Craticulina is a genus of true flies

Brachycera

Brachycera is a suborder of Diptera. It is a major suborder consisting of around 120 families. The most distinguishing characteristic of the suborder is reduced antenna segmentation...

 in the family Sarcophagidae. They are cleptoparasites of various species of sand wasp

Sand wasp

The Bembicini, or Sand wasps, are a large tribe of crabronid wasps, comprising 20 genera. Bembicines are predators on various groups of insects. The type of prey captured tends to be rather consistent within each genus, with flies being the most common type of prey taken...

s. They are ovoviviparous, laying larvae instead of eggs. The larvae share the prey of the sand wasp, and though they are much smaller than the wasps, there may be many fly larvae to each wasp larva, causing food to run short. Sometimes the fly larvae also attack the wasp larvae while they are small, especially if there are many fly larvae, and too little food.

The flies are sand coloured, which no doubt helps camouflage

Camouflage

Camouflage is a method of concealment that allows an otherwise visible animal, military vehicle, or other object to remain unnoticed, by blending with its environment. Examples include a leopard's spotted coat, the battledress of a modern soldier and a leaf-mimic butterfly...

them, but at least some species of host take evasive action when Craticulina are present, though it is not clear how effectively.