Cuticular cavities: storage chambers for cyanoglucoside-containing defensive secretions in larvae of a Zygaenid moth.

As a defensive reaction against predators the larvae of Zygaena trifolii Esper. 1783, release highly viscous fluid droplets out of cuticular cavities. The fluid appears on the cuticular surface upon contraction of the irritated segments, with no specialized muscles being involved. Two morphologically different types of cavities have been found: the larger ones are located beneath pigment spots, the smaller ones occupy the remaining surface except in the ventral region. Both types have complicated cuticular opening structures. The defensive fluid contains the cyanoglucosides linamarin and lotaustralin, the amino acid beta-cyano-L-alanine, proteins and water. Although a considerable amount of fluid (3-6 microliter per sixth-instar larva) is stored in the cuticle, fine structural examinations of the epidermis do not show any specific cells or cell areas with morphological adaptations for secretion. Further, there do not exist any major cytological differences between the cells below the cavities and in the ventral region, where those cavities are absent.