We do not know what life is, but we can at any rate record its manifestations; and we know that it is always associated with an extremely complex substance called by Purkinje ‘protoplasm.’ This substance Huxley described as ‘the physical basis of life.’ Protoplasm, though we know of what elements it is composed, defies accurate analysis, and, indeed, is never the same for two minutes together. It is constantly changing, it is in a state of flux and is, in effect, a stream into which matter is continuously entering and continuously leaving.
Protoplasm may be living, or it may be dead; and when dead it soon undergoes dissolution; but there is no life without protoplasm. Somewhere or other Dr. David Sharp has stated that of the total amount of protoplasm ‘in being’ in the world, the active volume of the life-material of our globe, at least one-half is wrapped up in the body of insects. But insects only form one sub-group out of the several which make up the great group Arthropoda, or those animals which are distinguished from others by possessing externally jointed legs—that is, jointed appendages. This group includes also the Crustacea, the multi-segmented Centipedes, and the Arachnids or spider-like animals.
Insects, like aeroplanes, dominate the air; Crustacea, like submarines, inhabit the water; the poet has passionately asked:—
Ah! who has seen the mailèd lobster rise,
Clap her broad wings and soaring claim the skies?
But the answer, in the language of those curious mammals the politicians, is ‘in the negative.’ Crustaceans are essentially aquatic. On the other hand, centipedes and spiders are earth-loving animals but some have unhappily developed parasitic or pseudo-parasitic habits.
The last-named sub-group, the Arachnids, comprise many subdivisions. There are the spiders, the harvest-men, the scorpions, the king-crabs, and so on. But one of the most numerous of the subdivisions of the group are the mites and ticks (Acarina). I have for years been trying to find some organ or structure shared by insects and mites and ticks, and not found in any other group of arthropods. If I could do this I would invent a long polysyllabic word—with lots of Greek in it—which would really be a short way of designating those arthropods which convey disease to man.
Fig. 29—Trombidium holosericeum. Female, dorsal view. × 20. (After Railliet.)
The acarines are for the most part small, and they differ from spiders in having no waist. In fact, the three divisions into which the body of an arthropod is normally divided—head, thorax, and abdomen—are indistinguishable in mites, the body forming an unconstricted whole. As a rule, these little creatures breathe, as do insects, by tracheae, or, if these be absent, by the general surface of the body. They live for the most part on vegetable and animal juices, and their mouth-parts are, as a rule, piercing and suctorial; but in some species the appendages of the mouth are capable of biting as well as piercing. The adults have typically eight legs. The larval stages are very numerous, and at times six distinct moults of the skin are recognisable. With few exceptions the larva emerges from the egg as a six-legged creature. In fact mites undergo a metamorphosis which varies in complexity and in completeness in different groups, and it is often one of the larval stages which causes the greatest trouble to man.