The text below is grayed out because it is not intended to be read. It is a necessarily imperfect OCR of the original and is only used by a search engine.
224
Journal of the Lepidopterists' Society
GREGARIOUS SEASONAL ROOSTING OF
SMYRNA KARWINSKII ADULTS IN EL SALVADOR
(NYMPHALIDAE)
Alberto Muyshondt
101 Avenida Norte #322, San Salvador, El Salvador
AND
Alberto Muyshondt, Jr.
Escuela Nacional de Agricultura, San Andres, Depto. La Libertad, El Salvador
Nocturnal gregarious roosting is known to exist in a variety of reputedly unpalatable species of butterflies (Carpenter, 1931; Crane, 1955; Jones, 1930; McFarland, 1970; McNeil, 1937; Poulton, 1931; and our personal observations on the phenomenon in adults of Dry as iulia iulia (Fabricius), Heliconius petiveranus Doubleday and H. charitonius L.). According to A. M. Young (pers. comm.), one case of nocturnal gregarious roosting of a nymphalid, Marpesia hernia (Hewitson), has been studied. But apparently a semi-permanent or seasonal, 24 hours a day, six months a year kind of roosting has not been known to exist in any kind of butterfly.
As stated in our preliminary report on communal resting of Smyrna karivinskii (1973), we have observed, annually since 1962, adults of Smyrna karivinskii (Geyer) resting during the day in groups ranging from 10 to more than 100 individuals, of mixed sex ratio, in cavities of lava walls and tree trunks, and on the underside of concrete slabs roofing alleys between cabins at Cerro Verde (a mountain of about 2000 m elevation overlooking the WNW slope of the Izalco Volcano, about 50 km from San Salvador). In total we have observed the phenomenon 37 days during the months of January, February, March, April, August, November and December in the course of eleven years. In each of these months, in different years, the observations have been repeated several times, except in August (only one time during 1965). In all instances the roosting aggregations observed were situated in the shade, away from direct sunlight. The locations chosen by the groups were always protected from the northern winds, which are usually quite strong from December through February, principally on mountain tops.
On 31 March 1972 we observed five roosting aggregations very closely. One contained 15 adults, a second 20, two about 50 each and a fifth more
Volume 28, Number 3
225
than one hundred. Monthly trips were made to the same place from June to November, but not a single group was found during that period. On 23 December, two groups were located, one consisting of 123 individuals (Fig. 1), and a second, some three meters from the first, of 39 (Fig. 2). As usual, both males and females were present, but no sexual activity was noticed. From time to time individuals would depart from the groups, presumably in search of food resources, and sporadically individuals would join the groups.
In order to determine the sex ratio in the aggregations, the one that contained 123 individuals was captured early in the morning of 23 December 1972, using a big bag made of stiff plastic sheet, that when flattened acted as a big paper envelope, immobilizing the butterflies without damaging them. To cause the butterflies to move into the bag, the mouth of it was applied to the concrete, enclosing the whole group, and then moved back and forth until all individuals, except 15 that escaped, were inside. The bag was flattened, keeping the butterflies motionless on their sides, and one by one they were extracted, quickly sexed and counted. Sexing this species is easy due to their sexual dimorphism. The total was 42 males and 66 females. As they were freed, they darted to the surrounding wood, alighting on tree trunks and rocks. During the following 5 hours, there was much flying activity: males chasing females and other males. Some individuals started to alight singly under the roofing slabs, scattered over an area of roughly 40 m. By the time we left we counted 38 individuals. None joined the smaller group, nor had they started to form a new aggregation.
We tried to mark the second smaller group with red spray from a distance, but the group started to disintegrate when the mist reached it, so we stopped this method. Out of the original 39 butterflies in the group, only 18 stayed. We tried then to mark the remaining butterflies individually by capturing each one by hand, giving it a light spray on the right rear underwing and putting it back in place, but the first individual so treated did not stay with the group and flew away. The rest were then left alone for future observations.
On 28 January 1973, about a month later, we visited the same place. The large group was not found. The smaller, though, was at the same place it had been on 23 December, and it had about the same number of individuals as when first sighted. Even the marked one was there, standing conspicuously in the second row. Unfortunately on the following trip to the locality this group was not found again, having been disturbed the week before by some curious tourists, according to the report of one of the guards.
226
Journal of the Lepidopterists' Society
On various occasions females were captured from aggregations and dissected. None had eggs in the abdomen, but considerable amounts of fat tissue were present.
It has been noticed that during the wet season the species is found in lower lands, where the foodplants (several species of Urticaceae), are very abundant. During the wet season it is rare to find adults of Smyrna karwinskii up in Cerro Verde. Not so during the dry season.
We have collected eggs and larvae of this species during the wet season in the same habitat we have collected eggs and larvae of its more common close relative Smyrna blomfildia datis Friihstorfer, which is seldom found at high altitudes (the highest record for S. b. datis in El Salvador is one adult captured on the slopes of Cerro Verde, ca. 1600 m, by S. R. Steinhauser in October 1967, pers. comm.), yet we have never found eggs or larvae of S. karwinskii in Cerro Verde. It is to be noted that the larvae of S. karwinskii are easily distinguished from the larvae of S. blomfildia datis by their color. S. kanoinskii have the body and spines brown, while S. blomfildia datis presents four different morphs: body and spines greenish white, body greenish white with black spines, body mostly black with light spines and body mostly black with black spines. In shape and head the two species are much alike.
Discussion
Since the phenomenon was first observed, back in 1962, it had been our assumption that the butterflies were grouped at places where water filtrated and that they were drinking there. But on 31 March 1972 we had a chance to observe five groups from a short distance for a period of seven hours. Not once did we see any individual uncoil its proboscis, nor did we notice any moisture in the concrete, thus eliminating that, as well as the alternative explanation that the grouping was formed to allow use of each other's excreta to recycle fluids, as has been reported by Hessel (1966) for single individuals of Agathymus aryxna (Dyar). The fact that the groups are always formed in the shade, away from direct sunlight, also eliminates the possibility that receiving solar heat plays a role in causing the aggregations.
In the unpalatable species, gregarious nocturnal roosting has been viewed as an evolutionary behavior acquired to enhance their unpleasant scent and therefore the chances to effectively deter any approaching predator. Smyrna kanoinskii adults are not reputed to be distasteful to predators, but no experiment that we know of has been carried out in this respect. We have observed that the larvae when molested extrude a gland located anterad of the prothoracic legs, as its relatives S.
Volume 28, Number 3
227
blomfildia datis, Colobura dirce L. and Historis odius Fabricius do, emitting a scent very faint to humans, presumably to repel potential vertebrate predators (Hemiptera have been found feeding on Smyrna spp. larvae). This makes us doubt the palatability of the adults. If after appropriate experiments, this species proves to be distasteful to such predators, the communal roosting habits could be explained for the reasons given above. This mechanism would act only as a chemical repellent, having a passive role, as it is evident that the individuals in the congregation do not have the ability to communicate to the other members of the group when danger is imminent, as individuals can be captured by hand from any place in the group and at any time of the day without causing a reaction from the rest of the individuals. It is necessary to be rather rough to obtain a mass response from the whole aggregation. When this is done the individuals disperse in all directions producing an audible rustling noise with the wings, somewhat like Hesperiidae.
Another possible benefit that the congregated individuals seem to derive from their communal roosting is the mimetic effect obtained: the groups look like a dried moss or lichen formation, at least to humans.
The fact that the aggregations are formed at the beginning of the dry season, persist through it and dissolve at the beginning of the wet season, plus the presence of excessive fat tissue and the absence of eggs in the females, seems to point to a case of aestivation in a state very close at least to diapause. Individual diapause would serve the purpose of living through the dry season by itself, but the communal aestivation would have the additional advantage of keeping the sexes together, thus guaranteeing an effective and early encounter, optimizing the chances of early copulations and consequently the production of fertile eggs when weather conditions are once again favorable for larval development.
The fact that individuals abandon the groups from time to time, and the fact that we have witnessed individuals feeding at tree wounds in the neighboring woods, seem to indicate that this is not a case of complete diapause, but a partial one that calls for a close and reliable source of food, even if only periodically needed by organisms whose metabolism is grealty slowed down.
There seems to be a degree of organization in the groups with some kind of discrimination between individuals belonging to different ones. The organization is suggested by the consistent way the groups are formed: there is a nucleus of several individuals with the heads pointing inwards, sometimes so close as to have their upraised antennae almost touching, surrounded by tightly packed rows, forming circles or partial
228 Journal of the Lepidopterists' Society
Figs. 1 and 2. Roosting assemblies of Smyrna karwinskii: 1, 123 adults; 2, 39 adults. Both photographed at Cerro Verde, 23 December 1972.
circles of individuals with the heads again pointing inwards, and with the antennae touching a member of an inside row (see Fig. 1). The discrimination is deduced from the results of the disruption of the large group when it was counted: none of the dispersed individuals came to join the members of the smaller group, but kept by themselves. Probably they later formed another group elsewhere composed of the
Volume 28, Number 3
229
same individuals. One month after the smaller group was partially disturbed, it again had about the same number it had had originally, including the one individual marked with an unfamiliar color.
It is our opinion that Smyrna karwinskii adults have acquired this unusual social behavior as an adaptation tending to minimize the losses of individuals through the dry season resulting from prcdation on the one hand and excessive activity on the other, and to maximize the chances of early egg production when conditions are favorable for the dispersal of the species. This adaptive strategy nevertheless seems to be disadvantageous when compared with the one adopted by S. blomfildia datis, whose larval polymorphism seems to indicate a more flexible ability to adapt itself to adverse ambient conditions.
Acknowledgments
We are greatly indebted to Dr. Alexander B. Klots of the American Museum of Natural History, who besides encouraging the authors to publish the results of their observations, took time out of his busy schedule to read the manuscript and give much valuable criticism. We also thank Dr. Allen M. Young who shared important information with us, Viktor Hellebuyck who helped the authors in parts of their observations, and the rest of the family Muyshondt for their sustained efforts in the study of the Salvadorian butterflies.
Literature Cited
Carpenter, G. D. H. 1931. Acraeinae butterflies congregating in a small area
for the night's rest. Proc. Roy. Entomol. Soc. London 6: 71. Clench, H. K. 1970. Communal roosting in Colias and Phoebis (Pieridae). J. Lepid.
Soc. 24: 117-120. Crane, J. 1955. Imaginal behavior of a Trinidad butterfly, Heliconius erato hydara
Hewitson, with special reference to the social use of color. Zoologica 40: 167-
197. Hessel, J. M. 1966. Fluid recycling in Agathymus aryxna (Megathymidae). J.
Lepid. Soc. 20: 242. Jones, F. M. 1930. The sleeping Heliconians of Florida. Nat. Hist. 30: 635-644. McFarland, N. 1971. A specialized case of communal roosting in Pieris rapae
(Pieridae). J. Lepid. Soc. 25:144-145. McNeil, F. A. 1937. Notes on the gregarious resting habit of Danaeus melissa
hamata W. S. Maclery, in the Whitsunday Islands off the East coast of
Queensland. Proc. Roy. Entomol. Soc. London 12:108. Poulton, E. B. 1931. The gregarious sleeping habit of Heliconius charitoniua, L.
Proc. Roy. Entomol. Soc. London 6: 71.