Seasonal Water Balance in Orthoporus Ornatus, A Desert Millipede
1978
Crawford, Clifford S.
Seasonal H₂O 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 H₂O, cuticle—tissue H₂O, gut—tissue and content H₂O, and remaining H₂O (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 H₂O was relatively high in spring and summer; lower in fall and winter. Cuticle—tissue H₂O remained low from June (time of annual subterranean molt) to early winter, then increased to its maximal value during spring. Gut H₂O was highest during summer feeding, then declined until after molt. Remaining H₂O was highest in recent postmolt millipedes surfacing early or imbibing from a moist substrate, and lowest in early winter. Seasonal levels of cuticle—tissue H₂O and gut H₂O were similar between sexes at any midsegment width. Total body H₂O loss was increased by molting and appeared relatively independent of O₂ consumption. Less than half of total body H₂O loss during the feeding season appeared due to excretion; other avenues were cuticular transpiration and secretions from repugnatorial glands. Total body H₂O gain was little affected by metabolic—H₂O production, but gain due to H₂O ingestion during feeding and to inferred transcuticular uptake during dormancy was considerable. Such uptake should have been active because diffusion gradients of H₂O activity (aw) between hemolymph and soil were generally unfavorable to dormant animals gaining cuticle—tissue H₂O.
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