Fig. 616.—Transformation of the bumblebee, Bombus, showing the transfer of the 1st abdominal larval segment (c) to the thorax, forming the propodeum of the pupa (D) and imago: n, spiracle of the propodeum. A, larva; a, head; b, 1st thoracic, c, 1st abdominal, segment. B, semipupa; g, antenna; h, maxillæ; i, 1st, j, 2d leg; k, mesoscutum. l, mesoscutellum; m, metathorax; d, urite (sternite of abdomen); e, pleurite; f, tergite; o, ovipositor; r, lingua; q, maxilla.
In what may be termed the 3d stage (Fig. 616, C), though the distinction is a very arbitrary one, the change is accompanied by a moulting of the skin, and a great advance has been made towards assuming the pupal form. The abdomen is very distinctly separated from the thorax, the propodeum being closely united with the thorax, and the head and thorax taken together are nearly as large as the abdomen, the latter now being shorter and perceptibly changed in form, more like that of the completed pupa, while there are other most important changes in the elaboration of the parts of the thorax, particularly the tergites, and of the head and its appendages. Meanwhile the ovipositor has been completed and nearly withdrawn within the end of the abdomen.
The next to study the transformations of the Hymenoptera was Ganin, who discovered the early remarkable pre-eruciform larvæ, as we may call them, of certain egg-parasites (Proctotrypidæ). He discovered the imaginal buds of the wings in the third larva of Polynema (Fig. 185), but his observations, and those of Ayers, need not detain us here, as they have little to do with the subject of the normal metamorphosis of the Hymenoptera, and will be discussed under the subject of Hypermetamorphosis.
To Bugnion we owe the first detailed account of the internal changes in the Hymenoptera, his observations being made on a chalcid parasite, Encyrtus fuscicollis, a parasite of Hyponomeuta. The apodous larva (Fig. 618) moults but once, the next ecdysis being at the time of pupation. It passes through a semipupal stage.
Fig. 617.—Encyrtus larva: 1, 2, 3, ganglia in front of the brain; m, mouth; s. gl, silk-gland; br, brain; n, nervous cord; w, bud of fore, w′, bud of hind, wing.
Bugnion observed in the larva of Encyrtus three pairs of lower thoracic or pedal imaginal buds, two pairs of upper or alary buds, a pair of ocular or oculo-cephalic buds destined to build up all the posterior part of the head, a pair of antennal buds, and three pairs of buds of the genital armature (ovipositor). He also detected the rudiments of the buccal appendages under the form of six small buds (Fig. 619), which do not invaginate, and are not surrounded by a semicircular pad. Also in the abdomen, behind each pair of stigmata, there is a group of hypodermic cells (Fig. 617), which, without doubt, correspond to the wing-buds, but are not differentiated into a central bud and its pad, and does not merit the name of imaginal bud. In fact, except the eye-buds, which are unlike the others, he only observed the imaginal buds of the legs, wings, and ovipositor. The antennal buds are, like those of the buccal appendages, without an annular zone.
The pedal buds were detected in the middle of larval life. They each form a central bud surrounded by a circular thickening. They gradually elongate and become tongue-like and somewhat bent; soon a linear opening or slit appears, forming the mouth of a cavity which communicates with that of the body, allowing the passage into them of the tracheæ, muscles, and nerves, and afterwards of the blood. Finally, the buds grow longer and slenderer, are bent several times, and show traces of the articulations; and soon under the old larval skin, now beginning to rise in anticipation of the moulting, we see the coxa, femur, tibia, and tarsus of the perfect insect, the tarsal joints not yet being indicated.
