The image of a butterfly often involves vibrant wings in a sunlit meadow, distinguishing them from their moth relatives associated with the night. While butterflies are known to be active during the day (diurnal) and moths at night (nocturnal), nature presents exceptions. The discovery of butterflies that operate under the cover of darkness challenges these distinctions.
When Butterflies Embrace the Night
The concept of a nocturnal butterfly was firmly established within the last few decades. The primary group of these night-flyers belongs to the family Hedylidae, often called American moth-butterflies, found in Central and South America. This family consists of a single genus, Macrosoma, with 36 species.
Despite their moth-like appearance and nocturnal habits, they are classified as true butterflies. While most butterflies rest at night under leaves or between rocks, the Hedylidae are just beginning their activity. The existence of nocturnal butterflies and day-flying moths shows that the division between these insects is more complex than a simple day-or-night activity pattern.
An Ancient Shift to Nighttime Activity
Research suggests the earliest ancestors of both butterflies and moths were active during the day. A major shift occurred when the ancestors of most modern Lepidoptera became nocturnal, likely to avoid daytime predators and competition. While most moths retained this nocturnal lifestyle, butterflies are thought to have re-evolved to be active during the day.
The nocturnal Hedylidae butterflies are a unique part of this evolutionary puzzle. One hypothesis is that they represent a lineage that reverted to nocturnality from a diurnal ancestor, a secondary adaptation back to a nighttime existence.
Specialized Senses for Darkness
To navigate and survive in the dark, nocturnal butterflies developed specialized sensory adaptations. The Hedylidae are the only known butterfly family to possess true hearing organs, known as tympanal organs, located at the base of their forewings. These “ears” are attuned to detect the ultrasonic calls of bats, their major predators.
In addition to hearing, their vision is adapted for low-light conditions. The eyes of nocturnal butterflies have a design more similar to moths than their diurnal relatives. This structure, known as superposition optics, is more efficient at gathering light. Genomic studies have shown their visual genes are more similar to those of nocturnal moths than to other butterflies, suggesting an evolution to suit a nighttime environment.
Butterfly or Moth Navigating the Night
Given their nocturnal habits and drab, moth-like appearance, why are Hedylidae not classified as moths? The distinction comes down to anatomical features that align them with butterflies. One difference between butterflies and moths is antennae shape; butterflies have slender, club-tipped antennae, while moths have feathery or tapered ones. The Hedylidae, however, have clubless antennae, adding to their moth-like look.
Despite this, other characteristics place them in the butterfly group. Their caterpillars have long, horn-like projections on their heads, similar to some Nymphalid butterflies. Their pupal stage is a chrysalis, characteristic of butterflies, not the silken cocoon spun by most moths. These biological traits confirm their status as true butterflies, showing behavior alone does not define their classification.