. Comparative anatomy and physiology. Chap. III.] INSECTA. 75 developed behind the region of the thorax; one pair of appendages form the prse-oral antennae, and the metameres do not exceed twenty in number. They are sharply divisible into two great sub- divisions, according as they are or are not provided with wings ; with the latter, of course, we must as- sociate those in which wings are found in one sex only, or are rudimentary, or of whose ancestral ex- istence (as in the case of parasites), we have sufficient evidence. A. Aptcra, or true wingless forms such as the spring- tails (Podura),
. Comparative anatomy and physiology. Chap. III.] INSECTA. 75 developed behind the region of the thorax; one pair of appendages form the prse-oral antennae, and the metameres do not exceed twenty in number. They are sharply divisible into two great sub- divisions, according as they are or are not provided with wings ; with the latter, of course, we must as- sociate those in which wings are found in one sex only, or are rudimentary, or of whose ancestral ex- istence (as in the case of parasites), we have sufficient evidence. A. Aptcra, or true wingless forms such as the spring- tails (Podura), and bristle-tails (Lepisma). In the simplest of these the mouth or- gans can work either from side to side, or from before back- wards ; the trachece, however well de- veloped, and they are Fig. Si.—OrcTicsella cincta, enlarged, often only poorly so, never anastomose with one another (Fig. 34). B. Pterygota.—Here belong all the remaining insects, which are either winged, that is, provided with two pairs of membranous dorsal outgrowths in the region of the thorax, which can be moved by. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Bell, F. J. (Francis Jeffrey), 1855-1924. London, Cassell
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