Process of pupation.—Notwithstanding the great number of persons who have reared Lepidoptera, close and patient observations as to the exact details are still needed. Gonin, who has made the closest observations on Pieris, pertinently asks why the antennæ, which are appendages of the head, are visible in the abdominal region, and why the tongue (maxillæ) is extended between the legs as far as the 3d abdominal segment. To answer these questions he made a series of experiments. Selecting some caterpillars which were about to pupate, he produced an artificial metamorphosis by removing the cuticula in small bits. Exposing the appendages in this way, they preserved the position which they are seen to take during growth. Each wing appeared within the limits of the segment from which it grew out (Fig. 610), not extending beyond, as it does in the normal pupa, so that Réaumur was wrong in saying that “the wings are here gathered on each side into a kind of band, which is large enough to lie in the cavity which is between the 1st and 2d segment.” (8e Mém., p. 359.)
All these parts are coated with a viscous fluid secreted by special glands, which hardens after pupation upon exposure to the air. So long as the parts are soft, they can easily be displaced. Gonin drew one of the antennæ like a collar around the head, and one half of the tongue upon the outer side of the wing.
“When pupation is normal, the integument splits open on the back of the thorax, and the pupa draws itself from before backwards. Owing to the feeble adherence which the chitinous secretion gives it, it draws along with it the underlying organs. The legs, antennæ, the two halves of the tongue (maxillæ) retained by their end, each in a small chitinous case, can only disengage themselves from it when in elongating they have acquired a sufficient tension. The curves straighten out and the folds unbend. The chitinous mask of the head in withdrawing from the larval skin follows the ventral line; the tongue and labial palpi free themselves from its median part; the antennæ disengage themselves from the two lateral scales. Between these different appendages a space is left on the surface of the head for the eyes, and on the thorax for the legs. These are not completely extended on account of the lack of freedom of the femoro-tibial articulation; the femur preserves its direction from behind forwards, and the knee in the two first pairs remains at the same height. The wings overlie them and cover the under side of the two basal abdominal segments; their surfaces in becoming united increase much in size.”
As the chitinous frame of each spiracle gradually detaches itself, we see a tuft of tracheæ passing out of the orifice. It is at this moment that the provisional tracheal system is cast off, and it is easy to see that the process is facilitated by the simultaneous elongation of all the appendages. The permanent tracheæ can follow this elongation because they are sinuous, and need only to straighten their curves. It is, however, not the same with the tracheoles, as we have seen in the case of the wings (p. 133), and their extension or stretching is thus explained by a very simple mechanism.
“The position which the organs assume in the chrysalis is not due to chance, everything is determined in advance, and the microscope shows us that the structure of the hypodermis is specially modified in all the parts which remain external. It is a fact well known to those who rear Lepidoptera that if this normal arrangement is disturbed there are few chances that the perfect insect will survive. A leg lifted up, or an antenna displaced, leaves a surface illy protected against external influences. Almost always this accident causes a drying of the chrysalis.
“Several interesting experiments may be cited as bearing on this subject. If during transformation the chitinous mask of the head is separated from the integument beneath, it is arrested half-way in its development, and the antennæ and tongue are not fully extended. When the case or skin of the caterpillar is drawn, not from before backward, but in the opposite direction, all the appendages of the thorax are placed perpendicularly to the body. Dewitz and Künckel d’Herculais, in stating that the skin of the caterpillar splits open along its whole length, show that they were ignorant of the mechanism; for it is precisely because the chitinous larval skin splits open only in front that it preserves sufficient adherence to the organs beneath to draw them after it in the direction of the abdomen.
“To only read modern authors, one would suppose that the mechanism of pupation had remained hitherto unknown. In reality, it did not escape the notice of Swammerdam or of Réaumur, both of whom have described it with care. The first attached too much importance to the flow of blood, the action of which would be rather to push the organs out than to extend them over the surface of the thorax; the second insists on the movements of the insect. This factor, very admissible in caterpillars, ‘whose under side is situated on a horizontal plane’ (iii, 9e Mémoire, p. 395), cannot be invoked for those which suspend themselves by the tail, as in the genus Vanessa.” (Gonin.)
b. The Hymenoptera
In the Hymenoptera, Ratzeburg was the first to figure and describe the numerous intermediate stages between the larva and pupa, his subjects being the ants, Cynips, and Cryptus, which pass through five stadia before assuming the final pupal shape.
In the bees, as we have observed in the larvæ of Bombus (Proc. Bost. Soc. Nat. Hist., 1866), after hardening a series in alcohol of young in different stages of development, it will be found difficult to draw the line between the different stages since they shade insensibly into each other, those represented in Fig. 616 being selected stages. The head of the incipient semipupa distends the prothoracic segment of the larva whose head is pushed forward and the thoracic segments are much elongated, while the appendages and wings are well developed, and have assumed the shape of those of the pupa. Development both in the head and thorax begins in the most important central parts, and proceeds outwards to the periphery. During this period the “median segment,” or 1st abdominal, has begun to pass forward and to form a part of the thorax.