I have a small decomposition box made out of improvised plastic machine oil tubs. I have been adding kitchen wastes to it for the past couple weeks along with dry leaves.
Recently I added okara to it and while turning the compost, I noticed grubs growing specifically in the okara. There are grubs in multiple sizes, so I am not sure if this is the only species either.
This looks like a leather jacket, a crane fly larva. To answer your question, no, they are not one of the decomposers. These guys eat decomposed organic matter and in this case the crowns of grasses. Thus finding them in your decomposing compost.
The worst thing you could do is use any pesticide for this, I am dead serious. I can explain if you ask another question about it.
It is neither helpful or harmful to your compost. It might just be helping by giving those larvae food to keep them off the lawn.
Whilst larvae or maggots won't do any harm in the composting material, if the container is near your home or indoors, you might want to discourage those. Larvae eventually turn into things that fly, and you might not want those in or near the home. If you want to get rid of them, pick out all you can see, then apply a layer 2-4 inches thick of dry browns, such as shredded newspaper, dry dead leaves or dry grass cuttings, see here https://homeguides.sfgate.com/eliminate-maggots-compost-bins-42936.html
