Featured species: Papilio hornimani

IUCN Red List Conservation Status: Growing Concern? 

Although this large and striking East African endemic swallowtail is not on the IUCN Red List, being treated in effect as ‘Least Concern’ (2), given the ever-increasing impact of human population growth, agriculture and climate change on the mid-montane forests of East Africa, it is easy to imagine this species, or at least some of its recognised subspecies, becoming ‘Near Threatened’ or even ‘Vulnerable’ in the near future. Moreover, P. hornimani has the potential to act as a flagship for at least 20 other swallowtails (3) that have populations in the area comprising the ‘eastern arc’ Pare, Usambara and Udzungwa mountains, Kilimanjaro and its associated ranges in northern Tanzania, and the southernmost montane forests of Kenya (Chyulu and Teita hills). Papilio (Nireopapilio) desmondi is particularly interesting in this regard, with four separate subspecies recognised across the region, including the Chyulus and Teita Hills (4,5,6).

Description and Relationships

A medium to large-sized swallowtail (male forewing length in the range 45–62 mm; female 55–66 mm), both sexes are broadly similar in appearance. In the male, much of the upperside is velvety-black, with both wings crossed centrally by an iridescent blue or blue-green band. Small spots of the same colour occur at the apex of the forewing, and in a row parallel to the outer margin of the hindwing. In females, the black and bluish-green colours are less intense, and there is an additional row of marginal spots on the hindwing. The undersides are dark, with marbled patterning on the discal and basal areas of the hindwing – better developed in females. Males have a row of bright, silvery/whitish submarginal spots on the hindwing; the females, as with many species of the nireus group, lack these high-contrast spots. Both sexes have a well-developed, clubbed hindwing tail (1,4).

 

There are over 60 species of Papilio in the afrotropics, all of which (apart from members of the Papilio machaon complex in the Arabian Peninsula) are currently divided among just three subgenera: Druryia (which includes Papilio dardanus and P. antimachus), Princeps (Papilio demodocus and its closest relatives), and Nireopapilio – to which Papilio hornimani belongs (7,8). Like most of the 26 species in this subgenus (5), P. hornimani belongs to the nireus species group, almost all members of which have similar coloration. However, most of these greenish-blue banded swallowtails have reduced hindwing tails; in Tanzania and Kenya, P. hornimani is the only blue-banded nireus group species that has a fully-developed tail.

Distribution

As already indicated, Horniman’s Swallowtail only occurs in eastern and northern Tanzania, and in the Chyulu Hills, southern Kenya (1,9,10). The nominate subspecies, P. hornimani hornimani (type locality: Magila), flies in the East and West Usmabaras and the South Pare mountains (1). A separate population, P. h. mwanihanae Kielland, 1987 (illustrated; type locality: Sanje), occurs to the south, in Mwanihana National Park (Udzungwa Mountains) (9). The third currently recognised subspecies, P. h. mbulu Kielland, 1990 (type locality: Hasama Forest), is found in the montane forests of northern Tanzania, in the Mbulu district, Ngorongoro, Oldeani, Mt Kwaraha, Mt Meru and Mt Kilimanjaro (9). There has been uncertainty regarding the Chyulu Hills population in Kenya (5,10), but both phenotype and geography (the Chyulus being only about 70 km northeast of Mt Kilimanjaro) strongly suggest it represents an outlier of subspecies mbulu (1). 

 

Habitat and Ecology

Very little is known about the life-cycle, natural history or biology of this beautiful swallowtail. Apparently it has been bred or reared on at least one occasion, but it seems nothing has been published (1). Even its larval hostplants are uncertain – most probably one or more species of Vepris (Teclea in older literature; Rutacaeae); possibly it will also feed on Citrus (9). Some species of Vepris are known to have pharmacologically active secondary compounds (1), which could be significant for the development and physiology of the butterflies. The adults, said to be powerful flyers, are always associated with low to mid-montane forests, having been observed in the range of 300–2200 metres (9). Males are frequent mud-puddlers, attracted to damp soils or mud – especially, so it has been said, if moistened with elephant urine (11)—elephant dung has been found in the Udzungwa Mountains at 2600 m (12), and elephants may go even higher on Mt Kilimanjaro. The adults also visit flowers – but there appear to be no specific records. Nothing at all is on record concerning the natural enemies of P. hornimani, including parasitoids.  

Threats 

Relatively recent photographs posted online through iNaturalist and Tanzania Birds indicate that P. hornimani mbulu and P. h. mwanihanae continue to thrive, at least in parts of their ranges. At the turn of the century, some 25 years ago, P. hornimani hornimani was confirmed still to occur in the Mlinga Forest system (1). In 2007, the butterfly farming project at Amani was listing P. h. hornimani at $2.50 per specimen (13), if available (pupa or adult was not made clear). The stated ethos of the project was that butterflies on offer were ranched or bred – but, even if so, no information regarding the life history of P. hornimani appears to have come from this source. The Amani Butterfly Project continues today, with the strapline “a non-profit organisation that helps 400 rural Tanzanians from 6 villages in the East Usambara Mountains farm and market native butterflies.” (14)

The mission of the Amani Butterfly Project is “to reduce poverty and create incentive for forest conservation” (14) – suggestive that the major current threat to the continuing existence of Papilio hornimani comes from deforestation. Much of the region between the East Usambaras and the coast lacks forest cover. Victorian-era descriptions suggest that a wide stretch of this lowland region was, even then, dry coastal savannah rather than forest, perhaps made so by fires for hunting (1). 

However, some commentators are convinced that much deforestation of the East Usambara lowlands has been recent. Unlike the forest and woodland areas cleared for sisal plantations during the British and German colonial period, more recent degradation has been caused by fires, subsistence agriculture and charcoal burning (1). More generally, fragmentation is probably the major threat to the remarkable biodiversity of Tanzania’s mid-montane forests (e.g. 15). A field-based and stakeholder survey published in 2020 (co-authored by the original director of the Amani Butterfly Project, now associated with the Tanzania Forest Conservation Group) concluded that, in reality, agriculture is now the primary deforestation driver, having a much greater impact than charcoal production, with timber harvesting and clearing for livestock making only minor impacts (16).

Conservation Measures

Policy-makers in Tanzania have sought to lessen deforestation by reducing demand for charcoal. However, agriculture it seems, not charcoal, is demonstrably the main driver of local deforestation. Beyond protected areas, it has been suggested that there is no clear policy limiting the conversion of forests to agricultural land. So, reducing deforestation in Tanzania will require coordination between the agriculture, livestock, land, energy and forest sectors (16).

Thus the overarching threat to lower to mid-montane forest insects such as Papilio hornimani is not something that an organisation like SBBT can directly address. All three subspecies of this iconic East African swallowtail seem to be thriving in some localities – albeit in national parks and various non-designated forest areas that have not yet been degraded. In anticipation that many of these populations are likely to be faced with further forest fragmentation and even total loss, what SBBT could and perhaps should do is support and encourage local scientists in Tanzania to carry out basic research. Where possible this could be done in collaboration with members of the global entomological community. The first need is to resolve basic details of the swallowtail’s life cycle and ecological requirements, including its larval hostplant and adult resource needs. Once this has progressed sufficiently, research focus could then be switched to trying to understand its population dynamics. Anyone in a position to initiate such a programme, which could include opportunities for citizen science and the volunteer sector, is encouraged to contact SBBT.  

Richard I. Vane-Wright 

Associate Member, Durrell Institute of Conservation and Ecology (DICE), University of Kent, Canterbury CT2 7NR, UK

Scientific Associate, Insects Division, Department of Science, Natural History Museum, London SW7 5BD, UK

Honorary Advisor, Swallowtail and Birdwing Butterfly Trust 

Acknowledgements 

The author is very grateful to iNaturalist (Trude Peterson) and Stefane Akame for permission to use some of their images of Horniman’s Swallowtail. Adam Cotton, Clive Huggins, Sophie Hall, Martin Partridge and Mark Collins kindly read and commented on the first draft.  

References 

1. Vane-Wright, R.I. & Liseki, S.[D]. (2008). The type material, taxonomy and conservation of Horniman’s Swallowtail, Papilio hornimani (Lepidoptera: Papilionidae). Journal of Natural History 42(19/20): 1333–1348. 
2. Collins, N.M. & Morris, M.G. (1985). Threatened Swallowtail Butterflies of the World. The IUCN Red Data Book. Gland (Switzerland): IUCN.
3. Papilio constantinus, P. dardanus, P. demodocus, P. desmondi, P. echerioides, P. fuelleborni, P. mackinnoni, P. nireus, P. nobilis, P. ophidicephalus, P. pelodurus, P. phorcas, P. rex, P. sjoestedti, Graphium angolanus, G. antheus, G. colonna, G. leonidas, G. philonoe, G. policenes.
4. Liseki, S.D. & Vane-Wright, R.I. (2011). Butterflies (Lepidoptera: Papilionoidea) of Mount Kilimanjaro: introduction and family Papilionidae. Journal of Natural History 45(37/38): 2375–2396. doi: 10.1080/00222933.2011.596635 [including TNAH 596635 Supplementary material.pdf].
5. Williams, M.C. (2023). Afrotropical butterflies. Genus Papilio Linnaeus, 1758. Swallowtails. Online at: https://metamorphosis.org.za/articlesPDF/1041/011%20Genus%20Papilio%20Linnaeus.pdf [accessed 17.v.2025]
6. Collins, N.M. & Clifton, M.P. (1984). Threatened wildlife in the Taita Hills. Swara 7(5): 10–14.
7. Cotton, A.M. & Nakae, M. (2020). Replacement name for Eques Kirby, 1896 (Lepidoptera: Papilionidae). Butterflies (84): 47. 
8. Condamine, F.L., Allio, R., Reboud, E.L., Dupuis, J.R., Toussaint, E.F., Mazet, N., Hu, S.J., Lewis, D.S., Kunte, K., Cotton, A.M. & Sperling, F.A. (2023). A comprehensive phylogeny and revised taxonomy illuminate the origin and diversification of the global radiation of Papilio (Lepidoptera: Papilionidae). Molecular Phylogenetics and Evolution 183: 107758. 
9. Kielland, J. (1990). Butterflies of Tanzania. Melbourne: Hill House. 
10. Larsen, T.B. (1991). The Butterflies of Kenya and their Natural History. Oxford: Oxford University Press.
11. Williams, J.G. (1969). A Field Guide to the Butterflies of Africa. London: Collins. 
12. Nowak, K., Jones, T., Lee, P. & Hawkins, D. (2010). Savanna elephants in montane forest: assessing the population of a landscape species in the biodiverse Udzungwa Mountains. Proceedings of the Tanzania Wildlife Research Institute Seventh Scientific Conference, Arusha, Tanzania. Arusha: TAWIRI, 8 pp. https://www.stzelephants.org/download/udzungwa-elephants/Savanna%20elephants%20in%20montane%20forest_TAWIRI_2009.pdf [accessed 17.v.2025] 
13. Anderson, W. & Saidi, S.A. (2011). Internationalization and poverty alleviation: practical evidence from Amani Butterfly Project in Tanzania. Journal of Poverty Alleviation and International Development 2(2): 17–45. 
14. The Amani Butterfly Project. Online at: https://www.minibeast.co/amani?srsltid=AfmBOooRba1m_pVmgt5HrDUFJxmgLX1Pq-KNRRF5dll5HZN7bm0UkmzE [accessed 17.v.2025] 
15. Ylhäisi, J. (2004). Indigenous forests fragmentation and the significance of ethnic forests for conservation in the North Pare, the Eastern Arc Mountains, Tanzania. Fennia – International Journal of Geography 182(2): 109–132. 
16. Doggart, N., Morgan-Brown, T., Lyimo, E., Mbilinyi, B., Meshack, C.K., Sallu, S.M. & Spracklen, D.V. (2020). Agriculture is the main driver of deforestation in Tanzania. Environmental Research Letters 15(3): 034028.