## 1 Introduction  
The wireless sensor networks (WSNs) technology have been widely applied in military, industry, agriculture and many other areas [1,2]. In the WSNs, a lot of nodes operate on limited batteries, making energy resources the major bottleneck. Therefore, an economical and frugal management of energy is essential for improving energy efficiency. Because energy consumption due to communication is the major part of the energy consumption in WSNs [3], a high performance routing protocol is often a key requirement in WSNs systems. The design of routing protocols in WSNs is very challenging due to their inherent characteristics of large scale, no global identification, dynamic topology, and very limited power, memory, and computational capacities for each sensor. Currently, many energy-efficient routing algorithms have been studied with the aim of saving energy [4, 5, 6].  
The existing routing protocols in WSNs can be categorized into flat routing protocols and hierarchical routing protocols, or single-path routing protocols and multipath routing protocols [7]. Recent research on WSNs routing protocols has proven that clustering and multipath are needed to improve energy efficiency and load balancing.  
When designing multipath routing algorithms, many parameters (e.g., path length and energy consumption of communication) also need be considered. The optimization of network parameters for WSNs routing processes might be considered as a combinatorial optimization problem. Our proposed approach benefits from the success of ant colony optimization (ACO) [8] in solving the problem. The ACO algorithm is a heuristic algorithm introduced by [PERSON] and his collaborators for solving some combinatorial optimization problems [9], such as traveling salesman problem (TSP) [10]. The ACO algorithm has some characteristics, such as distributed computing, self organization and positive feedback, suited for searching routing in modern communication networks.  
However, few of the existing works have considered the integration of clustering, multipath and ACO to maximize the network lifetime and achieve load balancing in WSNs. Motivated by the advantages of clustering, multipath and ACO, this paper proposes a multipath routing protocol (MRP) based on dynamic clustering and ACO for reactive WSNs. The main objective of our work is to maximize network lifetime and at the same time achieve load balancing. The main contributions of this paper are listed:  
(1) A novel distributed algorithm based on some parameters (such as signal strength, residual energy of node) is designed to form clusters among the nodes located in the event area.  
(2) An extended ACO algorithm based on many metrics (such as residual energy, path length, energy consumption of communication) is applied to search for the multiple paths between the cluster head (CH) and sink node.  
(3) A load balancing function is further proposed to distribute the traffic over discovered multiple paths.  
The rest of this paper is organized as follows. Section 2 introduces some related routing algorithms. In section 3 we propose the system model and the motivation of our work. The details of the MRP algorithm are described in section 4. The performance evaluation of our scheme as well as a comparison with the previous typical routing algorithms is presented in section 5. Section 6 draws the conclusions.