Morphological adaptations to silk production by adult females in the pollen wasp genus Quartinia (Masarinae, Vespidae) - a keystone character for ground nesting in dry sand habitats.

While many pollen wasps nest in hard clayey soil or in rigid sand or use these kinds of substrates to build aerial earthen cells, all representatives of the genus Quartinia, in which nesting behavior has been studied so far, construct their nests in habitats with loose sand. The walls of the burrow are stabilized by a silky excretion that is applied on their inner surfaces during nest construction. The lining-behavior of Quartinia females is described. Representatives of the genus Quartinia and other pollen wasp taxa have been studied comparatively using histology, microcomputed tomography and scanning electron microscopy for identifying the structures correlated with lining-behavior. Thus, we can document a change in the morphology of the maxillary gland for females of the genus Quartinia, as well as the existence of a process of the galea which likely serves the production of the silky threads. The fact that these modifications are missing in males is corroborating their function in brood care. Two possible ways of silk thread production are discussed. The newly discovered structures are key adaptations of the genus Quartinia that enabled the inhabitation of new habitats and thus probably facilitated the species diversification of this genus.