Parental care of post-hatching offspring is widespread in insects, but the role of communication in parent-offspring interactions remains largely unknown. I have found that, in the subsocial treehopper
, aggregated nymphal offspring produce substrate-borne, vibrational signals in synchronized bursts that elicit the mother's antipredator behavior. In this study I describe the signals used by nymphs and explore their role in mother-offspring interactions and within-brood communication. Nymphs were stimulated to signal in the laboratory in response to light contact, simulating the approach of a predator. Signals of nymphs at the site of disturbance triggered a rapid wave of signaling by many individuals within the aggregation. This coordinated signaling was associated with the mother's defensive behavior. Signaling was limited to the vibrational channel: when transmission of vibrations was blocked between signaling nymphs and the mother, the mothers' response was abolished. Nymphs signaled not only in response to contact, but also in response to playback of signals from their siblings. Nymphs in otherwise undisturbed aggregations signaled only in response to signals coordinated into synchronized, group displays, and not to signals in random temporal patterns. However, nymphal signaling thresholds were lowered after a recent experience of simulated predation. After a period in which nymphs were stimulated to signal (by light contact simulating a predator's approach), playback of one individual signal could trigger a coordinated burst within the aggregation. It remains unknown if coordination among siblings to produce synchronized, group signals is completely cooperative, or if siblings compete for the mother's proximity. But it is clear that a complex system of communication among siblings, and between siblings and their parent, is an important feature of maternal care in these subsocial insects.