Dear Mr. Bug Man,
These live in my compost pile. They seem to be good for the decomposition, because they eat the contents of the pile and excrete them in a much-broken-down-form. But: what the hell? Big as my pinkie. Jerusalem Cricket?
Despite the suspiciously similar appearance to the killer "graboids" from the movie Tremors, your grub is just a grub, in this case the larval form of the Green Fruit Beetle (Cotinus mutabilis). Any observant insect watcher in Southern California, Arizona or Mexico has surely seen these enormous metalic green scarabs which take flight in August and September, buzzing noisily and circling clumsily in their search for fruit, namely figs, peaches, apricots, nectarines, grapes and cactus fruit which is the wild host plant. Originally native to Arizona and New Mexico, the beetle has moved west and is now relatively common in the Los Angeles Basin. Eggs are laid in compost piles, and the grubs, which can reach 2 inches in length, are sometimes called "crawly-backs" because of their method of locomotion, which involves undulating the body and pushing against the substratum with short stout bristles on the back of the thorax. The grubs feed on decaying vegetation, and are beneficial to the compost pile.