  Olfactory navigation is observed across species and plays a crucial role in
locating resources for survival. In the laboratory, understanding the
behavioral strategies and neural circuits underlying odor-taxis requires a
detailed understanding of the animal's sensory environment. For small model
organisms like C. elegans and larval D. melanogaster, controlling and measuring
the odor environment experienced by the animal can be challenging, especially
for airborne odors, which are subject to subtle effects from airflow,
temperature variation, and from the odor's adhesion, adsorption or reemission.
Here we present a method to flexibly control and precisely measure airborne
odor concentration in an arena with agar while imaging animal behavior.
Crucially and unlike previous methods, our method allows continuous monitoring
of the odor profile during behavior. We construct stationary chemical
landscapes in an odor flow chamber through spatially patterned odorized air.
The odor concentration is measured with a spatially distributed array of
digital gas sensors. Careful placement of the sensors allows the odor
concentration across the arena to be accurately inferred and continuously
monitored at all points in time. We use this approach to measure the precise
odor concentration that each animal experiences as it undergoes chemotaxis
behavior and report chemotaxis strategies for C. elegans and D. melanogaster
larvae populations under different spatial odor landscapes.
