In mobile ad hoc networks, data transmission is performed within an untrusted wireless environment. Various kinds of attack have been identified and corresponding solutions have been proposed. Wormhole attack is one of the serious attacks which forms a serious threat in the networks, especially against many ad hoc wireless routing protocols and location-based wireless security system. We identify two types of wormhole attacks. In the first type, malicious nodes do not take part in finding routes, meaning that, legitimate nodes do not know their existence. In the second type, malicious nodes do create route advertisements and legitimate nodes are aware of the existence of malicious nodes, just do not know they are malicious. Some researchers have proposed detection mechanisms for the first type. In this paper, we propose an efficient detection method called Delay Per Hop Indication (DelPHI). By observing the delays of different paths to the receiver, the sender is able to detect both kinds of wormhole attacks. This method requires neither synchronized clocks nor special hardware equipped mobile nodes. The performance of DelPHI is justified by simulations.