Cannibalism of pupae by larvae has been documented In many species of Insects, but the features of larval cannibalism and pupal defensive mechanisms against larval cannibalism have been largely Ignored. Pupae of tenebrionld beetles rotate their abdominal segments in a circular motion in response to the tactile stimulation of appendages, including legs, antennae, maxillary pulps, and wings. When the pupal abdominal rotation responses of Tenebrio molitor and Zophobas atratus were completely blocked by transecting the ventral nerve cord (VNC) of the pupae, the appendages of the paralytic pupae became initial, major targets for attack by larval cannibals. The majority of 20 paralytic pupae was cannibalized by 100 larvae within 6 h, and almost all the pupae were killed within 2-3 days. In contrast, only a few pupae of Z. atratus and several pupae of T. molitor were cannibalized when the VNC was Intact. The abdominal rotation response of the pupae thus functions as an effective defense against larval cannibalism.

Pupae of the beetle Zophobas atratus Fab. (Coleoptera: Tenebrionidae) have jaws called gin traps on the lateral margin of their jointed abdominal segments. When a weak tactile stimulation was applied to the intersegmental region between the two jaws of a gin trap in a resting pupa, the pupa rapidly closed and reopened single or multiple gin traps adjacent to the stimulated trap for 100200 ms. In response to a strong stimulation, a small or large rotation of the abdominal segments occurred after the rapid closure of the traps. Analyses of trajectory patterns of the last abdominal segment during the rotations revealed that the rotational responses were graded and highly variable with respect to the amplitudes of their horizontal and vertical components. The high variability of these rotational responses is in contrast with the low variability (or constancy) of abdominal rotations induced by the tactile stimulation of cephalic and thoracic appendages. Since the closed state of the gin traps lasts only for a fraction of a second, the response may mainly function to deliver a “painful” stimulus to an attacker rather than to cause serious damage.

Background: Pupae of the tenebrionid beetles exhibit different defensive behaviors in response to tactile stimulation of different parts of their bodies. Little is known about the mechanosensory mechanisms that initiate these defensive behaviors. To understand the mechanisms, we examined the distribution and function of the mechanoreceptive sensilla on the pupal body of Zophobas atratus. Methods: The sensilla on the surface of the pupal cuticle were observed using light and scanning electron microscopy. The tactile sensitivity of various parts of the pupal body was tested using three probes with different bending forces. The function of particular sensilla was examined by stimulating them electrically. Results: Hair (trichoid) sensilla were classified according to their length, ie, long hair sensilla (80-200 µm) were mostly located on the lateral regions of each body segment and short hairs (5-50 µm) were distributed across almost the entire surface of the pupal body, except for the elytra and intersegmental membrane of the abdomen. In addition to the trichoid sensilla, almost all parts of the pupal body, including the elytra and the intersegmental membrane, had campaniform sensilla (CS) with a dome-like structure in the cuticular layer; these sensilla detected strains in the cuticle. The CS were concentrated on the articulation of segmented appendages and the lateral region of the intersegmental membrane. The defensive response (abdominal rotation) to electrical stimulation was greatly reduced when afferent nerves from CS clustered on the apical region of the tarsus were transected. Conclusion: CS may be responsible for the high sensitivity of pupal appendages to tactile stimuli. The CS located on the appendages and abdominal intersegmental membrane may trigger specific and effective defensive behaviors. Both types of mechanoreceptive sensilla may enable pupal perception of the external environment, including the location of potentially harmful stimuli, and are involved in the control of defensive posture and movement.