Though Swammerdam knew that the rudiments of the wings were already present under the skin of the larvæ, we are indebted, for our present knowledge, to the thorough and profound investigations of Weismann on the metamorphosis of the Diptera, and also to the researches of Ganin and others who have worked on the pupæ of Muscidæ, in which the development is most complicated and modified. In the more generalized and primitive Diptera, such as Corethra, the processes of formation of the pupa and imago are much simpler than in the muscids and Pupipara. These processes are still simpler in the Lepidoptera and Hymenoptera, and for this reason we have given a summary of what has been done on these organs by Newport, Dewitz, and especially by Bugnion.

Our knowledge of this subject is still very imperfect, only the more salient points having been worked out, and, as Korschelt and Heider state, there is still lacking certain proof as to how far the relations of the internal changes known to exist in the Muscidæ also apply to other orders of insects, though it must be considered that in the pupa of Lepidoptera, Hymenoptera, perhaps also the Coleoptera, and we would add in the Neuroptera as well as the male Coccidæ, very similar metamorphic processes take place.

a. Development of the outer body-form

The form of the imago is completely marked out in the pupa, so that the transition from the pupa to the imago is comparatively slight and only depends on the modification and development of the parts already present.

Fig. 622.—Anterior part of young larva of Simulium sericea, showing the thoracic imaginal buds: p, prothoracic bud (only one not embryonic); w, w′, fore and hind wing-buds; l, l′, l″, leg-buds; n, nervous system; br, brain; e, eye; sd, salivary duct; p, prothoracic foot.—After Weismann.

In most cases the modification in question consists of the changes occurring during the passage from the larval form to the imago, the reformation of parts already present being most marked, while the new rudiments only participate in a limited way in the process. Thus, for example, the head of the caterpillar together with the antennæ and mouth-parts, also the thoracic limbs, pass directly and unchanged from the larva into the pupa. The compound eyes and the wings are, however, new formations, the latter arising from imaginal buds. The same is the case with many other Heterometabola, where the passage of the larva into the pupa in general is due to a transformation of parts already present. The changes in the brain, the fusion of certain ganglia of the ventral nervous cord, the changes in the abdomen, involving the reduction in the number of segments and the remodelling of the end of the body, and the formation of the ovipositor or sting, and in the higher Hymenoptera the transfer of the 1st abdominal segment to the thorax, and the origin of the genital armature,—all these should here be taken into account.

It should be observed that in every case where the larvæ are footless, as in Diptera, all the Hymenoptera except the phytophagous ones and certain coleopterous larvæ, the limbs of the imago stage are, in the earliest stages, indicated as new structures in the form of imaginal buds.

Formation of the imago in Corethra.—Corethra may serve as an example of such a relatively simple metamorphosis. Its larva belongs to the group of eucephalous dipterous larvæ. The head of the perfect insect is already indicated in the larva, and its parts, with certain modifications, pass directly into the pupa. The compound eyes, and this is a rare exception among the Holometabola, are present in the larva. On the other hand, the thoracic legs, the wings, and halteres are developed out of new rudiments which are present in the last larval stage, before pupation. Each thoracic segment has four of them, two ventral and two dorsal (Fig. 622); the ventral buds becoming the legs. Of the dorsal pairs, that of the mesothorax develops into wings, that of the metathorax into halteres, while from the corresponding rudiments of the prothorax in Corethra arise the stigma-bearing dorsal or respiratory processes of the pupa, and in Simulium a tuft of tracheal gills (Fig. 623, ra; see also Fig. 582).