Though the study of the embryology of insects is as yet in its infancy, still with the facts now in our possession we can state with tolerable certainty that at first the embryos of all insects are remarkably alike, and the process of development is much the same in all, as seen in the figure of Diplax (Fig. 186), the louse (Fig. 187), the spider (Fig. 188) and the Podura (Fig. 189), and we could give others bearing the same likeness. We notice that at a certain period in the life of the embryo all agree in having the head large, and bearing from two to four pairs of mouth organs, resembling the legs; the thorax is merged in with the abdomen, and the general form of the embryo is ovate. Now this general embryonic form characterizes the larva of the mites, of the myriopods and of the true insects. To such a generalized embryonic form to which the insects may be referred as the descendants, we would give the name of _Leptus_, as among Crustacea the ancestral form is referred to Nauplius, a larval form of the lower Crustacea, and through which the greater part of the Crabs, Shrimps, Barnacles, water fleas, etc., pass to attain their definite adult condition. A little water flea was described as a separate genus, Nauplius, before it was known to be the larva of a higher water flea, and so also Leptus was thought to be a mature mite. Accordingly, we follow the usage of certain naturalists in dealing with the Crustacea, and propose for this common primitive larval condition of insects the term Leptus. [Illustration: 190. Zoea.] The first to discuss this subject of the ancestry of insects was Fritz Mueller, who in his "Fuer Darwin,"[15] published in 1863, says, at the end of his work, "Having reached the Nauplius, the extreme outpost of the class, retiring farthest into the gray mist of primitive time, we naturally look round us to see whether ways may not be descried thence towards other bordering regions. * * * But I can see nothing certain. Even towards the nearer provinces of the Myriopoda and Arachnida I can find no bridge. For the Insecta alone, the development of the Malacostraca [Crabs, Lobsters, Shrimps, etc.] may perhaps present a point of union. Like many Zoeae, the Insecta possess three pairs of limbs serving for the reception of nourishment, and three pairs serving for locomotion; like the Zoeae they have an abdomen without appendages; as in all Zoeae the mandibles in Insecta are destitute of palpi. Certainly but little in common, compared with the much which distinguishes these two animal forms. Nevertheless, the supposition that the Insecta had for their common ancestor a Zoea which raised itself into a life on land, may be recommended for further examination" (p. 140).