Fritz Müller has expressed the opinion[68] that this vermiform type is of comparatively recent origin. He says: “The ancient insects approached more nearly to the existing Orthoptera, and perhaps to the wingless Blattidæ, than to any other order, and the complete metamorphosis of the Beetles, Lepidoptera, &c., is of later origin.” “There were,” he adds, “perfect insects before larvæ and pupæ.” This opinion has been adopted by Mr. Packard[69] in his “Embryological Studies on Hexapodous Insects.”
M. Brauer[70] also considers that the vermiform larva is a more recent type than the Hexapod form, and is to be regarded not as a developmental form, but as an adaptational modification of the earlier active hexapod type. In proof of this he quotes the case of Sitaris.
Fig. 57, Larva of Cecidomyia (After Packard). 58, Lindia torulosa (after Dujardin).
Considering, however, the peculiar habits of this genus, to which I have already referred, and also that the vermiform type is altogether lower in organization and less differentiated than the Campodea form, I cannot but regard this case as exceptional; one in which the development has been, as it were, to use an expression of Fritz Müller’s, “falsified” by the struggle for existence, and which therefore does not truly indicate the successive stages of evolution. On the whole, the facts seem to me to point to the conclusion that, though the grublike larvæ of Coleoptera and some other insects, owe their present form mainly to the influence of external circumstances, and partially also to atavism, still the Campodea type is itself derived from earlier vermiform ancestors. Nicolas Wagner has shown in the case of a small gnat, allied to Cecidomyia, that even now, in some instances, the vermiform larvæ possess the power of reproduction. Such a larva (as, for instance, Fig. [57]) very closely resembles some of the Rotatoria, such for instance as Albertia or Notommata, which however possess vibratile cilia. There is, indeed, one genus—Lindia (Fig. [58])—in which these ciliæ are altogether absent, and which, though resembling Macrobiotus in many respects, differs from that genus in being entirely destitute of legs. I have never met with it myself, but it is described by Dujardin, who found it in a ditch near Paris, as being oblong, vermiform, divided into rings, and terminating posteriorly in two short conical appendages. The jaws are not unlike those of the larvæ of Flies, and indeed many naturalists meeting with such a creature would, I am sure, regard it as a small Dipterous larva; yet Dujardin figures a specimen containing an egg, and seems to have no doubt that it is a mature form.[71]
For the next descending stage we must, I think, look among the Infusoria, through such genera as Chætonotus or Ichthydium. Other forms of the Rotatoria, such for instance as Rattulus, and still more the very remarkable species discovered in 1871 by Mr. Hudson,[72] and described under the name of Pedalion mira, seem to lead to the Crustacea through the Nauplius form. Dr. Cobbold tells me that he regards the Gordii as the lowest of the Scolecida; Mr. E. Ray Lankester considers some of the Turbellaria, such genera as Mesostomum, Vortex, &c., to be the lowest of existing worms; excluding the parasitic groups. Haeckel[73] also regards the Turbellaria as forming the nearest approach to the Infusoria. The true worms seem, however, to constitute a separate branch of the animal kingdom.
Fig. 59, Prorhynchus stagnaus.[75]