## 1 Introduction  
The integrated use of mobile and static sensor nodes is a promising approach that allows the deployment of _advanced surveillance systems_[1, 2]. On one hand, simple and cheap (low-end) static ground sensor nodes allow massive deployment, making it possible to cover large areas [3]. On the other hand, mounting more expensive and more sophisticated (high-end) sensors on mobile platforms [4, 5] allows their usage in different locations of a covered area [6]. Combining these two types of sensors, the low-end sensors can trigger the usage of the high-end ones in different locations upon demand, which allows having a lower number of these expensive sensors, thus resulting in a positive impact in the overall cost of the system, without losing functionality. In respect to the mobility feature, despite the usefulness of bi-dimensional mobile platforms on the ground, such as _Unmanned Ground Vehicles_ (_UGVs_), the sensor's operational area may be reduced due to either the terrain's geography or other obstacles [7]. Three or two dimensional mobile platforms in the air (two by moving at a fixed height), such as _Unmanned Aerial Vehicles_ (_UAVs_) [8], may provide better results as they have the ability to freely move the sensor to the desired locations, avoiding obstacles. Small UAVs, such as those presented by MLB Spyplanes [9], are much cheaper than large UAV platforms, such as Predator and Globalhawk [10] and are able to perform missions in regions which large platforms would not be able to access, such as urban environments [11, 12]. Based on these arguments, the interest for small UAV platforms to act as the mobile sensors is reasonably rational. In order to make this above discussed combined WSN setup efficiently work, this paper presents a networking strategy to allow the cooperation of low-end static sensors on the ground and high-end mobile sensors in the air, applied to the scenario of large areas surveillance. The proposed strategy aims at providing efficient communication among the sensor nodes, reducing the number of messages exchanged in the network. The goal of this reduction is to decrease the overhead in terms of resource consumption of the low-end static sensor nodes, as communication is known as a resource intense task in WSNs and an important concern for these sensor nodes [13]. In other words, we propose a method to deliver alarm messages issued by the static sensor nodes to the UAVs, so that the desired cooperation needed to perform resource efficient area surveillance is achieved. To address this alarm messages delivery problem, the approach proposed in this work uses a decentralized strategy, based on artificial _pheromones_, inspired by the biological mechanisms used by ants to track food in the nature [14]. Artificial pheromones have previously been applied to distributed coordination by means of stigmergy, the indirect communication using environment cues [15]. Pheromone marks are deposited in the environment forming a trail while biological entities such as ants are moving. The pheromone provides information to other entities when they pass over it. Artificial pheromones have the same feature, _i.e._, to provide information, as well as the same behavior; they also lose their strength over time, modelling the evaporation of the real pheromones. These are the main characteristics explored in the proposed approach. The _main contributions_ of this paper can be summarized as follows:  
1. A detailed model of WSN composed of static low-end ground sensor nodes and small UAVs applied to a scenario of large area surveillance is proposed;
2. Reusing a known concept of pheromone-based communication for the studied scenario, we propose a novel protocol for the alarm message delivery from static to mobile sensor nodes,including three different mechanisms: trail-follow and trail-search as well as pheromone distribution over the ground sensors. The innovative aspect of this approach is supported by the way the stigmergy concept is used to guide the communication and coordinate the sensor nodes.