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. [Illustration: 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. We may advantageously compare our Beetle with Peripatus, and note the points of agreement and of difference. Now, if our captive Beetles are to yield us the greatest possible amount of profit, we shall keep them under observation for some time, so as to watch their habits. In keeping these Beetles we shall not require a large aquarium. A small gathering of aquatic weed will be necessary to keep the water in good condition and the aquarium ready for its tenants. My interest in these Beetles was quickened by a letter in the _Field_ (Oct. 28, 1893), in which a correspondent at Weybridge asked ‘for information as to what animal or bird bisects so neatly the shells of the Water Snail (_Planorbis_).’ I thought then, and know now, that the shells were ‘bisected,’ if that is the proper word, by Water Beetles. From that time I have had, and still have, several living in small aquaria, but for a long time was unable to get direct evidence on the subject. [Illustration: FIG. 16.--Shells of Molluscs broken up by Dytiscus. (From a photograph by Cherry Kearton.)]