Seasonal Water Balance in Orthoporus ornatus, a Desert Millipede

Crawford, Clifford

Seasonal H"2O balance was studied in Orthoporus ornatus (Girard) (Spirostreptidae) inhabiting a volcanic escarpment in central New Mexico. Dormant, subterranean millipedes were collected periodically between November and June; feeding, surface-active millipedes mainly between July and October. For each specimen weights and total body H"2O, cuticle-tissue H"2O, gut-tissue and content H"2O, and remaining H"2O (assumed to be mostly hemolymph) were recorded. Fluctuation of these parameters for each collection was determined by regressing individual values against corresponding midsegment widths. Total body H"2O was relatively high in spring and summer; lower in fall and winter. Cuticle-tissue H"2O remained low from June (time of annual subterranean molt) to early winter, then increased to its maximal value during spring. Gut H"2O was highest during summer feeding, then declined until after molt. Remaining H"2O was highest in recent postmolt millipedes surfacing early or imbibing from a moist substrate, and lowest in early winter. Seasonal levels of cuticle-tissue H"2O and gut H"2O were similar between sexes at any midsegment width. Total body H"2O loss was increased by molting and appeared relatively independent of O"2 consumption. Less than half of total body H"2O loss during the feeding season appeared due to excretion; other avenues were cuticular transpiration and secretions from repugnatorial glands. Total body H"2O gain was little affected by metabolic-H"2O production, but gain due to H"2O ingestion during feeding and to inferred transcuticular uptake during dormancy was considerable. Such uptake should have been active because diffusion gradients of H"2O activity (a"w) between hemolymph and soil were generally unfavorable to dormant animals gaining cuticle-tissue H"2O.

Seasonal Water Balance in Orthoporus ornatus, a Desert Millipede

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