Heliconius hermathena Hewitson is a little-known butterfly which is restricted to certain non-forest habitats in the Brazilian Amazon. Unlike forest species of the genus, H. hermathena has not undergone subspeciation in areas of presumed Quaternary forest refugia, but rather shows a unique racial distribution apparently related to present occurrence of isolated non-forest habitats. Three subspecies [including H. h. hermathena] (2 previously undescribed [including H. h. sheppardi ssp. nov.]) are non-mimetic and little differentiated, whereas a 4th, H. h. vereatta, is strongly mimetic of sympatric H. m. melpomene and H. erato hydara. The non-mimetic subspecies are apparently widespread, although of patchy occurrence and often at very low population densities. In contrast, the mimetic subspecies is very restricted geographically, being known only near Faro, Para (though its models are widespread and even microsympatric with non-mimetic H. hermathena in many other areas). It occupies essentially continuous habitats within its limited range, and attains population densities and sizes much greater than do non-mimetic races. Adult morphology places H. hermathena near H. erato and H. charitonia, but the pupal characters suggest that the species is relatively primitive, near to the melpomene-group of the genus. Both H. erato and H. charitonia are widespread and successful species, whereas the distribution of H. hermathena suggests that it is a specialized relict. The larva is uniquely colored bright red or orange, possibly for the purposes of signaling (aposematism linked to Muellerian, mimicry, or ease of location by adults). Imaginal H. hermathena are very sedentary, with individuals apparently establishing restricted home ranges, generally within a radius of 100 m of their nocturnal roosting sites. Roosting behavior is unique among Heliconius in that the butterflies aggregate on the undersides of leaves and grass blades about 1/2 m from ground level. This behavior seems to afford maximum protection from weather and predators in their specialized habitats. As larval host plants, H. hermathena uses only 2 woody sclerophyllous species of primitive Passiflora, P. faroana Harms and P. hexagonocarpa, characteristic of natural Amazonian scrub-vegetation communities on deep sandy soils. Alluvial sands supporting these communities are widely distributed over the Amazon, especially along the outwashes of the Brazilian and Guianan shields. The genetic basis for mimicry in H. h. vereatta is apparently a single factor with essentially dominant effects. The mimetic subspecies intergrades with a non-mimetic race north of Faro, resulting in a narrow hybridization zone. The co-mimics of H. h. vereatta become progressively rarer northwards across this zone, and are decreasingly microsympatric with H. hermathena where the non-mimetic phenotype exists in higher frequencies. Mimetic selection appears even strong enough to maintain local differences in genic frequencies, with mimetic forms becoming more common in closed habitats where models fly, and non-mimetic forms (perhaps profiting by flash-disruptive coloration) predominating in adjacent more-open habitats.