A safety band prevents falling of the suspended pupa of the butterfly Inachis io (Nymphalidae) during moult. A comparison with the girdled pupa of Pieris brassicae (Pieridae, Lepidoptera)

Abstract

During the pupal moult the suspended prepupa of the butterfly Inachis io completely removes its larval skin (exuvia) from the body. In doing this, the attachment to a silken pad has to be transferred from the anal prolegs of the prepupa to the cremaster of the pupa without losing the hold at any time. Falling is prevented by a short safety band which is drawn off from the innermost layer of the exuvia as it adheres to two prominent, pupal projections when the exuvia is slipped backwards. The projections which lie on the ventral side of the last abdominal segment are covered with numerous, minute spines each orientated in the opposite direction to the removal of the exuvia. The transparent safety band has a gelatinous consistency and is strong enough to withstand the fivefold load of a pupa. By contrast, the prepupa of Pieris brassicae is additionally held in place during moult by a girdle around the back. It usually rests in an upright position. Nevertheless, the pupa also shows the prominent projections covered with spines. The exuvia is held fast by the projections, but only a fold is formed. The formation of a safety band is never observed. The connection of the exuvia to the projections is sufficient to hold the tip of the abdomen within reach of the pad for hooking the cremaster therein. However, the pupae of those species in Satyrinae which pupate lying in cocoons or cells lack any kind of projections as well as spines.