The ambush bug is a heteropteran predatory insect with cryptic coloration. Unusual among insects, males remain coupled with a female for several to many hours after mating, resulting in a large investment in each mating opportunity for males. Unpaired males, unpaired females, and coupled males and females were collected on goldenrod in late summer in east central Minnesota over two seasons. Each insect was weighed, and the pronotum width, abdomen width, abdomen length, and rear leg length were measured with a digital stereoscopic microscope. Unpaired males and coupled males were not significantly different for any of the parameters. Abdomen width was significantly greater in coupled females than single females the first year, although mass, pronotum width, and abdomen length of paired and unpaired females were not different. In the second year, paired females were not larger than single females with pronotum width significantly greater in single females. Females were larger than males for all parameters. Size-assortative mating was not observed in either year. Scramble competition for mates occurs in male ambush bugs. Since darker (melanistic) males may reach a temperture necessary for flight more quickly under cool but sunny conditions, we assessed the degree of thoracic pigmentation in coupled and single males with image capture software. The coloration of paired and unpaired males did not differ with respect to size of pigmented area or hue.