Vibratory communication in the soil: pupal signals deter larval intrusion in a group-living beetle Trypoxylus dichotoma
Pupae of several insect species are known to generate air-borne sounds and/or substrate-borne vibrations, but the functions of the sounds/vibrations are not well understood. Here, we present the first evidence of vibratory communication between pupae and larvae of a group-living Japanese rhinoceros beetle Trypoxylus dichotoma which inhabits humus soil. The last-instar larvae of this beetle construct their own pupal cells to ensure normal pupation and eclosion. These cells are fragile and subject to damage from burrowing larvae because pupae and larvae co-inhabit the same patches of humus. In laboratory experiments, we demonstrated that pupal cells harboring live pupae were less likely to be broken by larvae than those harboring dead pupae. It was also demonstrated that pupae produced vibrations in response to larvae approaching the pupal cells. High-speed video and vibration analyses showed that pupae emitted 3–7 pulses at 1.3-s intervals by beating their pronotum against the inner wall of the pupal cell. The pupal vibration was of low frequency with a maximum energy at ≈ 100 Hz. The drumming behavior was more frequently observed in the presence of an approaching larva than in its absence. When pupal vibrations were played back near to vacant artificial pupal cells, these cells were rarely disturbed by the larvae. These results provide evidence that pupae generate vibrations to deter conspecific larvae, thereby preventing damage to the cells. This larval response to pupal vibrations may have evolved through preexisting anti-predator and/or sib-killing-avoidance behavior.