Intraspecific differences in olfactory sensilla in relation to diet breadth in Uroleucon ambrosiae (Homoptera: Aphididae)

The olfactory plate organs found on the proximal segments of aphid antennae, the secondary rhinaria, were studied in apterae of the aphid Uroleucon ambrosiae. The numbers and shapes of these structures are important taxonomic characters in Aphididae, but they also show intraspecific variation within a morph. In this study, numbers were found to vary with aphid size but, independent of size, were greater in aphids collected in the eastern than in the southwestern United States. In addition, the sizes of rhinaria were greater in eastern than in southwestern aphids. These differences correlate with previously recorded differences in diet breadth and the efficiency of finding and settling on food: eastern aphids specialize on the host Ambrosia trifida, on which they settle relatively efficiently, while southwestern aphids are more generalized and less efficient. Greenhouse experiments demonstrated that differences in rhinaria are partly determined genetically. Southwestern aphids showed bilateral asymmetry in length of antennal segments though not in numbers of rhinaria, but only on certain adopted hosts that were apparently inferior for growth. The significance of these intraspecific differences is discussed in relation to the physiology, ecology, and evolution of host range.