CHITIN AND BODY WATER ISOTOPIC COMPOSITION IN GRASSHOPPER SPECIES OF NORTHERN COLORADO

Insects comprise over three quarters of all animal species, yet studies of oxygen and hydrogen isotopic composition are limited to only a few genera. Studies suggest that variability in isotopic compositions of insect body water is controlled by dietary water sources, modified by respiratory and metabolic processes. Insect chitin (exoskeleton/wing) is chemically inert after synthesis, recording isotopic compositions at time of formation. Chitin isotopic compositions are dependent on diet and local precipitation, so are useful in migration and paleoclimate studies. To date, there are no published studies of isotopic compositions in grasshopper chitin or body water. The purpose of this work was to examine δD and δ¹⁸O variability in grasshopper chitin and body water between sexes and species as well as how these variables change along an elevation gradient.

Four species of grasshopper were collected during summer 2012 from five sites near Boulder, CO, ranging in elevation from 1575 m to 3500 m. Chitin samples were cut from grasshopper wing and leg segments and were equilibrated with laboratory atmosphere to normalize exchangeable hydrogen compositions prior to analysis with TCEA-IRMS. Body water was distilled from frozen specimens using a vacuum extraction line and water samples were analyzed for δD and δ¹⁸O (LGR Liquid Water Isotope Analyzer).

Measured grasshopper chitin and body water isotopic compositions were both highly variable. Chitin δD values ranged from -134‰ to -54‰(VSMOW), and δ¹⁸O from +22‰ to +46‰ (VSMOW), while body water δD values ranged from -54‰ to +109‰ and δ¹⁸O from +5‰ to +25‰. Much of this variation is attributable to differences among species. Female specimens exhibited broader ranges in both chitin and body water isotopic compositions, with more depleted average values than males. Discrepancies between male and female isotopic compositions may reflect physiological differences related to oviposition. Preliminary data suggest isotopic enrichment in grasshopper chitin at higher elevations, consistent with decreasing humidity in the arid subalpine environment. Relationships between elevation and body water isotopic composition reflect altitude and humidity gradients in meteoric water compositions but are complicated by behavioral differences among species.