Moisture gradients form a vapor cycle within the viscous boundary layer as an organizing principle to worker termites

• Published in: Insectes sociaux Vol. 66; no. 2; pp. 193 - 209
• Main Authors: Soar, R., Amador, G., Bardunias, P., Turner, J. S.
• Format: Journal Article
• Language: English
• Published: Cham Springer International Publishing 01-05-2019; Springer Nature B.V
• Subjects: Air temperature; Behavior; Biomedical and Life Sciences; Boundary layers; Clustering; Conveyors; Entomology; Excavation; Feedback loops; Life Sciences; Macrotermes michaelseni; Mass transfer; Mud; Research Article; Soil conservation; Soil moisture; Soil temperature; Substrates; Temperature effects; Temperature gradients; Termites; Vapors; Viscosity; Workers (insect caste); Worker termite behavior; Spatiotemporal precursor; Construction; Viscous boundary layer; Moisture gradient; Template
• ISSN: 0020-1812; 1420-9098
• DOI: 10.1007/s00040-018-0673-0

Studies of termite mound building have considered the mud they prepare, its properties and its composition. Here we consider the behaviors of the mound building termites
Macrotermes michaelseni
, (Sjostedt), in the presence of the viscous boundary layer (VBL), which spontaneously forms over any surface that air passes over. We looked how soil moisture and air vapor are coupled to form a feedback loop and a spatiotemporal precursor to worker termites in the presence of mound material. We explored residency and activities of workers when presented with a VBL and either varying substrate temperature gradients or a soil moisture transition within the soil substrate. We report the emergence of a ‘vapor conveyor’, which forms around a neutral evaporative equilibrium point (NEEP) at the soil/air interface, where the soil-borne moisture temperature (along the gradient) and the 100% saturated air-borne vapor temperature coincide within the VBL, forming a bubble of neutral mass transfer which, we propose, worker termites are sensitive to as viscosity changes within. We found, on average, that 67% (std. dev 27%) of behavioral events (clustering, excavation, and deposition) occurred within 1 °C either side of the NEEP. We found negative correlation (− 0.78) between the substrate temperature gradient (0.1–0.9 °C mm
−1
) and the extents of behavioral activity, suggesting coupling between soil-borne moisture and air-borne vapor advection within the VBL. We recorded unique behaviors relating to interaction with the viscosity of vapor-saturated air at this scale. We speculate that workers may exploit the VBL to overcome a classic trade-off, i.e., how to push activities forward into potentially desiccating environments while conserving moisture in both the termites and the soil they build with.