A mobile robot needs locomotion mechanisms that enable is to move unbounded through-out its environment. However, there is a large variety of possible movement ways and so the selection of a robot’s approach to locomotion is an important aspect of mobile robot design. Most of the locomotion mechanisms have been inspired by their biological counterparts.
Biological systems succeed in moving through a wide variety of harsh environments. Therefore, it can be desirable to copy their selection of locomotion mechanisms.
However, the drawbacks in replicating nature in this is extremely difficult for several reasons beginning with the mechanical complexity. For example, insects achieve a level of robustness that human fabrication techniques are not able to achieve. Moreover, a major problem is the energy storage system and the muscular and hydraulic activation systems used by large animals and insects for achieving a certain amount of torque, response time and efficiency that is far exceed similarly man-made systems. These limitations lead us to develop mobile robots using wheeled mechanisms which is a well-known human technology for vehicles or articulated legs inspired by spiders’ locomotion mechanism as well as some mammals. In general, legged locomotion requires higher degrees of freedom and therefore greater mechanical complexity than wheeled systems.
Read more about locomotion : Wikipedia