Hi Alison, these two species are very similar in body and mandible shape, and without having them under the microscope you can never be sure. But in terms of field characters, M. nigriceps always have very distinctly darker head and gaster compared with obvious light reddy brown body and yellowish mandibles. M. pyriformis is similar, but is overall darker in body (often nearly black) and with relatively dark mandibles. Cheers, Jon