The most archaic Arthropod—Perip´atus—must be mentioned. It is not found in Britain, nor even in Europe; so that, unless we travel, we shall only know it from books, or from museum specimens. But it is an extremely interesting creature, for it is of worm-like aspect, and breathes by air-tubes, opening all over the body, which has no external segments. The limbs are imperfectly jointed, and each of them bears two claws. Most naturalists make this genus a Class by itself, while some put it with the Centipedes. There are about a dozen species, four of which are African, two Australian, and the rest are found in South America and the West Indies. Besides these there are some doubtful species.
In habit they resemble the Centipedes, and they ensnare the insects on which they feed by ejecting sticky slime from the small processes near the mouth. The left process is shown in the illustration, just below the antenna of that side.
Professor Sedgwick, who described these animals in the Quarterly Journal of Microscopical Science (1888), and, more popularly, in the Cambridge Natural History, says, that ‘the exquisite sensitiveness and changing form of the antennae, the well-rounded plump body, the eyes set like small diamonds on the side of the head, the delicate feet, and, above all, the rich colouring and velvety texture of the skin, all combine to give these animals an aspect of quite exceptional beauty.’
Unfortunately, an illustration in black-and-white can only render form. We must take the beauty of the colouring for granted. One thing, however, cannot escape the most cursory examination of the picture—the resemblance of the creature, in some respects, to a worm, and, in others, to a caterpillar, which, as everybody knows, is the larval stage of a butterfly. If this resemblance sets us thinking how it came about, and what it means, Peripatus will, for the present, have done its work for us.
With these general notions of Arthropods, we may pass on to put our pocket lens to some practical use. Our first subject shall be the Margined Water Beetle (Dytis´cus margina´lis), which can be taken in almost any open pond in the country. Water covered with duckweed should be avoided in hunting for these beetles, which prefer ponds with a clear surface, so that they may easily come to the top to breathe.
Every one has a good general notion of the principal Insect-groups, technically called Orders—Beetles, Cockroaches and Grasshoppers, Butterflies, Bees and Wasps, and Flies. Insects may be defined as animals with hollow-jointed limbs, and divided into three regions—head, thorax, and abdomen. The head bears a pair of antennae; the thorax carries three pairs of legs, and (generally) two pairs of wings; the abdomen is without appendages. Insects when adult breathe by tubes that open to admit air. In Chapter VI we shall see that many larvae obtain an air supply in different ways.
Fig. 15.—Margined Water Beetle (male).
Beetles may be taken as very good types of true Insects. They constitute the Order Coleop´tera, or Insects with sheathed wings, only the hinder pair being used for flight (Fig. 18), and at other times they are folded under the wing-cases, or el´ytra, as in Fig. 15.