Synchronized contractive movement of Amaurobius ferox spiderlings

During the post-matriphagy period, Amaurobius ferox spiderlings (Araneae, Amaurobiidae) show synchronous movement, contracting their bodies simultaneously. This paper describes this behavior for the first time and identifies influencing factors. The spiderlings’ contractions triggered by web vibration caused by intruders result in a strong pulsation of the whole web that a single individual would not be able to induce by itself. Repetition of the contractions was synchronized among individuals (n = 60 clutches). The movement appeared on the first day after matriphagy. The proportion of participants was maximum on the third day post-matriphagy, when on average 60.7% of the individuals were involved; thereafter the synchronicity progressively decreased. The spiderling groups performed contractions at the highest frequency on the fourth day post-matriphagy, and stopped contracting after the second molt. Experiments using mechanical stimuli produced by an electronic vibrator and a cricket’s movement showed that the vibrational intensity of the external stimuli was positively correlated with the number of contractions performed. Nestmate presence increased the number of contractions performed by individuals, and members of densely packed groups showed more contractions per individual than those in less dense groups. Contractions appeared only during the period when the mother was absent (after matriphagy, or when the mother was removed after the first molt of spiderlings and before matriphagy), and the young were not yet capable of capturing prey. Contractions may function as an antipredatory behavior.