Seasonal Water Balance in Orthoporus Ornatus, A Desert Millipede

Abstract

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.