Tissue and Cell 37 (2005) 223–232 Fine structure of the midgut and Malpighian papillae in Campodea (Monocampa) quilisi Silvestri, 1932 (Hexapoda, Diplura) with special reference to the metal composition and physiological significance of midgut intracellular electron-dense granules G. Pigino a,∗ , M. Migliorini a , E. Paccagnini a , F. Bernini a , C. Leonzio b a Department of Evolutionary Biology, University of Siena, via A. Moro 2, 53100 Siena, Italy b Department of Environmental Sciences “G. Sarfatti”, University of Siena, via P.A. Mattioli 4, 53100 Siena, Italy Received 25 July 2004; received in revised form 1 February 2005; accepted 7 February 2005 Available online 19 April 2005 Abstract The fine structure of the midgut and the Malpighian papillae in Campodea (Monocampa) quilisi Silvestri, 1932 (Hexapoda, Diplura) specimens was described. We observed the presence of electron-dense granules (EDGs) in the midgut epithelial cells, similar in genesis, structure and aspect to the type A spherocrystals described in the midgut epithelium of Collembola and Diplopoda. Energy-dispersive X-ray microanalysis was used to detect the chemical composition of the granules and to relate it to the concentrations of some potential toxic heavy metals (Pb, Cu, Zn) in soil and litter. Chemical composition of the granules seems strongly influenced by the presence and bioavailability of heavy metals in the external environment. Specimens from a contaminated abandoned mining and smelting area (Colline Metallifere, southern Tuscany) were able to accumulate Fe, Mn, Zn, Pb and Cu in their midgut EDGs. In addition, we observed that C. (M.) quilisi was able to excrete the metal-containing granules into the external medium by the moulting of the intestinal epithelium. This confirms that the process of ionic retention of midgut cells is particularly significant in animals lacking Malpighian tubules. © 2005 Elsevier Ltd. All rights reserved. Keywords: Diplura; Ultrastructure; Metal-containing granules; X-ray microanalysis; Midgut; TEM 1. Introduction Most species of Campodeidae (Hexapoda, Diplura) feed on decaying vegetable material displaying functions that are related to the cycle of organic matter (Wallwork, 1983; Dindal, 1990). Due to their habitat in the upper horizons of soil and their feeding behaviour, Diplura can be exposed to metal contamination caused by anthropogenic sources (sewage sludge, fumes, mining activity, etc.) (K¨ ohler, 2002) and, similarly to Collembola, they can survive in severely metal contaminated sites (H˚ agvar and Abrahamsen, 1990; Migliorini et al., 2004). In saprophagous invertebrates, this ability is often correlated with some detoxification mech- anism (Triebskorn and K¨ ohler, 1996), including the ac- ∗ Corresponding author. Tel.: +39 0577 234391; fax: +39 0577 234476. E-mail address: pigino@unisi.it (G. Pigino). tivation of metal-binding proteins (e.g. metallothioneins) (Dallinger, 1996; Hensbergen et al., 2000, 2001) and the pre- cipitation of metals as intracellular electron-dense granules (EDGs) (Humbert, 1974, 1977; Hopkin and Martin, 1982; van Straalen et al., 1987; Prosi and Dallinger, 1988; Hopkin, 1989; Pawert et al., 1996; K¨ ohler et al., 1996). In particular Dallai and Burroni (1981) and Xue et al. (1990), studying the structure of the gut of Diplura (Campodea sp., Japix sp. and Lepidocampa weberi Oudemans), observed the presence of EDGs in the midgut epithelial cells and stated that their main function could be the maintenance of the mineral balance and the regulation of ion excess. It has been demonstrated, by van Straalen et al. (1987) and Posthuma et al. (1992), that Collembola first accumulate potential toxic metals in EDGs at the level of the midgut and then excrete them by the pe- riodic renewal of the gut epithelium at each moulting. Due to similarities in the gut structure of Diplura and Collembola 0040-8166/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.tice.2005.02.001