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Revista de Biología Tropical

On-line version ISSN 0034-7744

Abstract

MONGE-NAJERA, Julián et al. Spatial distribution, territoriality and sound production by tropical cryptic butterflies (Hamadryas, Lepidoptera: Nymphalidae): implications for the "industrial melanism" debate. Rev. biol. trop [online]. 1998, vol.46, n.2, pp. 297-330. ISSN 0034-7744.

Neotropical butterflies of the genus Hamadryas, noted by the emission of sound, spend much time perching on trees and are believed to be cryptically patterned and colored with respect to tree trunks and branches they use as perching sites, but the subject had not been studied previously. This paper describes spatial distribution, territoriality and sound production in five species, under natural conditions: Hamadryas amphinome (Lucas, 1853), H. februa (Godart, 1824), H. feronia (Fruhstorfer, 1916), H. glauconome (Bates, 1864) and H. guatemalena (Bates, 1864). Tree characteristics and use by butterflies were recorded under natural conditions in open habitats (grassland thinly covered with trees) in Costa Rica and Panama, avoiding the problems that affected previous natural selection studies in Biston betularia (the "industrial melanism" moth). Males perched on the trees and used them as courting territories. The butterflies perched more often on some individual trees, and did not use others. The general tree bark ("background") color tended to match wing coloration, while presence of food, position of trees along flight routes, tree size, bark texture, and lichen cover were not associated with the frequency of perching on the trees. Most individuals that perched in the study sites were males. Species differed in perching height and populations of H. februa perched at the same heights in both countries; H. feronia moves to higher perches near day's end. The relative use of branches and trunks is not related to the time of day but reflects the typical perching height of each species. The northern side of trees is less used and cardinal side distribution is independent of time of day. Perches exposed to direct sunlight are less used in hot days. All species perch with the head downwards. Perching males frequently fly towards other Hamadryas as well as towards tethered cardboard models. Trees with experimentally removed males were taken by newcomers 32 times more often than trees with resident males. Each marked H. feronia male was seen perching on 1-4 trees daily, without difference between seasons, and each tree used had a minimum daily mean of 1.5 perching butterflies. Most H. feronia interactions occur from 13:00 through 15:00 hours and are more frequent in the rainy season. At night males share perches. Sound emission was studied by using non-destructive experimental methods (N=858) and with a scanning electron microscope. Both sexes emit sound and the sound apparatus, located in the forewing, is percussive, not stridulatory. At the end of the upward wingstroke, the wings are clapped and modified r-m1-2 veins meet at a speed of approximately 1420 mm/s, producing the characteristic clicks. Wingbeat frequency of free-flying individuals is 20-29 Hz. There is some wing deformation during movement. Clicks last a mean of 1.38 ms with mean intervals of 43.74 ms and the component frequencies concentrate around 2.4 kHz, matching Hamadryas hearing capacity and being appropriate for the acoustic conditions of habitat. The swollen Sc vein is present exclusively in Hamadryas; has a serpentine structure inside and probably acts as resonance box. Growth of the sound apparatus may be checked by its effect on flight capacity, physiological costs and ecological reasons. All Hamadryas have a membrane, shaped as an elongated cupola, in the costal cell, that acts as ear. A second and smaller ear has four chambers and may detect predatory bats when the insects are perching at night. Field observations showed that Hamadryas spp. emit audible clicks when approached by potential predators, to defend territories from other Hamadryas and in at least one species also during courtship. Severe wing damage, common in wild Hamadryas almost never affects the section with the sound mechanism. A review of the literature shows that more than 50 species of lepidopterans (11 families) emit sound audible to humans and suggests that sound mechanisms evolved several times. In general, lepidopteran sound is used basically as a warning to predators and for intraspecific communication.

Keywords : Behavior; protective coloration; crypsis; ndustrial melanism; territoriality; spatial distribution; sound; stridulation; mechanism; wing structure; defense; mating; courtship; communication.

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