We study formation of mutual insurance networks in a model where agents who obtain more resources share a fixed amount of resources with all directly linked agents that obtain fewer resources. We identify the pairwise stable networks and effcient networks in a basic model where agents are identical. Then, we introduce in the model two types of heterogeneity: an exogenous one, where agents differ in their income or in their preferences over the transfer scheme, and an endogenous heterogeneity where the costs of linking to an agent depends on the number of links the latter has already formed in the network. We examine the impact of these heterogeneities on stability and efficiency.