Oecanthus alexanderi n. sp. Walker occurs in the lower Rio Grande valley of Texas and in Mexico as far south and west as Michoacan. Its song and morphology establish it as a member of the Oecanthus rileyi species group. Other members of this group of New World species include O. rileyi Baker, O. allardi Walker & Gurney, and O. fultoni Walker. Calling songs of the rileyi group consist of chirps or brief trills in which trios of pulses are produced in such rapid succession as to be indistinguishable to the human ear. The chirps/trills are easily countable and have a highly regular rate that varies linearly with temperature. Extrapolated regression lines tend to converge at 4°C and an expected chirp rate of O. Because of these relations, simple formulas can convert counts of chirps vs time into surprisingly close approximations of ambient temperature—justifying the group's designation as “thermometer crickets”.