Wireless Sensor Networks (WSNs) comprise spatially distributed sensor nodes to cooperatively monitor physical conditions in harsh environments that make packet transmission unreliable. Some studies try to adapt the well-known Transmission Control Protocol (TCP) for reliable data transport in WSNs. However, a long multi-hop connection usually leads to poor performance, such as low packet delivery ratios and throughputs. The connection can be split into multiple shorter fragments by selecting some intermediate nodes as TCP proxies to improve performance. This letter proposes a scheme called Adaptive Split-TCP (AS-TCP) which can adaptively split a TCP connection into fragments of varied lengths (i.e., numbers of hops) based on wireless channel conditions.