The rectal sac as a second liver for detoxification in the metamorphosing silkworm

The insect larval fat body is the primary organ for detoxification of environmental pesticides and metabolic waste. During lepidopteran metamorphosis, the larval fat bodies undergo degradation and reprogramming for development, leaving the process of detoxification largely unknown at this stage. Using integrative approaches in the silkworm, Bombyx mori, we demonstrate that the pupal rectal sac, a greatly enlarged hindgut structure, serves as a central detoxification hub during the non-feeding metamorphic period. We show that it actively sequesters metabolic waste (uric acid) and, remarkably, clears exogenous toxins directly from the hemolymph. Proteomic and functional analyses reveal that the rectal fluid is enriched with a suite of active enzymes, including prophenoloxidase, laccase, and esterase, which create a stage-specific detoxification microenvironment. This organ thereby functions analogously to a vertebrate liver, providing remote clearance of toxins when feeding and excretion cease. Our findings uncover a key adaptive strategy in Lepidoptera and open new avenues for developing stage-specific pest management strategies by targeting this essential detoxification center.