We present the design and implementation of Arachne, a threads system that can be interfaced with a communications library for multithreaded distributed computations. In particular, Arachne supports thread migration between heterogeneous platforms, dynamic stack size management, and recursive thread functions. Arachne is efficient, flexible, and portable-it is based entirely on C and C++. To facilitate heterogeneous thread operations, we have added three keywords to the C++ language. The Arachne preprocessor takes as input code written in that language and outputs C++ code suitable for compilation with a conventional C++ compiler. The Arachne runtime system manages all threads during program execution. We present some performance measurements on the costs of basic thread operations and thread migration in Arachne and compare these to costs in other threads systems.