Reproductive isolating mechanisms evolve to reduce the sharing of alleles among groups where reproduction does not result in offspring, or results in offspring with lower fitness. Occasionally, individuals from genetically different populations mate and produce hybrid offspring. Post-zygotic reproductive isolating mechanisms affect the hybrid individual and can be intrinsic (developmental problem) or extrinsic (individual is incompatible with its environment). The viceroy (Limenitis archippus) and red spotted purple (Limenitis arthemis astyanax) are two closely related species of butterfly native to North America that are capable of hybridizing. This study looks at the proboscis morphology of Limenitis archippus and Limenitis arthemis astyanax, as well as their hybrid “rubidus” form. Proboscis architecture relates to its function, and each structure has an impact on fluid uptake abilities (nutrient acquisition), making its architecture fundamental to the individuals fitness. We hypothesize that hybrid butterflies might have proboscises with reduced functionality, impairing feeding ability, thus acting as a post-zygotic reproductive isolating mechanism. To test this, we measured eleven structures on each proboscis and compared measurements among species groups. Recorded measurements supported the hypothesis that hybrid Limenitis butterflies might have reduced proboscis functionality, which would hinder fluid uptake, and ultimately reduce hybrid fitness.