The spindle-shaped larva in its first stage roughly resembles a trochosphere of a worm rather than the larva of an insect so high in the scale as a Hymenopter. It is active, but after moulting the second larva is oval, still without segments. Dr. Ayers gives a profusion of details and figures of the first and second stages of our Teleas, the second strongly resembling the Cyclops stage of Ganin. He describes three stages, and though he did not complete the life-history of the insect, he thinks it changes to an ovoid flattened form which succeeds the Cyclops stage in other Pteromalidæ, and that there are at least four ecdyses.
It is difficult to account for these strange larval forms, unless we suppose that the embryos, by their rich, abundant food, have undergone a premature development, the growth of the body-walls being greatly accelerated, the insects so to speak having been, under the stimulus of over-nutrition and their unusual environment, and perhaps also the high temperature of the egg, hurried into vermian existence on a plane scarcely higher than that of an active ciliated gastrula.
Further observations, difficult though they will be, are needed to enable us to account for the singular prematurity of the embryo of these parasites. That these stages are reversional and a direct inheritance from the vermian ancestors of these insects is not probable, but the forms are evidently the result of adaptation in response to a series of stimuli whose nature is in part appreciable but mostly unknown.
Fig. 653.—Third larva of Polynema: at, antenna; fl, imaginal bud of wing; l, rudimentary legs; tg, buds of one of the three pairs of styles of the ovipositor; fk, fat-body; eg, ear-like process.
Fig. 654.—A-D, development of Teleas; A, stalked egg; B, C, D, the 1st larval stage: at, antenna; md, hook-like mandibles; mo, mouth; b, bristles; m, intestine; sw, the tail; ul, under lip or labium. E, larva of another parasite, Ophioneurus.—This and Figs. 650–653 after Ganin.
It may be noted, however, that the appearance of a primitive band in the second larval stage suggests the origin of these forms, as well as that of insects in general, from a Peripatus-like, and again from an earlier leech-like Annelid ancestor. Hence the first larval or Cyclops stage is due to a precocious development caused by the unusual environment, and is simply adaptational, and not of phylogenetic significance.
SUMMARY OF THE FACTS AND SUGGESTIONS AS TO THE CAUSES OF METAMORPHISM
An explanation of the causes of metamorphosis is one of the most difficult undertakings in biology, and the phenomenon has been considered as one of the chief difficulties in the way of the acceptance of the theory of descent.