Light attracted butterflies : a review from the Indian sub-region with an inventory from West Bengal , India

Some of the nocturnal Lepidoptera, mostly moths (Heterocera) are negatively heliotropic but positively phototactic, being impelled by an irresistible attraction to a bright lamp (Willey 1867). Phototaxis (attraction to artificial lights) has, however, been explained to be somewhat different from heliotropism (attraction to natural/sunlight) by Willey (1867). On the basis of exploiting natural light/sunlight, butterflies (Rhopalocera) can be categorized in two broad groups, viz., (i) the diurnal ones, with day flying habit and retiring at dusk, and (ii) the crepuscular ones, with a flying habit at dusk and even low-light conditions. For the night, they usually take shelter among bushes and trees. However, several observations regarding their occasional response to artificial light have been reported from both old and new worlds.


Light attracted butterflies: a review from the Indian sub-region with an inventory from West Bengal, India
Soumyajit Chowdhury 1 & Rahi Soren 2 1 School of Oceanographic Studies, Jadavpur University, 32 Raja S. C. Mullick Road, Kolkata, West Bengal 700032, India 2 Ecological Research Unit, Department of Zoology, University of Calcutta, 35 Ballygunge Circular Road, Kolkata, West Bengal 700019, India Email: 1 wildlifesc@gmail.com(corresponding author), 2 rahisoren@gmail.comSome of the nocturnal Lepidoptera, mostly moths (Heterocera) are negatively heliotropic but positively phototactic, being impelled by an irresistible attraction to a bright lamp (Willey 1867).Phototaxis (attraction to artificial lights) has, however, been explained to be somewhat different from heliotropism (attraction to natural/sunlight) by Willey (1867).On the basis of exploiting natural light/sunlight, butterflies (Rhopalocera) can be categorized in two broad groups, viz., (i) the diurnal ones, with day flying habit and retiring at dusk, and (ii) the crepuscular ones, with a flying habit at dusk and even low-light conditions.For the night, they usually take shelter among bushes and trees.However, several observations regarding their occasional response to artificial light have been reported from both old and new worlds.
The occasional phenomenon regarding attraction of butterflies to artificial lights in the Indian subregion have been reported in the literature.The first report was published by J.I. Alfrey in a paper by Best (1951) concerning the sighting of Lime Butterfly (Papilio demoleus demoleus Linnaeus) attracted to "Kitson Oil Lamps" at night at the railways station in Jhansi (southern Uttar Pradesh State, India) during a migration.Later, more detailed observations were published by Usman (1956), Donahue (1962), Shull (1964), Shull & Nadkerny (1967), Nadkerny & Shull (1968), Sharma & Chaturvedi (1999), Nair (2001) and Sharma & Chaturvedi (2005).
The present article deals with a review on the earlier observations on light-attracted butterflies in the Indian subcontinent with additions of four new species responding to the same from West Bengal.The primary objective is thus to elaborate the diversity and seasonal inclination of light-attracted butterflies in the Indian sub-region along with a possible explanation for such an unusual phenomenon.
Materials and Methods: The primary review work regarding the cases of light-attracted butterflies in the Indian sub-region from the period between 1951 and 2005 was accomplished through literature surveys.
Both inadvertent observations as well as designed experiments were undertaken during that period.However, the records for light-attracted butterflies in the urban (Kolkata) and forested regions (Samsing and Buxa of the Dooars) of West Bengal are new from this region and resulted from incidental observations during the period 2008 to 2009.No intentional lighttraps were designed in those places for attracting butterflies at night.
Results: The light-attracted butterflies recorded by the earlier authors from 1951-2005 revealed 27 species belonging to five families.Table 1 shows a list of the species, along with the place; season of observation and the type of artificial light source to which they were attracted.The observations by the present authors (2008)(2009) report six butterfly species that were attracted to light at night from Kolkata and Dooars (Samsing and Buxa) regions of southern and northern West Bengal, India.Four species reported here, viz.Papilio polytes Linnaeus, Tanaecia lepidea Butler, Neptis sp. and Pelopidas  2. A total of 31 species of butterfly resulting from the past and present observations showed occasional response to a variety of artificial lights in different parts of the Indian region.Among these, nymphalids (35.48%) outnumber the rest, followed by pierids (22.58%), lycaenids (19.35%), hesperiids (12.9%) and papilionids (9.68%) (Fig. 1).The observations (Tables 1 & 2) also reveal a seasonal inclination for such a phenomenon, as the maximum incidents were recorded during the monsoon months (June-October) in the Indian sub-region (Fig. 2).
Discussion and Conclusion: Crepuscular species, like Melanitis leda, are more active in the dark than the sun-loving, diurnal species, and thus are attracted more frequently to artificial lights (Donahue 1962).
The phenomenon of light attraction among butterflies has been found to be inclined more in the monsoon season (June-October) in the Indian subregion.Heavy downpours during the monsoon months may disrupt the night-time shelters of some butterfly   individuals, inducing them to move from their former preferred site to a new one.In the absence of optimum light, they fail to do so using visual cues.In that case, a source of artificial light with an intensity enough to stimulate them to get attracted may serve the purpose of utilizing their visual potency to search for a safe shelter.Throne (1961) also suggested that a butterfly may fly at light if it is disturbed at night and is near the light in the first place.Heitzman (1965) was certain that a large percentage of the specimens collected at light were startled from their resting places in nearby trees or bushes by the collector or some larger insects attracted towards light.Donahue (1962), however, noted that the butterfly numbers were greatest during the monsoon months, particularly in arid areas (like New Delhi, where his observations were made) when there is an abundance of food.He therefore pointed out the coincidence of the population peak with the rainy season, and either one or both these factors might influence the activity of certain species at light.He also indicated the probable effect of temperature upon the nocturnal activity of the observed species, since most of his observations were in the monsoon months, with no record of any species in winter.Furthermore, as in most cases of butterfly attraction towards a light source near their resting places, occasional incidents were reported where specimens were drawn from a considerable distance (Heitzman 1965).Donahue (1962) also argued that in some instances the butterfly would have to expend some effort to reach that light.Moreover, approaching that source, many of them tend to settle near the light.As light is an important cue that attracts insects to sources of heat (Schowalter 2006), the warmth obtained on getting nearer the light may also help in thermoregulation.
Further observations and experimentations on Indian butterflies being attracted to light at night may provide a less speculative explanation of phototactic and nocturnal behaviour.

Figure 1 .
Figure 1.A graphical representation showing the relative richness for light-attracted butterfly species in terms of their families (1951-2009).

Table 1 . A summary of the published data on butterfly species attracted to artificial light sources (1951-2005).
Donahue (1962)re additions to the known butterflies subject to this phenomenon in India (Table1), while Chilades lajus Stoll.andMelanitisledaLinnhave also been reported earlier bySharma & Chaturvedi (2005)from Maharashtra and byDonahue (1962)from Uttar Pradesh respectively.All the species were observed as single individuals during a single light-attraction event.No mass attractions were thus observed for the species in West Bengal.The observations by the present authors are detailed in Table