In locusts and other insects, odorants are transduced into electrical signal by the olfactory receptor neurons and transmitted to central circuits for further processing. Previous studies have shown that exogenous variables (e.g., flow rates, humidity, temperature, odor mixtures, etc.) can influence the responses of the sensory neurons and therefore modulate the central circuits. However, how the sensory neuron activity is manipulated to achieve adaptive gain control in the following circuit is yet to be understood. It is possible that the magnitude of the stimulus-evoked response in the receptor neurons, their spontaneous activity, or both of these factors can change how information about a chemical cue is processed downstream. To this end, I studied the effects of modulating two different factors on the olfactory system (flow rate and relative humidity) at four levels of the olfactory system: individual olfactory receptor neurons (first-order neurons), the whole antenna (electroantennogram recordings), individual projection neurons in the antennal lobe of the brain (second-order neurons), and population antennal lobe activity as assayed by local field potential recordings in the mushroom body. We found that flow rate changes altered the magnitude of the stimulus-evoked responses in the antenna without altering the spontaneous activity levels. Whereas, changes in the relative humidity elicited a decrease in both response magnitude and baseline activity. Intriguingly, only the humidity modulation experiments brought about significant compensatory change in the spontaneous and odor-evoked activity of the second-order neurons in the antennal lobe. Therefore, our data and analysis suggest that baseline activity of receptor neurons seems to play a key role in adapting the gain of the locust brain's central circuit.