GIANT WHITE GRUBS

I was digging through a heap of kraal manure, which had just begun decaying. I came across numerous giant grubs in the hot pale depths of the manure. They seem to have grown in size relative to those commonly known to me. Their lengths are just above 6 cm.
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The smallest one in this picture is exactly the size i am familiar with.

I learned from the internet that they are called white grubs. The wonder to me is why they have grown so large. The ones I was familiar with before hand are around 2 cm length and relatively thinner.

The interesting part is that they are more sensitive to light and burrow their way into the dark once placed onto the surface. They live under the kraal manure regardless of how dry or wet the environment may be.

What do they feed on? Their defecation is similar in colour to the manure they live in, and this gives me reason to think they devour the manure, perhaps this assists in decomposition of the manure. Part of me tells me to realise that organic matter can be really rich and important to life support. I believe the worms grow so well because they feed in a naturally nutritious environment.

I managed to pick a few just for the photo and returned them into their dark to let them perform whatever duty they have assigned themselves to.