The secretory activity of the two branched malpighian tubules (MTs) of the second-instar larva in Aeolothrips intermedius is described. MTs of adult thrips have the typical ultrastructure of excretory epithelium with apical microvilli containing long mitochondria and a rich system of basal membrane infoldings. In the second-instar larva just before pupation, the ultrastructure of MT epithelial cells is dramatically different, and there are numerous huge Golgi systems in the cytoplasm. These cells are involved in an intense secretory activity to produce an electron-dense product which is released into the MTs lumen. This secretion is extruded from the hindgut and used by the second-instar larva to build an elaborate protective cocoon for pupation. Electron-spray-ionization mass spectrometry analysis of the cocoon revealed the presence of a β-N-acetyl-glucosamine, the main component of chitin, which is also present in the cocoons of Neuroptera and some Coleoptera. J. Morphol., 2010. © 2009 Wiley-Liss, Inc.