Death Feigning in Larvae of Scorpionflies (Mecoptera: Panorpidae): Frequency and Postural Changes Based on Larval Instars

J Insect Behav https://doi.org/10.1007/s10905-024-09859-6 RESEARCH Death Feigning in Larvae of Scorpionflies (Mecoptera: Panorpidae): Frequency and Postural Changes Based on Larval Instars Ryo Ishihara · Kentarou Matsumura · Takahisa Miyatake Received: 2 March 2024 / Revised: 17 July 2024 / Accepted: 26 July 2024 © The Author(s) 2024 Abstract Death feigning is thought to have evolved primarily as a predator avoidance behavior, and has been reported in 10 of the 31 orders of insects. However, there have been no reports of death-feigning behavior in Mecoptera species. We found that larvae of two scorpionfly species, Panorpa japonica and P. pryeri, showed death feigning in response to external stimuli by brush poking stimulation. First, we examined the frequencies of death-feigning postures. The two species showed two different postures of death feigning, “straight” and “ball.” Most of the 1st instar larvae of P. japonica and P. pryeri adopted the straight death-feigning posture. Next, we examined duration of death feigning. As the larval instar progressed, the death-feigning posture shifted from straight to ball in both Panorpa species. In P. japonica, the longest durations of death feigning were found in the 2nd to 3rd instars, while the longest duration of death feigning was found in the late 4th instar in P. pryeri larvae. Keywords Anti-predator behavior · freezing · larvae · thanatosis · tonic immobility R. Ishihara · K. Matsumura · T. Miyatake (*) Graduate School of Environmental, Life, Natural Science and Technology, Okayama University, Okayama, Japan e-mail: Introduction Death feigning is defined as immobilization in response to external stimuli (e.g., Miyatake et al. 2004; Ruxton et al. 2018). The adaptive significance of death feigning in insects has been much studied, and the behavior is thought to have evolved primarily as a predator avoidance strategy (e.g., Humphreys and Ruxton 2018; Skelhorn 2018). For example, in the red flour beetle, Tribolium castaneum, it has been reported that the frequency of predation by Adanson’s jumper spider, Hasarius adansoni, was lower for individuals of the strain with a longer death-feigning duration than for individuals of the strain with a shorter death-feigning duration (Miyatake et al. 2004, 2009). This behavior of temporary immobility without postural change is called freezing, while temporary immobility with postural change is called death feigning or thanatosis (Sakai 2021). Postures of death feigning are sometimes unique and vary not only among species (Miyatake 2021), but also between local populations (Ohba and Matsuda 2021). Furthermore, even within a species, the posture varies according to developmental stage; for example, the immobile posture of the red flour beetle differs between adults and larvae, with larvae in a straight position, while adults turn over on their backs and fold their legs (Miyatake et al. 2008; Matsumura et al. 2017). However, to our knowledge, no other study has reported on variations of death-feigning postures during larval instars. Vol.: (0123456789) J Insect Behav Death feigning has been found in 10 of the 31 orders of insect taxa (Miyatake 2021): Odonata (Gyssels and Stoks 2005), Plecoptera (Moore and Williams 1990), Orthoptera (Nishino and Sakai 1996; Honma et al. 2006), Phasmatodea (Godden 1972), Neuroptera (Sendova-Franks et al. 2020), Mantodea (Edmunds 1972; Lawrence 1992), Hemiptera (Holmes 1906; Villet 1999; Kang et al. 2016; Ohba and Matsuda 2021), Coleoptera (Bleich 1928; Konishi et al. 2020), Lepidoptera (Tojo et al. 1985; Dudley 1989; Larsen 1991) and Hymenoptera (Hölldobler and Wilson 1990; van Veen et al. 1999; King and Leaich 2006; Cassill et al. 2008; Cardoso et al. 2024). We found that when larvae of two species of Mecoptera insects are stimulated, they adopted death feigning in different postures depending on their larval instar. This paper is the first report of death feigning in Mecoptera. We also observed the frequency and duration of death feigning in the two species of Mecoptera in the laboratory. We identified two types of death-feigning posture in the scorpionfly larvae: one where they stayed straight, and the other where they curled up into a ball shape, and also found that the frequency of these postures varied with larval stages. Material and Methods Insects Two scorpionfly species (Mecoptera: Panorpidae), Panorpa japonica and P. pryeri were used in these experiments (Fig. 1). These two species have a life cycle during larval development in which they spend Fig. 1  Adults of two Mecoptera species used in this study: A: Panorpa japonica and B: Panorpa pryeri Vol:. (1234567890) the day underground and are active on the surface at night. Predators of the larvae are unknown. The larvae pupate in the soil and then emerge above ground in early spring. In the present study, 54 larvae were derived from adults collected in April 2020 at a bank of the Asahikawa River, Okayama City (34° 6′ 8′′ N, 133° 9′ 3′′ E) for P. japonica. For P. pryeri, larvae were derived from adults collected in June 2022 at Koguri-yama Mountain, Hirosaki City, Aomori Prefecture (40° 32′ 44′′ N, 140° 28′ 39′′ E). Fifty-five larvae of P. pryeri were used in the experiment, each larva was reared in a Petri dish (9 cm diameter × 2 cm high) lined with a moderately moistened nonfat cotton. The food was commercially available dried sewage worms. Observation of Death Feigning Individual scorpionfly larvae were placed in a Petri dish (9 cm diameter, 1.5 cm height) and lined with filter paper and stimulated with a bristle handle end brush (Pentel, ZBS 1–15, Tokyo). If the larvae did not move after 5 s after stimulation while maintaining the "straight" or "ball" posture described below, they were considered to have entered death feigning; those that moved within 5 s were not considered to be in death feigning. The duration of death feigning from the start to the end (when the larvae started moving again) was measured with a stopwatch, and if the larvae did not become inert after 10 or more stimulations, it was recorded as not feigning death. Both species exhibited two types of postures while feigning death, named “straight” and “ball” (Fig. 2). Measurements of death feigning were taken at five larval instar stages: 1st, 2nd, 3rd, early 4th, and late 4th instars when the larva had matured to a white Fig. 2  Death-feigning postures, straight (A) and ball (B). Examples of P. japonica are shown J Insect Behav body color. Death-feigning posture and duration of death feigning were compared for each larva. In this experiment, death feigning was observed in 39 P. japonica and 44 P. pryeri, and five replicate measurements were taken for all individuals. Statistics A generalized linear mixed model was used to test the respective proportions of individuals that immobilized in a straight posture, those who immobilized in a balled posture, and those who did not immobilize. In this analysis, we used an it (...truncated)