RESUMEN
The activities of social insect colonies are supported by exocrine glands and the tremendous functional diversity of the compounds that they secrete. Many social wasps in the subfamilies Vespinae and Polistinae have two sternal glands-the van der Vecht and Richards' glands-that vary in their features and function across the species in which they are found. Field observations suggest that giant hornets use secretions from the van der Vecht gland to chemically mark targeted nests when workers initiate group attacks on social insect prey. However, descriptions of giant hornets' sternal glands and details about their recruitment behavior are lacking. We describe the morphology of the sternal glands of the giant hornet Vespa soror du Buysson and consider their potential to contribute to a marking pheromone. We also assess the gastral rubbing behavior of workers as they attacked Apis cerana F. (Hymenoptera: Apidae) colonies. V. soror workers have well-developed van der Vecht and Richards' glands on their terminal gastral sternites, with morphologies that robustly support the synthesis, storage, and dissemination of their secretory products. Observations confirm that the van der Vecht gland is exposed during gastral rubbing, but that the Richards' gland and glands associated with the sting apparatus may also contribute to a marking pheromone. Workers briefly but repeatedly rubbed their gasters around hive entrances and on overhead vegetation. Colonies were heavily marked over consecutive attacks. Our findings provide insight into the use of exocrine secretions by giant hornets as they recruit nestmates to prey colonies for group attacks.
RESUMEN
Although the contact calls of birds have been studied for their acoustic properties, limited research has investigated their repetitive nature. The rate of contact calls could be related to movement, with recruiting birds signalling their location, or it could help maintaining spacing between group mates, or give information about the environment where both signaller and receiver are located. If maintaining spacing, higher call rates would be expected in denser vegetation; alternatively, if birds gain information about predation risk from the cessation of contact calling, then open areas might elicit higher call rate. We studied how contact call rate in groups of Swinhoe's White-eyes (Zosterops simplex) was influenced by vegetation, collecting a total of 800 recordings. After statistically controlling for group size, the vegetation effect was weak and inconsistent. However, flying individuals produced a distinct flight call consisting of repeated notes similar to contact calls, and group-level contact call rate increased before flights, particularly when birds flew into the group. Therefore, we believe that contact call rate indicates information about individual or group movements, and could function as a continuous signal about the need for recruitment. We encourage further studies investigating how habitat, risk and audience influence contact call rate.