Epithelia form barriers within the body of animals and protect the body against the environment. They are exposed to signals from both sides, and these signals provoke specific responses within the epithelial cells. For example, nervous and hormonal signals elicit secretory activity in exocrine glands; the epithelial cells then secrete electrolytes, water, proteins and/or mucins. We examine the intracellular signaling pathways that mediate between membrane-bound receptors for hormones or neurotransmitters, respectively, and the mechanisms of transepithelial ion transport. Moreover, we are interested in the mechanisms involved in the development and maintenance of polarity in epithelial cells. For these studies, we use exocrine glands of insects, particularly the fruitfly Drosophila melanogaster and the blowfly Calliphora vicina.