Fig. 618.—Older Encyrtus larva, lateral view, showing the buds of the antennæ (ant), legs, and wings (w, w′): oe, œsophagus; q1, q2, q3, buds of the genital armature; o, rudiment of the sexual gland (ovary or testis); ur.t, urinary tube; st, stomach; i, intestine (rectum); n, ventral nervous cord; r, rectum; sp1-sp9, spiracles.
Fig. 619.—A still older larva, ready to transform. The imaginal buds of the antennæ (f), eyes, wings (a1, a2), and legs have become elongated: ch, chitinous arch; b, mouth; o, eye-bud; g, brain; e, stomach; x, rudiment of the sexual glands (either the ovary or testis).—This and Figs. 617 and 618, after Bugnion.
The wing-buds (a1, a2) appear at the same time as those of the legs, as racket-shaped masses of small cells situated directly behind the 1st and 2d pair of stigmata, in contact with the tissue ensheathed by the corresponding tracheal vesicle (Fig. 618). Afterwards they have exactly the form of those of the Lepidoptera (Fig. 619).
The proliferation of the hypodermis is not limited to the thorax, but takes place at corresponding points in the first seven abdominal segments. These abdominal agglomerations of cells do not give rise to true buds, but serve simply to reconstitute the hypodermis of the abdominal segments at the time of metamorphosis.
Ocular or oculo-cephalic buds.—The eye of insects, as is well known, is a modification of a portion of the integument, the visual cells being directly derived from the hypodermis, the cornea being a cuticular product of this last, like chitinous formations in general.
The ocular buds appear towards the end of larval life as a simple mass of hypodermic cells, and form a compact layer on the dorsolateral face of the prothoracic segment, and clothe the cephalic ganglion or brain like a skull-cap. The central portion only is destined to form the eye, while the peripheral pad, continuing to thicken, gives rise to a voluminous and rounded mass, which meets on the median line that of the opposite side, and forms the integument of all the posterior part of the head.
Bugnion also observed on the median line a group of small hypodermic cells which he regarded as the rudiment of the anterior ocellus, but he did not detect those of the posterior ocelli.
The antennal buds.—These appear at an early date under the cuticle of the head, as two distinct rounded cellular masses, with a central cavity, but no annular zone (Fig. 619, f). Each one grows longer in a transverse sense, and its summit, extended from the outer side, curves downward. It now forms a hollow tube folded at the end, and terminated by a disk whose centre is perforated (Fig. 619, f). Afterwards, when the larva is ready to transform, it grows longer, becomes folded on itself in its cavity, and, passing beyond on each side the limits of the larval head, encroaches on the prothoracic segment.
The buds of the buccal appendages.—Towards the end of the larval period, the buds of the mouth-parts appear as small digitiform projections, situated on each side and below the mouth. Formed of small epithelial cells pressed against each other, they are all directed anteriorly, and possess no furrow or pad.