When about to emerge the pupa works its way up the vertical tunnel by means of the above-mentioned hooklets, forces open the trap-door, and wriggles along the horizontal burrow until it reaches the air, only the last three or four segments remaining in the tree. Its anterior portions then break open and the moth crawls out and expands its wings in the ordinary way, resting on the trunk of the tree, until they are of sufficient strength and hardness for flight.
The perfect insect appears in October and November. Although it must be common, it is rarely seen; specimens are consequently best obtained in the pupa state and reared in captivity. The easiest way to find the pupa is to pass a straw into the horizontal burrow, and move it about until it touches the trap-door. The collector is at once apprised of this circumstance by a distinct hollow sound, produced by the straw when it comes in contact with the lid, which acts like a miniature drum. If no such sound is heard after moving the straw into every possible position, it may be assumed either that the insect has left the burrow, or that it is inhabited by a larva only. When, however, a pupa is actually discovered, a section of the tree-trunk should be cut out, extending from about two inches above the horizontal burrow to about one foot below it, and the log, thus obtained, taken home. Should a number of pupæ be found in one tree the whole trunk may then be taken, if practicable, and kept in a well-lighted room or a conservatory, until the enclosed insects emerge. The specimens usually come out of the pupa at about five or six o'clock in the evening, and if intended for the cabinet should be killed before dark, as they very soon injure themselves when flying.
The best time of year to obtain the pupa of this insect is during August and September, as most of the specimens are then in that condition. Apart from the indications above described, burrows containing larvæ may often be known by the fresh pellets of excrement which are present near the opening. The vacated burrows frequently have the remains of the old pupa shell at the entrance, and generally look gnarled and weather-worn. These indications are useful as guides to the collector before exploring the burrow with a straw in the manner above described.