The larva of Promachus vertebratus are typically cream-colored and live in the soil or in decaying wood in which they hatch. To begin feeding, the larvae pierce the body of their prey and suck the bodily fluids from the initial wound. These larvae have been observed feasting on grasshopper eggs and white grubs because these insects will generally feed on eggs or any soft-bodied insects that they have access to (Mahr S. 2009).
    According to observation, the larva of Promachus vertebratus is the most likely of the asilids known to attack white-grubs. Typically, the larva of Promachus vertebratus will engage theses grubs when they have become less inactive and are in the process of making cells for pupation (Malloch J. R. 1922).

The adult Promachus vertebratus  are predominantly aggressive, generalist hunters. Because they are generalist hunters, they have been documented attacking butterflies, wasps, bees, dragonflies, grasshoppers, beetles, and other flies. Overall, this species will take down the vast majority of flying insects that it can get its proboscis into. Not notoriously harmful to humans, some of the larger species of robber flies can even inflict a painful bite if handled carelessly (Mahr S. 2009).
This picture of an adult Promachus vertebratus taken from, shows how this insect takes down and digests its prey.

        With the help of a short, strong proboscis, this robber fly stabs its victim, injecting saliva usually containing neurotoxic enzymes (look at the picture above!). The robber fly can then suck its captured meal through a straw, as these enzymes paralyze and digest the insides of the prey.

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