Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/63

Rh shape, and color. The question now arises, Do the Tracheata come from the Crustacea, or are they modified Annelida? Plausible arguments have been advanced for the first of these views; but the second, or that of the Tracheata coming from the Annelida, will be the one here adopted. On seeing for the first time the minute worm resembling an Annelid (Fig. 47), moving through the water, it would surprise the observer to learn that it was the larva, or undeveloped young, of an Insect. The numerous segments of which the immature Insect and Spider are composed gradually coalesce, until finally the perfect Insect exhibits only three pieces, the Spider two. In the young of the Ephemera, or the one-day fly, and of the Libellula, small respiratory tufts are found externally, exactly as in the Annelida, which were alluded to in speaking of the brilliant colors of the Nereis. The existence in the larva of these external respiratory tufts, the manner of the development of the young of Insects and Spiders, furnishes the clue to their origin.



The development, however, of the Myriapoda is just the reverse of that of the Insects, the Centipedes, etc. increasing instead of diminishing the number of their segments. Originally the body is composed of a congeries of cells, segment after segment being added, exactly in the same manner as in the case of the Annelida, with which in structure they closely agree, being adapted, however, to live on land. The Annelida seem then to be the ancestors of the Myriapoda, Insects, and Spiders, the Myriapoda retaining much of the Annelid structure through life, whereas the Insect is an Annelid or Myriapod only when in a larval or undeveloped condition. That the development of the different kinds of insects has been gradual. Geology seems to show, the evidences for which will be brought forward in the chapter on that subject. By looking at Tree IV. the Hymenoptera will be seen very high up; this family includes the Bees, Ants, etc., whose economy has always been the