Cylindrical Click Beetle larvae, particularly the economically important ones, are called Wireworms.
The one shown here was feeding under bark. Other species feed in the soil, where they may eat the roots of developing plants. Some attack the roots and tubers of commercially grown vegetables.
Typically in the family Elateridae there are three to five larval instars, and development takes one to three years depending on the quality and quantity of food available.
One particularly interesting Click Beetle group is the tribe Pyrophorini, in which the larvae are bioluminescent. Adult Pyrophorines are luminescent too, and may attract mates with their flash patterns.