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The physiological parameters mortality, mass, oxygen consumption and amylase activity, and microanatomical features of the digestive tract, mesenchym and reproductive organs were used to characterise starvation in Galumna elimata. The mites were reared in sterilised plastic vials containing moistened zeolite at 25 °C and a 12:12 photoperiod. The control group was kept under the same conditions, but pieces of bark covered with the green bark alga, Desmococcus vulgaris (syn. Protococcus viridis), were added as food for the mites. The physiological parameters were recorded after 21 days, and the microanatomical after 21 and 42 days. The guts of the starved mites were empty or filled with mucoid substances, while the guts of control mites contained food boli formed from algal cells. The mortality was significantly higher in starved animals. The mortality after 42 days of starvation was higher in males than females. The fresh mass of starved individuals significantly decreased while the water proportion content of their body tissues increased. Oxygen consumption of the starved mites was lower. Starvation did not influence the activity of amylase. Glycogeneous granulae were characteristically absent, and mucoid substances present in the guts of mites starved for 21 days. The activity of mesenteral and caecal cells, proventricular glands and cells of salivary glands was reduced after 42 days of starvation. The cells of the seminal vesicles were reduced and contained no spermatic cells in males starved for 42 days. Starved females probably resorbed immature oocytes, but had eggs in their oviducts. Starvation induces ovovivipary or larvipary in Galumna elimata. Dijkman, 1993; Urbášek & Starý, 1994; Šustr & Starý, 1998). It is not always easy to interpret these results because: (i) The difficulty of establishing outdoor conditions indoors (Woodring & Cook, 1962a). (ii) Food preference tests are influenced by the spectrum of diets offered. Galumna elimata preferred the green bark alga, Desmococcus vulgaris, in the laboratory but this species is not present in soil (Hubert et al., 1999; Hubert & Lukešová, 2001). (iii) The quantification o f food consumption (Saichuae et al., 1972) or defecation (Hubert et al., 1998) does not provide information about assimilation efficiency. Scheloribates laevigatus has a high defecation rate when fed Desmococcus algae and Penicillium spores, but the spores and some o f the algal cells pass through the gut undamaged (Hubert et al., 1999; Hubert & Lukešová, 2001). A higher food intake could compen­ sate for energetically poor diets. (iv) The presence o f substrate specific enzyme activity indicates, but is not proof o f substrate utilisation. For example, Achipteria coleoptrata and Scheloribates lae­ vigatus differ in chitinase activity (Siepel & RuiterDijkman, 1993), but neither species digest fungal cell walls, where chitin is the main compound (Hubert, in prep.). The physical and chemical conditions in the oribatid gut are unknown. Enzyme activity, measured in vitro, does not correspond to the amount o f the sub­ strate digested in vivo. There are indications that there are qualitative differences in the chitinase activity in two populations o f Steganacarus magnus (see Smrž, INTRODUCTION The feeding biology o f oribatid mites has been inten­ sively studied, especially the interactions between oribatids and the micro-organisms responsible for plant litter decomposition (reviewed by Wallwork 1983 and Seastedt 1984). In order to achieve a better understanding of the role of oribatids in soil systems and litter decompo­ sition, several authors have described their feeding habits and assigned them to feeding guilds (Schuster, 1956; Luxton, 1972; Kaneko, 1988; Siepel & Ruiter-Dijkman, 1993). However, the food resources used by oribatids in soil are difficult to investigate due to the heterogeneity of soil systems. Many experiments and observations have been made on their feeding habits under laboratory conditions. In addi­ tion such information was also obtained from rearing experiments (Cleat, 1952; Sengbush, 1954; Woodring & Cook, 1962a; Stefaniak & Seniczak, 1976). Food prefer­ ence tests were used by Wallwork (1958), Hartenstein (1962), Luxton (1972), Trávníček (1989), Siepel (1990), Rihani et al. (1995), and Maraun et al. (1998). The micro­ organisms have been identified and the structural changes that occurs in the food during its passage through the gut have been determined by dissection and the cultivation of gut contents (Schuster, 1956; Hoebel-Mavers, 1967; Behan & Hill, 1978; Behan-Pelletier & Hill, 1983; Smrž, 1992a; Smrž, 1996). The presence o f digestive enzymes gives some information on the compounds they utilise (Zinkler, 1971 and 1972; Luxton, 1972; Dinsdale, 1974; Zinkler et al., 1986; Siepel, 1990; Siepel & Ruiter-

The effect of starvation on the metabolic rate and microanatomy of Galumna elimata (Acari: Oribatida)