In ants, midgut doesn't join the hindgut until the end of the larval stage. The food that is not digested is retained in the form of a meconium which is visible as a dark area through the body of the larva.

The big larva at left is a prepupa (or semipupa), who has expelled the meconium (which is removed to the refuse pile by workers) and is ready to begin pupation. Note the diference with the big larva at right, who has not expelled it. White dots are urate crystals.

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1. 1. Wheeler G, Wheeler J .  1976.  Ant larvae: review and synthesis.. Memoirs of the Entomological Society of Washington. 7:1-108.