24-h variation of vigilance in the cockroach Blaberus giganteus

Our objective was to investigate whether sleep-like states occur in the cockroach, Blaberus giganteus by applying the methods formerly used for another cockroach species, Leucophaea maderae and the scorpions, Heterometrus and Pandinus. The behaviour of isolated animals (n = 10) kept under LD 12:12 h was recorded by time-lapse video for three consecutive 24-h periods. Nine behavioural states were scored for 1-min real-time epochs. Rest was subdivided into 4 sub-states on the basis of body posture and the position of the antennae. The cockroaches showed a nocturnal behaviour exhibiting a bout of locomotion at dark onset which lasted several hours, and a preference for rest in the light period. Immobility with both the body and the antennae touching the substrate (state 1) was the predominating state in the light period. In order to establish whether the sub-states of rest represented different levels of vigilance the arousal threshold was measured by determining the latency of a behavioural response to a vibration stimulus. The levels of arousal differed significantly in four behavioural states in the light period but not in the dark period. In state 1 the animals exhibited the lowest arousal whereas in the activity states arousal was the highest. The state with the highest arousal threshold occurred in the beginning of the light period. Thereafter, arousal progressively increased and remained relatively high during the dark period. The effect of 6-h deprivation of rest by the gentle shaking of the cages whenever the animals were immobile, resulted in a reduced latency to state 1, a small increase of state 1 and a more prominent initial increase of activity during recovery. In conclusion, this study provides evidence for the existence of a 24-h variation of vigilance in the cockroach. It further indicates that a 'rest deficit' gives rise to a compensatory response. The data support the notion that sleep-like states are present in these insects.