Swarmlets are applications and services that leverage networked sensors and actuators with cloud services and mobile devices. This article offers a way to construct swarmlets by composing "accessors," which are wrappers for sensors, actuators, and services, that export an actor interface. Actor semantics provides ways to compose accessors with disciplined and understandable concurrency models, while hiding from the swarmlet the details of the mechanisms by which the accessor provides sensor data, controls an actuator, or accesses a service. This architecture can leverage the enormous variety of mechanisms that have emerged for such interactions, including HTTP, Websockets, CoAP, and MQTT. Recognizing that these standards have emerged because of huge variability of requirements for bandwidth, latency, and security, accessors embrace heterogeneity instead of attempting to homogenize.