Chapter Five The Malpighian tubules and cryptonephric complex in lepidopteran larvae

Lepidopterans (butterflies and moths) are an ecologically and agriculturally important group of holometabolous insects. Their larvae and adults exhibit trophic partitioning, which is reflected by the various modifications of their digestive and excretory systems. Adults are capable of flight and feed mostly on the nectar of plants, acting as pollinators. Larvae are voracious leaf eaters whose extremely alkaline midgut (pH≈11) is an adaptation to high tannin levels in the host plants. Morphologically and functionally regionalized Malpighian tubules of the larva modify fluid as it flows through them. The larvae also exhibit the so-called cryptonephric condition, where the distal end of the tubule is juxtaposed to the rectum and enveloped by the perinephric membrane. The distal part of the free tubule adjacent to the ileum, termed the distal ileac plexus, is characterized by a high density of secondary cells. Recent studies have identified several unusual aspects of ion transport physiology of the distal ileac plexus: (i) gap junctional coupling of principal and secondary cells that allows them to transport ions in opposite directions, (ii) the ability to switch between K+ secretion and K+ reabsorption depending on the input from the cryptonephric tubule, (iii) the presence of voltage-gated, ligand-gated and mechanosensitive ion channels and (iv) coordinated regulation of water and septate junction permeability during the reversal from secretion to reabsorption (presumably aimed at retention of water and solute content in the distal ileac plexus lumen, while Na+ and K+ are being reabsorbed). We describe recent advances in understanding ion-transporting and regulatory mechanisms in the Malpighian tubules of larval Lepidoptera with a special emphasis on the distal ileac plexus segment.