Abstract
The antipredator behaviour, tonic immobility (TI) is a valuable defence that enables insects to increase their chance of survival and is a trade-off between fleeing and protection. How the TI strategies of insects respond to environmental factors, however, remains a largely understudied subject. In this paper the effect of four factors (mechanical stimulation, light, sound and temperature) and metabolic responses were used to evaluate TI behavioural and physiological adaptions in Eucryptorrhynchus scrobiculatus (ESCR) and Eucryptorrhynchus brandti (EBRA). In the behavioural experiment, the metaventrite, which is the stimulus-sensing region in ESCR and EBRA, was subjected to mechanical stimulation. Light lengthened the TI duration in ESCR males, while sound had the opposite effect in ESCR and EBRA. The effect of temperature on the duration of TI was variable: in ESCR, the duration was shorter at low (15°C) and high (32°C) temperatures, but in EBRA, it was longer at the low (15°C) temperature. In the metabolism experiment, ESCR and EBRA metabolic rates (MR), was significantly dependent on whether they were in a state of TI or not. The TIMR declined to 76.90% in ESCR and 71.40% in EBRA. These results indicate that TI in ESCR and EBRA differed under different external conditions and contributes to the understanding of the physiological regulation of ecological traits of insect TI.
Highlights
When some insects are disturbed by predators they curl up and remain motionless or fall from their original resting places and behave as if dead, and in the absence of further stimulation recover and resume activity after a few minutes (Humphreys & Ruxton, 2018)
The duration and frequency of tonic immobility (TI) induced by mechanical stimulation were associated with weight (EBRA: n = 25, P = 0.789; Eucryptorrhynchus scrobiculatus (ESCR): n = 25, P = 0.697) and sex (EBRA: n = 25, P = 0.196; ESCR: n = 25, P = 0.782) (Table 1, Fig. 4)
Whether body weight and sex affected TI, which is the case for C. chinensis, C. maculatus and T. freemani (Hozumi & Miyatake, 2005; Miyatake et al, 2008a; Konishi et al, 2020)
Summary
When some insects are disturbed by predators they curl up and remain motionless or fall from their original resting places and behave as if dead, and in the absence of further stimulation recover and resume activity after a few minutes (Humphreys & Ruxton, 2018). This is called tonic immobility (TI), death feigning, thanatosis, animal hypnosis, playing dead, immobilization catatonia, playing possum or quiescence (Ruxton, 2006; Acheampong & Mitchell, 2010; Li et al, 2019) and is widely used by animals to limit injury and as a means of escape. The variation in TI in different species associated with environmental factors is worthy of further study
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