Evidence of Silk-production in the Malpighian Tubules of Campsurinae (Ephemeroptera: Polymitarcyidae) Nymphs.

Insect silk glands exhibit diverse origins, including epidermal, labial, and Malpighian tubule-derived structures. Polymitarcyidae mayfly nymphs (Ephemeroptera) are known to secrete silk for constructing and lining their tunnels. However, the organ responsible for silk production in this ancient insect lineage is poorly understood. This study provides the first histological, histochemical, and ultrastructural analysis of the Malpighian tubules in Polymitarcyidae (Campsurus truncatus, Campsurus sp., and Tortopsis canum), identified as the likely source of their silk. We found specialized cells with characteristics of intense protein synthesis and secretion in the distal regions of the tubules in all examined species. These findings represent the first detailed evidence confirming the Malpighian tubules as the site of silk production in Ephemeroptera, establishing a foundation for further studies on silk gland evolution in insects.