In this paper we present the design of jWebDust, a generic and modular application environment for developing and managing applications based on wireless sensor networks that are accessible via the internet. Our software architecture provides a range of services that allow to create customized web-based applications with minimum implementation effort that are easy to administrate. We here present its open architecture, the most important design decisions, and discuss its distinct features and functionalities. jWebDust allows heterogeneous components to interoperate and the integrated management and control of multiple such networks by defining web-based mechanisms to visualize the network state, the results of queries, and a means to inject queries in the network.