Structure, mechanism and mechanical properties of pupal attachment in Greta oto (Lepidoptera: Nymphalidae: Ithomiinae)
The structure and mechanism of pupal attachment are described for the nymphalid Greta oto using electron microscopy, and high‐speed and time‐lapse photography. The cremaster is composed of a 3‐D array of hooked setae that engage with silk fibers spun into layers in a pad on the lower leaf surface. Each seta comprises a shaft terminating in a strongly curved hook, tipped with two lateral barbs. These hook into the silk pad, which is densely laid and built‐up in the central portion, flattening out peripherally. Time‐lapse photography showed that silk pad construction by fifth instar larvae is completed in four distinct spinning movements, producing a random fiber arrangement. It is proposed that such a fiber arrangement provides isotropic strength, giving greater flexibility to the attachment. The cremaster is attached to the silk pad by a series of lateral movements of the pupa's posterior abdomen. This movement, together with the shape of the setal hooks, is thought to be integral to the attachment process. Tensile loading tests showed that attachment failure is due to the breakage of the silk pad, which undergoes gradual destruction before releasing the cremaster. The attachment was found to have high tensile strength and fracture toughness, both of which suggest that it has evolved for the dual purpose of preventing the pupa being pulled from the leaf by a predator and preventing the attachment being weakened by wind, which causes the pupa to swing.