By Jeff Mitton
Reading several papers on common wood nymphs, Cercyonis pegala, led me to pose two questions. Insects have six legs, but how many legs do butterflies have? Can butterflies hear, and if so, where are their ears?
Approximately 50 species of wood nymphs are recognized in North America, and they are in the family Nymphalidae, composed of about 6,000 species. Nymphalids are commonly called four-footed butterflies because the front pair of legs is not immediately apparent.
They are also called brush-footed butterflies because most of the species do not have feet on their front pair of legs, but structures bristling with hairs that look like brushes. Front legs are smaller than the middle and posterior pairs of legs, and they are usually pulled up and folded in front of the head.
They are furry protuberances beside the curled tubular mouth, the proboscis. The wood nymph in the photo appears to have just four legs, because the diminished front legs are usually mistaken for mouth parts or a nose.
Insects have six legs, but a common wood nymph seems to have only four. Photo by Jeff Mitton.
Some authors have stated that the front pair of Nymphalid legs are vestigial, meaning that they are diminished in form and function, like the appendix in humans. I suppose that they are vestigial when it comes to standing and walking, but forelegs have been co-opted to perform different functions.
Nymphalid mid and hind legs have fully functional feet with gustatory sensillae or sensory hairs for tasting. They literally taste the substrate to determine if it is edible. Sensory hairs on the brush feet have a similar function but a different purpose.
A study of oviposition site behavior in the red postman, a tropical butterfly, revealed the co-opted function of the brush feet. Females searching for host plants for oviposition drum their feet violently on plant surfaces to release plant saps, allowing the female to identify the specific species required for oviposition and larval feeding. So, sensory hairs on the middle and posterior feet are for finding food, while those on the brush feet on the front legs are for host plant selection.
Phylogenetic studies of the family Nymphalidae have revealed that the modification and co-option of the front legs evolved multiple times—this is more a story of convergent evolution of a beneficial trait than the functional loss of an unnecessary pair of legs.
Hearing is a critical sense for many insects, both for avoiding predators and communications among individuals. Insect auditory organs occur in many parts of the body, including mouthparts, wings and legs. Many but not all brush-footed butterflies are able to hear, but you will not find ears on their bodies. The structural organs that perceive sound in butterflies are called Vogel's organs and are on the ventral side and leading edge of each forewing. The organ is difficult to see because it is small (about 1 mm) and hidden by scales, but removing the scales reveals an ellipsoid, raised chitinous chamber.
Recent studies have examined hearing in common wood nymphs and the possible involvement of a conspicuously swollen subcostal wing vein immediately adjacent to the Vogel's organ. The swollen vein can be seen in the photo, close to the leading edge of the forewing, with the thickest portion directly behind the head.
A laser vibrometry study revealed that the hearing of common wood nymphs is tuned to low sounds, such as those made by the beating of birds’ wings. The long wavelengths of these sounds and the small size of the wood nymphs make it difficult to detect and locate these sounds.
Wing veins in butterflies do not carry blood; they are hollow tubes that provide structural support for the wing. The subcostal vein of the common wood nymph has puzzled lepidopterists for a century, for it is unusually thick or swollen. It is immediately adjacent to the Vogel's organ in wood nymphs.
Experimental studies tested the hypothesis that the subcostal vein provided an auditory chamber to enhance detection of low frequency sounds. If this hypothesis were correct, a study of numerous butterfly species would find that swollen veins occur only in species that had Vogel's organs and that the swelling would be pronounced in smaller species.
Furthermore, multiple veins would be involved in the smallest species. Finally, if swollen veins enhance detection of low-frequency sounds, then ablation of swollen veins would diminish sensitivity to low frequency sounds.
In a comparative study of 79 species, swollen veins were only found in species with ears, and swelling was more pronounced in smaller species. In the smallest species, multiple veins were swollen. Finally, ablating a swollen vein decreased the sensitivity to low-frequency sounds. Finally, electron microscopy revealed a direct, open connection between the swollen vein and the Vogel's organ.
In summary, wood nymphs have two pairs of legs used for walking and standing, plus another pair, quite reduced but with dense hairs and bristles capable of taste and smell and used to find host plants for oviposition. Vogel's organs, which detect sound, have tympanic membranes surrounded by a chitinous chamber, connected to the swollen subcostal vein, which acts as a supplemental sound chamber.
The common wood nymph tastes with its feet and hears with its wings.