Identified as Chauliognathus on BugGuide but not to species.
Imperial County, California, US
San Diego County, California, US
Santa Cruz County, Arizona, US
The larva that has snails for lunch.
It is definitely amazing that a creature as 'trivial' as a caterpillar could pose a huge threat to a creature larger than itself. This is a case similar to the proverbial ant that spooked the elephant.
Danger signals with a variety of colours; red, black and yellow are more familiar, and a combination of any two colours, or all, may well equate to being lethal. Creatures with these colours are often avoided like a plague. A tiny lesion-causing insect popularly known as 'papa' in this part, coloured exactly like this caterpillar is hard to forget. It must not be swatted against bare skin else... The coral snake is another classic example, with those ominous array of showy colours.
The terror inflicted on a snail on seeing the flesh-consuming larva approach is not hard to imagine. First, it may try to speed up (poor thing with no legs), or twirl its shell around an obstacle in order to dislodge a larva already onto it. Last resort is for the snail to retreat into its shell when it finds it impossible to outdo the relentless larva. But it often is a very bad idea that ends the long battle for survival if the snail does not withdraw leaving the face of its shell wedged against the ground. The larva would have the snail cornered, eat all of it and may even inhabit the shell for a while before going on subsequent hunts. The snail may however be toppled over depending on the size of its shell. Any snail that lives to see another is one that may have fallen off a height that had the larva removed on impact, or lost the caterpillar in the heat of the fight after rolling over an inclination.
This larva species, with black ants are largely responsible for snail mortality. They are the bane of snail farming.