Figure 2. Responses of different parasitic nematode species to CO₂.

A. A CO₂ chemotaxis assay. CO₂ is pumped into one side of a plate, and an air control is pumped into the other side. IJs are placed in the center of the plate and allowed to migrate in the CO₂ gradient for 1 hour. The number of IJs underneath the CO₂ inlet and the number underneath the air inlet are then counted, and a chemotaxis index (CI) is calculated according to the formula indicated. The chemotaxis index ranges from +1 to −1, with positive values indicating attraction to CO₂ and negative values indicating repulsion from CO₂. B. Responses of EPNs to CO₂ in a chemotaxis assay. All EPN species tested are attracted to CO₂ across concentrations. Data are from Dillman et al., 2012 [11]. C. CO₂ is required for normal attraction of EPNs to insect odor blends. Left, EPN responses to host odor blends in a chemotaxis assay. Right, EPN responses to host odor blends with CO₂ chemically removed. Attraction of EPNs to insect odor is reduced or eliminated in the absence of CO₂. Responses are shown as a heatmap; yellow indicates attraction and blue indicates repulsion. White boxes in the heatmap indicate EPN-host combinations that were not tested with CO₂ removed because they were not attractive with CO₂ present. Reproduced from Dillman et al., 2012 [11]. D. Responses of mammalian-parasitic nematodes to CO₂ in a chemotaxis assay. The skin-penetrating nematodes Str. stercoralis, Str. ratti, and Ni. brasiliensis are repelled by CO₂, while the passively ingested nematode Ha. contortus is attracted to CO₂. Data are from Castelletto et al., 2014 [26].