There are two forms of quiescent pupae among Insects: one in which the antennae, mouth-parts, limbs and wings are free, the other in which they are coherent to one another and to the body. Of the first kind of pupa two varieties are distinguishable. In one the larval skin is simply thrown off. It is known as incomplete, exavate or libera, and occurs in the Neuropterai Coleoptera, Hymenoptera and some Diptera. In the other variety, known as coarctate, and occurring only in Strepsiptera and Diptera, the larval skin is retained, and it either preserves the form of the larva, e. g. Strepsiptera, Stratomyidae, or contracts into a barrel-shaped structure, e. g. Muscidae. The second form of quiescent pupa, known as obtected, larvate, or signate, is characteristic of Lepidoptera. The cohesion between the limbs, etc, as seen here, is due to the hardening of a sticky fluid which covers the surface at the moment when the caterpillar skin is thrown off.
The obtected pupa is either angular, as in the majority of Lepidoptera with club-shaped antennae ( = Rhopalocera), and then often brightly tinted; or it is conical, as in Sphinx and other Lepidoptera with the antennae fashioned after various types ( = Heterocera). It is then, with rare exceptions, dark-brown in colour.
The pupa is plainly divisible into three well-marked and dissimilar regions, head, thorax, and abdomen. The head is globular and deflexed. The long antennae take origin from it laterally, and are bent backwards ventrally and towards the middle line. A lunate convexity below and in front of the base of the antennae marks the eye. A small median square piece in front is the labrum, and the angle projecting forwards from the convex surface that bears the eye, i.e. the gena, is formed under the larval mandible. It touches the outer inferior angle of the labrum. The pupal maxillae are large in size, and take origin below the labrum. Each has at its cephalic extremity a horn-like projection, the two projections uniting in the middle line. These projections usually but wrongly identified with the whole tongue, antliae, or maxillae of the imago, are absent in some Sphingidae, e. g. Acherontia, Macroglossa, and of great size in others, e.g. Sphinx Convolvuli.
The maxillae expand into an angle below the eyes, and are then prolonged as narrow bands lying side by side in the median ventral line, as far as the tips of the wings. The labium is hidden by the maxillae. Between the maxillae and antennae may be seen the tibial and tarsal portions of the pro- and meso-thoracic limbs, whilst the metathoracic are completely hidden by the wings. The mesothoracic, or first pair of wings, are large, and hide the metathoracic pair completely, save at their dorsal origin. Both wings and maxillae extend to the middle of the sternal region of the fourth abdominal somite.
The mesothoracic tergum, viewed dorsally, is of great size, whilst the prothoracic,- and especially the metathoracic, are much reduced. A depression behind the outer angle of the prothoracic tergum leads to the spiracle.
The abdomen consists of ten somites. The first and the ninth in particular are small. The first spiracle is completely, the second partially hidden by the wings. The third to the seventh spiracle inclusive are large, but the eighth is denoted only by a well-marked scar. The tenth, somite is prominent. It bears a dorsal spinous projection, the cremaster, which is differentiated within the anal valve of the caterpillar, and is covered with spines which vary much in different specimens. It is used by the pupa as a prop when it works its way up from its underground chamber previous to the emergence of the imago. A longitudinal depression, ventral to the cremaster, marks the anus; and the ridges on either side of it (=sustentors of Riley) correspond to the anal prolegs of the caterpillar. Irregular scars, more or less evident on the ventral aspect of the third to the sixth somites, denote the position of the other prolegs of the caterpillar.
In a male pupa the ring of the ninth somite is interrupted ventrally, and a depression with a more or less prominent tubercle on either side, marks the future aperture of the vas deferens. In a female, the eighth somite (as well as the ninth) is interrupted ventrally and bears a depression, the future aperture of the bursa copulatrix. These depressions appear to be constant in all Lepidoptera.
When June approaches the pupa becomes restless and writhes in its chamber. It works its way up to the surface of the ground by means of the abdomen, the only part of the body which possesses the power of motion. The last abdominal somites of the moth become free first of all from the pupa-skin: this skin then becomes brittle and is fissured longitudinally in the dorsal region of the thorax. A split runs ventrally along the fore-edges of the wings. The moth emerges early in the day and suspends itself vertically while its wings expand and dry. In two to three hours it becomes capable of flight.
The newly-formed pupa is soft: in colour creamy-white, and all its appendages contain large cavities filled by a blood-plasma which is extremely milky owing to the resolution of the fat body. A pupa preserved in alcohol at this stage retains its light colour. One in the possession of Mr. Poulton, has clearly defined though feebly coloured streaks, corresponding to the coloured streaks of the caterpillar. Under natural conditions the pupa hardens and becomes dark-brown. As this change of colour takes place underground, it cannot be due to the action of light.
Swammerdam was the first to point out that the appendages are readily separable in a newly-formed pupa, or may be dissected out from under the caterpillar skin when it is ready to be moulted. The operation is easy if the pupa or caterpillar are preserved in alcohol.
In the caterpillar it will then be found that the angle of the gena already mentioned in the pupa corresponds to the mandible: that the pupal antennae and maxillae are folded upon themselves: that the wings are mere tubercles. All these parts expand and assume their proper position as the caterpillar skin is being cast off.