Which organism's larva is commonly found associated with earthworms?

The correct choice is the cluster fly, as its larvae are commonly associated with earthworms. Cluster flies, primarily belonging to the family Calliphoridae, are known for laying their eggs in soil where they can find earthworms. When the larvae hatch, they typically enter the bodies of earthworms, feeding on them as they develop. This relationship allows the larvae to benefit from the nutrient-rich tissue of earthworms.

In contrast, bed bugs primarily reside in human habitations and do not have a life stage associated with earthworms. Ants, while they may interact with worms in the ecosystem, do not have larvae that depend on earthworms for development. Similarly, moth larvae, or caterpillars, typically feed on plant material and do not exhibit a relationship with earthworms. Hence, the association of cluster fly larvae with earthworms is a distinct biological relationship that highlights the ecological roles these organisms play.