A functional unit consisting of an eversible gland with neurosecretory innervation and a proprioreceptor derived from a complex sensillum in an insect

Summary The eversible sac of the antennal tip in Hypogastrura socialis (Collembola) has been reconstructed from serial ultrathin sections. The organ contains 3 specialized epidermal glandular cells and the dendrites of 2 sensory cells encapsulated by an enveloping cell. The following features of the system are particularly remarkable and have been analyzed in detail: (1) a neurosecretory innervation of the glandular cells, (2) the structure of the dendritic outer segments within the sac, and (3) the structure of the complex sensillum, from which these dendrites may be derived. The system may be thought of as providing an example of phylogenetic transformation of an exteroceptor into a mechanoreceptive proprioceptor. A functional model is proposed which involves control of the mechanism of evagination as well as of the secretory discharge.