Abstract
Eggs of the migratory locust, Locusta migratoria (Orthoptera: Acrididae), hatch synchronously when in a pod, but only sporadically when kept separately. Here, we aimed to detect the vibrational stimuli emitted by eggs that initiate synchronous hatching. First, we recorded the vibrations emitted by an egg pod and single eggs. One bout of vibrations consisted of 2 to 46 vibrations. The total number and amplitude of vibrations in single eggs increased as the time to hatch decreased. Eggs kept separately were continuously subjected during the last 2 days before hatching to recordings of vibrations from a single egg. Recordings made during the last 2.5 h before hatching caused these eggs to hatch significantly earlier than those not subjected to this treatment, the control. In contrast, eggs subjected to recordings made 8 to 10 h before hatching significantly delayed their hatching relative to the controls, which indicates that synchronous hatching of eggs is induced by age-dependent changes in vibrations from neighbouring eggs. Exposure to one large bout of vibrations (consisting of 40 vibrations in 101 s) was sufficient to induce synchronous hatching in eggs kept separately when applied 5 h before hatching, but not 36 or 11.5 h before hatching. Visual inspection of the spectra indicated that the vibrations had two peaks at about 100 Hz and 1.5 kHz. Only exposure to the latter altered the hatching time of eggs. The embryo moved the posterior part of its abdomen when emitting the vibrations. These results indicate that the synchronous hatching of locust eggs is induced when the embryos emit particular vibrations.
Highlights
Synchronous hatching has various advantages for animals laying batches of eggs, including the shortening of the time interval in which hatchlings as a group can be attacked by predators, swamping predators on emergence, reduction of egg cannibalism among siblings, helping hatchlings to escape from the ootheca or in forming an aggregation and enabling hatchlings to start feeding as a group (Morimoto & Sato, 1962; Provine, 1976, 1977, 1981; Arnold & Wassersug, 1978; Dehn, 1990; Warkentin et al, 2006; Nishide & Tanaka, 2016; Endo & Numata, 2017; Endo et al, 2019)
The recording of the egg pod started 46 h before hatching and the first bout of vibrations was detected at 22 h before hatching
Our study using video and vibration recordings revealed that the embryos of the migratory locust move the posterior part of their abdomens in a characteristic way when the vibrations are emitted
Summary
Synchronous hatching has various advantages for animals laying batches of eggs, including the shortening of the time interval in which hatchlings as a group can be attacked by predators, swamping predators on emergence, reduction of egg cannibalism among siblings, helping hatchlings to escape from the ootheca or in forming an aggregation and enabling hatchlings to start feeding as a group (Morimoto & Sato, 1962; Provine, 1976, 1977, 1981; Arnold & Wassersug, 1978; Dehn, 1990; Warkentin et al, 2006; Nishide & Tanaka, 2016; Endo & Numata, 2017; Endo et al, 2019). Synchronous hatching is well researched and is reported in both vertebrates and invertebrates (Spencer et al, 2001; Saigusa, 2002; Ikeda et al, 2006; Vergne & Mathevon, 2008; Webster et al, 2015; Aubret et al, 2016). Eggs are often laid in masses, which hatch synchronously. Most insect species do not attend their eggs, and there are only a few studies on the mechanism that underlie synchronous hatching in such species with the exception of the cockroach (Provine, 1976) and several species of bugs (Kiritani, 1964; Endo & Numata; 2017)
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