While most of these crabs have the light in only one place, one discovered by Sir Joseph Banks was luminous over its entire surface. Exactly what the luminous matter is, is not known, but in some instances it can be scraped off and will render the hands luminous when rubbed upon them. According to A. M. Norman, naturalist of the Porcupine expedition, the crustacean Ethusa, found at a depth of eighteen hundred feet, is blind, its eyestalk being spiny, and the eye replaced by a smooth, round termination which is supposed to be a light-emitting organ. Aristeus has phosphorescent eyes, which blaze with the yellow fire of a cat's eye, and this is true of many other crustaceans. Some have luminous backs; others have fiery bands upon the legs, while almost every portion of the body of some species is the seat of this wonderful light. That the lights are of some use there can be little doubt. In one little creature Dr. Gunther found a brilliant light stationed between its eyes, which certainly was a light to illumine its way in the deep, dark bed of the ocean.
XVIII. THE INSECTS
Among the best-known and attractive members of the lower animal kingdom are the insects, represented by the gorgeous butterflies, the iridescent beetles, the fierce spiders, and many others. The crustaceans may almost be called the insects of the ocean, as in general appearance they closely resemble these animals; but the real insects are higher forms.
Fig. 154.—Parts of a typical insect.
The skeleton (Fig. 154) of an insect is divided into three distinct parts instead of two. The head is distinct from the body, as in the crabs, and the skeleton, like that of the crabs, is external, and formed of a horny substance called chitin. As in the crabs, the body is made up of rings, or segments: four in the head generally, three in the thorax, and ten or eleven in the abdomen. It is interesting to glance at the various parts of insects, and later on, in reading about the many species, to note the many different purposes to which each is adapted.
The mouth is a very complicated organ in the crabs, and equally so in the insects. It is generally separated into four distinct parts: the upper lip, labrum, the jaws, or mandibles, a second pair of jaws smaller than the above, and the lower lip or jaws, labium. These are formed into sucking organs in the mosquito, biting organs in the ant, and tremendous graspers in the centipede, all displaying the most remarkable variety.
