[Illustration: A. After Wheeler, _Journ. Morph._ vol. viii., and Folsom, _Bull. Mus. Harvard_, xxxvi.
FIG. 18.--Embryos of Springtail (_Anuridamaritima_). Magnified. A, Head-region of germ band. B, Section through head and thorax. The neuromeres are shown in Arabic, the appendages in Roman numerals.
1, Ocular segment. 2, Antennal. 3, Trito-cerebral. 4, Mandibular. 5, Maxillular. 6, Maxillary. 7, Labial. 8, Prothoracic. 9, Mesothoracic. 10, Metathoracic.]
In the adult state no insect possesses more than six legs, and they are always attached to the thorax; in many Thysanura there are, however, processes on the abdomen that, as to their position, are similar to legs. In the embryos of many insects there are projections from the segments of the abdomen similar, to a considerable extent, to the rudimentary thoracic legs. The question whether these projections can be considered an indication of former polypody in insects has been raised. They do not long persist in the embryo, but disappear, and the area each one occupied becomes part of the sternite. In some embryos there is but a single pair of these rudiments (or vestiges) situate on the first abdominal segment, and in some cases they become invaginations of a glandular nature. Whether cerci, stylets and gonapophyses are developed from these rudiments has been much debated. It appears that it is possible to accept cerci and stylets as modifications of the temporary pseudopods, but it is more difficult to believe that this is the case with the gonapophyses, for they apparently commence their development considerably later than cerci and stylets and only after the apparently complete disappearance of the embryonic pseudopods. The fact that there are two pairs of gonapophyses on the ninth abdominal segment would be fatal to the view that they are in any way homologous with legs, were it not that there is some evidence that the division into two pairs is secondary and incomplete. But another and apparently insuperable objection may be raised--that the appendages of the ninth segment are the stylets, and that the gonapophyses cannot therefore be appendicular. The pseudopods that exist on the abdomen of numerous caterpillars may possibly arise from the embryonic pseudopods, but this also is far from being established.