Food, ingestion rates, and assimilation in the desert millipede Orthoporus ornatus (Girard) (Diplopoda)

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Abstract
Orthoporus ornatus feeds mostly on dead plant material and on superficia tissue of desert shrubs. Sand, small particles of rock, and parts of arthropods are also ingested. Millipedes could not be induced to feed in the absence of moist soil in the laboratory after an initial day of feeding. Field observations of apparent food preferences were made at several sites in the southwestern U.S. Millipedes at Big Bend National Park, Texas, fed regularly on bark of the following desert shrubs: cholla (Opuntia sp.), cresotebush (Larrea divaricata), and ocotillo (Fouqueria splendens). Millipedes on the West Mesa of Albuquerque, New Mexico, fed mostly on superficial tissues of dead Russian-thistle (Salsola kali). At the Jornada Validation Site, Dona Ana County, New Mexico, millipedes fed regularly on the bark of Mormon tea (two Ephedra spp.) and mesquite (Prosopis glandulosa). Otherwise, grazing on an assortment of surface litter was commonly observed. Ingestion rates and assimilation efficiencies were determined in the laboratory for millipedes feeding on Prosopis and Ephedra using an ash-free technique. Assimilation efficiencies and ingestion rates were temperature dependent, increasing with a rise in temperature. Ingestion rates for Ephedra ranged from 0.020 to 0.050 g ash-free dry wt per g dry wt tissue per day, with assimilation efficiencies ranging from 20–37%. Ingestion rates were lower for Prosopis than for Ephedra. Feeding experiments at 24° C were considered to depict feeding characteristics found in surface millipedes under field conditions. These millipedes ingested 0.034 g ash-free dry weight of Ephedra per g dry body weight per day with 31.4% of the food being assimilated. No significant differences occurred between assimilation efficiencies calculated by ash-free techniques and by using caloric values of food and feces.