant group within the arthropods. The body is in three parts: head, thorax and abdomen. The head carries the mouthparts, eyes and antennae. The thorax has three segments, each with a pair of legs, and in most insects there is a pair of wings on each of the two foremost thoracic segments. The remaining twelve segments form the abdomen which contains the main organs such as the alimentary canal and the gonads.

Animal names and systematics

In using a book of this type it is essential to understand the method of naming animals. The scientific study known as systematics is concerned with naming animals and plants, and with arranging them in groups which indicate their relationships with one another.

Modern systematics is based on the work of the Swedish naturalist Carl von Linne (1707-1778), also known as Linnaeus. He gave all the then known plants and animals a Latin name and arranged them in groups according to their physical appearance. He chose Latin because at that time this was the international language of science.

The principles of nomenclature established by Linnaeus are still in use today. The Latin or scientific name of an animal or plant consists of two parts, the first denoting the genus (plural genera), the second the species. In the present book it is necessary to use the scientific names because many of the animals mentioned do not have common English names, and when these do exist they are often not standard throughout the country. For example, the song thrush is known in Scotland as a mavis.

In 1859 a book was published which has had a profound effect on man’s understanding of the world around him.

This was The Origin of Species, by the English scientist Charles Darwin (1809-1882), who put forward the theory of natural selection, which seeks to explain how all existing species have evolved from species which have existed in past ages. Since Darwin’s time systematics has become more than a means of naming animals and plants; it has in many cases shown their relationships by grouping together those which are thought to have evolved from a common ancestor. The animal kingdom has been divided into some 14 major groups, known as phyla (singular phylum). Only a few of these have representatives occurring indoors, namely worms, molluscs, arthropods and vertebrates. The worms are represented in houses only by earthworms. The molluscs, a group that includes snails and bivalves, are not normally found in houses, except for certain slugs that occur in cellars.

The vertebrates or animals with a vertebral column include fishes, amphibians, reptiles, birds and mammals, including of course man.

Arthropods are animals with jointed legs. The group contains the crustaceans, scorpions, millipedes, centipedes, mites, spiders and last but not least the insects. About one million animal species have been described and more than three- quarters of these are arthropods. It is not surprising, therefore, that most of the animals found in houses belong among the arthropods.