Atmospheric convection is a process which involves the more or less vertical motion of air currents. During the discussion of winds (Part II), the writer indicated that winged insects inactivated by low temperatures, or wingless insects within any temperature zone, required vertical currents for support, and for assistance in horizontal transport, rather than horizontal currents alone, regardless of the strength of the latter currents. In the following pages, the process known as thermal convection will be considered as a means for the vertical and horizontal distribution of insects.