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To investigate if there is an energetic constraint influencing a nectar feeding ant’s decision to come back to the nest with partial loads, the energetic costs of running and carrying a load in the ant Camponotus rufipes are measured. Metabolic rates of individuals are measured in a running tube respirometer while they are unladen and laden at 25 °C. Workers voluntarily collect a load of 6 µL of a 30 % sucrose solution (mass = 6.8 mg), which results in an internal load of about 50% of the ant mass and is close to a full load for ants within this size range. The gross cost of unladen running is 264 J kg-1 m-1, while that of laden running is 225 J kg-1 m-1. The mass used to calculate the cost of laden running includes body mass of ant and load carried. Load carriage cost in C. rufipes foragers is calculated to be about 60% as much as body carriage per unit mass. Internal load carriage in C. rufipes is energetically cheaper compared with external carriage in other ant species. Such low carriage costs make it unlikely that the collection of partial crop loads in C. rufipes foragers is based on a minimization of foraging costs, as suggested for honeybees. more...

Partitioning the relative contributions of cuticular and respiratory water loss in a tracheate arthropod is relatively easy if it undergoes discontinuous gas exchange cycles or DGCs, leaving its rate of cuticular water loss in primary evidence while its spiracles are closed. Many arthropods are not so obliging and emit CO2 continuously, making cuticular and respiratory water losses difficult or impossible to partition. We report here that by switching ambient air from 21 to 100% O2, marked spiracular constriction takes place, causing a transient but substantial - up to 90% - reduction in CO2 output. A reduction in water loss rate occurs at the same time. Using this approach, we investigated respiratory water loss in Drosophila melanogaster and in two ant species, Forelius mccooki and Pogonomyrmex californicus. Our results - respiratory water loss estimates of 23%, 7.6% and 5.6% of total water loss rates, respectively - are reasonable in light of literature estimates, and suggest that the 'hyperoxic switch' may allow straightforward estimation of respiratory water loss rates in arthropods lacking discontinuous gas exchange. In P. californicus, which we were able to measure with and without a DGC, presence or absence of a DGC did not affect respiratory vs total water loss rates. more...

Fil: Rolandi, Carmen. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.

Fil: de la Vega, Gerardo José. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.