The first to discuss this subject of the ancestry of insects was Fritz Müller, who in his "Für 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 Zoëæ, the Insecta possess three pairs of limbs serving for the reception of nourishment, and three pairs serving for locomotion; like the Zoëæ they have an abdomen without appendages; as in all Zoëæ 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 Zoëa which raised itself into a life on land, may be recommended for further examination" (p. 140).
Afterwards Hæckel in his "Generelle Morphologie" (1866) and "History of Creation," published in 1868, reiterates the notion that the insects are derived from the larva (Zoëa, Fig. 190) of the crabs, though he is doubtful whether they did not originate directly from the worms.[16]
It may be said in opposition to the view that the insects came originally from the same early crustacean resembling the larva of a crab or shrimp, that the differences between the two types are too great, or, in other words, the homologies of the two classes too remote,[17] and the two types are each too specialized to lead us to suppose that one was derived from the other. Moreover, we find through the researches of Messrs. Hartt and Scudder that there were highly developed insects, such as May flies, grasshoppers, etc., in the Devonian rocks of New Brunswick, leading us to expect the discovery of low insects even in the Upper Silurian rocks. At any rate this discovery pushes back the origin of insects beyond a time when there were true Zoëæ, as the shrimps and their allies are not actually known to exist so far back as the Silurian, not having as yet been found below the coal measures.
The view that the insects were derived from a Zoëa was also sustained by Friedrich Brauer, the distinguished entomologist of Vienna, in a paper[18] read in March, 1869. Following the suggestion of Fritz Müller and Hæckel, he derives the ancestry of insects from the Zoëa of crabs and shrimps. However, he regards the Podurids as the more immediate ancestors of the true insects, selecting Campodea as the type of such an ancestral form, remarking that the "Campodea-stage has for the Insects and Myriopods the same value as the Zoëa for the Crustacea." He says nothing regarding the spiders and mites.
At the same time[19] the writer, in criticising Hæckel's views of the derivation of insects from the Crustacea (ignorant of the fact that he had also suggested that the insects were possibly derived directly from the worms, and also independently of Brauer's opinions) declared his belief that though it seemed premature, after the discovery of highly organized winged insects in rocks so ancient as the Devonian, and with the late discovery of a land plant in the Lower Silurian rocks of Sweden,[20] to even guess as to the ancestry of insects, yet he would suggest that, instead of being derived from some Zoëa, "the ancestors of the insects (including the six-footed insects, spiders and myriopods) must have been worm-like and aquatic, and when the type became terrestrial we would imagine a form somewhat like the young Pauropus, which combines in a remarkable degree the characters of the myriopods and the degraded wingless insects, such as the Smynthurus, Podura, etc. Some such forms may have been introduced late in the Silurian period, for the interesting discoveries of fossil insects in the Devonian of New Brunswick, by Messrs. Hartt and Scudder, and those discovered by Messrs. Meek and Worthen in the lower part of the Coal Measures at Morris, Illinois, and described by Mr. Scudder, reveal carboniferous myriopods (two species of Euphorberia) more highly organized than Pauropus, and a carboniferous scorpion (Buthus?) closely resembling a species now living in California, together with another scorpion-like animal, Mazonia Woodiana, while the Devonian insects described from St. John by Mr. Scudder, are nearly as highly organized as our grasshoppers and May flies. Dr. Dawson has also discovered a well developed milleped (Xylobius) in the Lower Coal Measures of Nova Scotia; so that we must go back to the Silurian period in our search for the earliest ancestor, or (if not of Darwinian proclivities) prototype, of insects."
Afterwards[21] the writer, carrying out the idea suggested above, "referred the ancestry of the Myriopods, Arachnids, and Hexapodous insects to a Leptus-like terrestrial animal, bearing a vague resemblance to the Nauplius form among Crustacea, inasmuch as the body is not differentiated into a head, thorax and abdomen [though the head may be free from the rest of the body] and there are three pairs of temporary locomotive appendages. Like Nauplius, which was first supposed to be an adult Entomostracan, the larval form of Trombidium had been described as a genus of mites under the name of Leptus (also Ocypete and Astoma) and was supposed to be adult."
In the same year Sir John Lubbock[22] agrees with Brauer that the groups represented by Podura and Campodea may have been the ancestors of the insects, remarking that "the genus Campodea must be regarded as a form of remarkable interest, since it is the living representative of a primæval type from which not only the Collembola (Podura, etc.) and Thysanura, but the other great orders of insects, have all derived their origin."
The comparison of the Leptus with the Nauplius, or pre-Zoëal stage of Crustacea, is much more natural. But here we are met with apparently insuperable difficulties. While the Nauplius (Fig. 191) has but three pairs of appendages, which become the two pairs of antennæ and succeeding pair of limbs of the adult, in the Leptus as the least number we have five pairs, two of which belong to the head (the maxillæ and mandibles) and three to the thorax; besides these is a true heed, distinct from the hinder region of the body. It is evident that the Leptus fundamentally differs from the Nauplius and begins life on a higher plane. We reject, therefore, the Crustacean origin of the insects. Our only refuge is in the worms, and how to account for the transmutation of any worm with which we are at present acquainted into a form like the Leptus, with its mandibulated mouth and jointed legs, seems at first well nigh impossible. We have the faintest possible indication in the structure of some mites, and of the Tardigrades and Pentastoma, where there is a striking recurrence, as we may term it, to a worm-like form, readily noticed by every observer, whatever his opinion may be on the developmental theory. In the Demodex we see a tendency of the mite to assume under peculiar circumstances an elongated, worm-like form. The mouth-parts are aborted (though from what we know of the embryology of other mites, they probably are indicated early in embryonic life), while the eight legs are not jointed, and form simple tubercles. In the Tardigrades, a long step lower, we have unjointed fleshy legs armed with from two to four claws, but the mouth-parts are essentially mite in character. A decided worm feature is the fact that they are hermaphrodites, each individual having ovaries and spermaries, as is the case with many worms.
191. Nauplius.