Do dragonflies migrate across the western Indian Ocean?

Abstract

In the tropical Indian Ocean, the Maldive Islands lack surface freshwater, so are unsuitable for dragonfly reproduction. Nevertheless, millions of dragonflies (Insecta, Odonata; mostly globe skimmer, Pantala flavescens) appear suddenly every year starting in October. Arrival dates in the Maldives and India demonstrate that the dragonflies travel from southern India, a distance of some 5001000 km. Dates of arrival and occurrence coincide with the southward passage of the Inter-tropical Convergence Zone (ITCZ). Circumstantial evidence suggests that the dragonflies fly with north-easterly tail winds, within and behind the ITCZ, at altitudes over 1000 m. It is proposed that this massive movement of dragonflies is part of an annual migration across the western Indian Ocean from India to East Africa. Arrival dates in the Seychelles support this hypothesis. Dragonflies also appear (in smaller numbers) in the Maldives in May, with the onset of the southwest monsoon, suggesting a possible return migration from Africa. These proposed migrations of dragonflies, regularly crossing 3500 km or more of open ocean, were previously unknown. It is known that these dragonflies exploit ephemeral rain pools for reproduction; the monsoons and ITCZ bring not only alternating, seasonal rains to India and Africa, but also appropriate winds for dragonflies to follow those rains. Several bird species migrate from India across the western Indian Ocean to wintering grounds in Africa. They do so at the same time as the dragonflies, presumably taking advantage of the same seasonal tail winds. Many of these birds also eat dragonflies; the possible significance of this was not previously appreciated.

Full text

Journal of Tropical Ecology (2009) 25:347–358. Copyright © 2009 Cambridge University Press
doi:10.1017/S0266467409006087 Printed in the United Kingdom
Do dragonflies migrate across the western Indian Ocean?
R. Charles Anderson
Manta Marine Pvt Ltd, P.O. Box 2074, Mal´
e, Republic of Maldives
(Accepted 9 March 2009)
Abstract: In the tropical Indian Ocean, the Maldive Islands lack surface freshwater, so are unsuitable for dragonfly
reproduction. Nevertheless, millions of dragonflies (Insecta, Odonata; mostly globe skimmer, Pantala flavescens) appear
suddenly every year starting in October. Arrival dates in the Maldives and India demonstrate that the dragonflies travel
from southern India, a distance of some 500–1000 km. Dates of arrival and occurrence coincide with the southward
passage of the Inter-tropical Convergence Zone (ITCZ). Circumstantial evidence suggests that the dragonflies fly with
north-easterly tail winds, within and behind the ITCZ, at altitudes over 1000 m. It is proposed that this massive
movement of dragonflies is part of an annual migration across the western Indian Ocean from India to East Africa.
Arrival dates in the Seychelles support this hypothesis. Dragonflies also appear (in smaller numbers) in the Maldives
in May, with the onset of the southwest monsoon, suggesting a possible return migration from Africa. These proposed
migrations of dragonflies, regularly crossing 3500 km or more of open ocean, were previously unknown. It is known
that these dragonflies exploit ephemeral rain pools for reproduction; the monsoons and ITCZ bring not only alternating,
seasonal rains to India and Africa, but also appropriate winds for dragonflies to follow those rains. Several bird species
migrate from India across the western Indian Ocean to wintering grounds in Africa. They do so at the same time as the
dragonflies, presumably taking advantage of the same seasonal tail winds. Many of these birds also eat dragonflies; the
possible significance of this was not previously appreciated.
Key Words: Amur falcon, dragonfly, Indian Ocean, ITCZ, Maldives, migration, monsoon, Odonata,
Pantala flavescens, Somali Jet
INTRODUCTION
Many species of dragonfly and land-bird migrate long
distances, but most avoid crossing wide bodies of water.
For example, several species of migratory dragonfly follow
coastlines rather than fly out over the sea, and there
are no known regular, seasonal transoceanic migrations
by dragonflies (Corbet 2004, May & Matthews 2008,
Wikelski et al. 2006). A relatively small number of land-
birds do make long ocean crossings, including several
which migrate from India to their wintering grounds in
eastern and southern Africa, across the western Indian
Ocean (Ali & Ripley 1987, Moreau 1938, 1972).
In the western Indian Ocean, the Maldive Islands do
not appear the most promising of locations for the study
of dragonflies. All 1200 islands are small coral cays, with
almost no surface freshwater, which dragonflies require
to complete their lifecycles. The annual appearance
Emails: anderson@dhivehinet.net.mv or charles.anderson11@
btinternet.com
of millions of dragonflies, although well known to
Maldivians, is therefore something of a mystery.
The original aims of this study were to document the
annual appearance of dragonflies in the Maldives and to
investigate their likely origins. It soon became apparent
that the dragonflies were arriving across the ocean from
southern India. This then raised questions as to how
they make the crossing (since the first dragonflies to
arrive in October each year do so when the seasonal
monsoon winds are still blowing towards India), and
why they make the crossing (since the Maldives lacks
surface freshwater and therefore seems an unlikely target
for migrant dragonflies).
MATERIALS AND METHODS
Study area
The area covered by this study encompasses the whole of
the tropical western Indian Ocean, and bordering land

348 R. CHARLES ANDERSON
masses, from western India to East Africa. However,
investigations were centred at Mal´
e, the island capital
of the Maldives. The Maldives is composed entirely of
coral atolls, which form a chain running north-south
from about 7◦N to about 0.5◦S, south-west of the
southern tip of India. Mal´
e lies at about 4◦N. The climate
of the Maldives, and indeed the entire study area, is
strongly influenced by the monsoons. In the Maldives
the south-west (or boreal summer) monsoon blows from
about May to October, while the north-east (or boreal
winter) monsoon lasts from about December to March.
The Inter-tropical Convergence Zone (ITCZ) marks the
boundary between these two wind systems. The ITCZ
migrates north and south seasonally, following the sun,
passing over the Maldives heading southwards during
the November intermonsoon and northwards during the
April intermonsoon.
The onset of the south-west monsoon in Maldives in
May is usually marked by a period of strong wind and
rain, associated with the northward passage of the Somali
or Findlater Jet (Findlater 1969). It is usually only during
the month of May that this seasonal, low-level (1000–
2000 m) jet passes over the Maldives; in earlier months it
is further south, while in later months it lies further north.
Dragonfly monitoring
Dates of first arrival of large numbers of dragonflies at
Mal´
e in October were recorded in 1983 and in every
year from 1996–2008 (n =14 y); dates of arrival of the
first individuals were recorded in 1996–97 and 1999–
2008 (n =12 y). Arrival dates of dragonflies during late
September to early November were recorded by local
observers at other localities in Maldives and India during
2002–2008, and on three vessels at sea, in 1983 (Smith
1984) and 1996 (i.e. during a total of 8 y in which arrival
dates were also noted in Mal´
e). For each locality it was
therefore straightforward to calculate relative arrival time
in days before or after arrival in Mal´
e. Dragonfly numbers
in Mal´
e were counted on a total of 770 d between October
2002 and September 2007 (i.e. on 42% of days), around
a standard 5.2 km circuit of the island. Residency time
in Mal´
e was estimated using dragonfly numbers from 3-d
periods (n =48) during which counts were made each
day, and for which there was no increase in numbers
from one day to the next (which would indicate additional
immigration).
Bird data
Birds that cross the western Indian Ocean include those
listed in Table 1. Records of these species in the western
Indian Ocean (Lakshadweep, Maldives, Seychelles and
Table 1. Migrant dragonflies that occur in the Maldives (Olsvik &
H¨
am¨
al¨
ainen 1992, pers. obs.), and migrant birds that regularly cross
the western Indian Ocean. An estimated 98% of dragonflies recorded
at Mal´
earePantala flavescens.
Migrant Odonata
Pale-spotted emperor Anax guttatus (Burmeister, 1839)
Vagrant emperor Anax ( =Hemianax) ephippiger
(Burmeister, 1839)
Globe skimmer Pantala flavescens (Fabricius, 1798)
Twister Tholymis tillarga (Fabricius, 1798)
Keyhole glider Tramea basilaris (Beauvais, 1805)
Voyaging glider Tramea limbata (Desjardins, 1835)
Blue percher Diplacodes trivialis (Rambur, 1842)
Transoceanic bird migrants
European roller Coracias garrulus Linneaeus, 1758
Blue-cheeked bee-eater Merops persicus Pallas, 1773
Pied cuckoo Clamator jacobinus serratus
(Sparrman, 1786)
Lesser cuckoo Cuculus poliocephalus Latham, 1790
Eurasian cuckoo Cuculus canorus Linnaeus, 1758
Amur falcon Falco amurensis Radde, 1863
Eurasian hobby Falco subbuteo Linnaeus, 1758
Lesser kestrel Falco naumanni Fleischer, 1818
Eurasian nightjar Caprimulgus europaeus Linnaeus, 1758
at sea) are from the literature (Anderson 2007, Skerrett
et al. 2001, and references therein), with additional
information from the databases of the Seychelles Bird
Records Committee and the Royal Naval Birdwatching
Society.
Meteorological information
Surface wind data were provided by the Maldives
Meteorological Service. Mean monthly wind direction
frequencies at Mal´
e were calculated from daily records
for 2002–2006 for two sectors: north-easterly (N–E, 0◦–
90◦) and SSW–NW (202.5◦–315◦). Dates of passage of
the Somali Jet were estimated from daily weather records,
and were considered to be periods of at least 5-d duration
between late April and early June during which there
was consistently lower than average mean pressure, and
higher than average wind speed, rainfall, cloud cover and
relative humidity.
Maldives does not have a regular radiosonde
programme. However, radiosonde data were collected
during the Japan Agency for Marine-Earth Science and
Technology (JAMSTEC) MISMO programme (Yoneyama
et al. 2008) during late 2006. Radiosonde data were
collected for Hulhule (an island adjacent to Mal´
e) from 18
October to 26 November 2006, and are freely available
at: www.jamstec.go.jp/iorgc/mismo.
Regional rainfall data are from Anon. (1991).
Indicative average monthly rainfall was estimated for
three regions, from four rainfall stations each: western
India (Ahmadabad, Hyderabad, Mumbai, Panaji), east

ITCZ migrations 349
Figure 1. Seasonality of dragonfly occurrence, monsoon winds and bird migration. Relative abundance of dragonflies at Mal´
e, Maldives, by month
(mean ±95% CI, October 2002 to September 2007, n=770 daily counts) (a). Predominant surface wind directions at Mal´
e, Maldives, by month
(dark cross-hatching =N/E; light stippling =SSW/NW; white =other directions) (b). Relative abundance of the migratory birds listed in Table 1 in
the western Indian Ocean, by month (n=248 records) (c).
central Africa (Dar es Salaam, Kampala, Lira, Nairobi)
and south-eastern Africa (Beira, Harare, Lusaka, Zomba).
Note that for East Africa, rainfall patterns are particularly
complex because of topographic effects (Griffiths 1969)
and the averages presented here are only approximations.
RESULTS
During different years of the study period, dragonflies
first appeared in Mal´
e between 4 and 23 October.
Small numbers appeared initially, with large numbers
appearing a few days later, usually between 14 and 28
October (in 12 out of 14 y), with 21 October being the
mean arrival date. Under the 2006 JAMSTEC MISMO
programme, radiosonde data collection at Mal´
e started on
18 October (the very day that dragonflies first appeared
in numbers in Mal´
e that year). The data indicate the
presence of one layer of ENE wind at 1200–2400 m
(with speeds of the order of 3–4 m s−1) and another at
2900–5000 m (5–6 m s−1). Each year dragonfly numbers
peaked in November–December (Figure 1a). Dates of first
arrival of dragonflies at different locations in Maldives
and south India are shown in Figure 2. Several species

350 R. CHARLES ANDERSON
Figure 2. Map of southern India and Maldives showing average, relative
arrival times of Pantala flavescens swarms during September to November
(n=83 observations made over 8 y). Units are days [mean (n)] relative
to arrival date in Mal´
e. Dots show sampling locations (triangles indicate
observations at sea); ellipses enclose locations for which arrival dates are
averaged (over sites, years or both). Scale: 2◦latitude =120 nautical
miles =222 km.
of dragonfly are involved (Table 1). The commonest was
the globe skimmer or wandering glider Pantala flavescens
Fabricius, which made up 98% of the total sampled in
Mal´
e; other species appeared slightly commoner on other
more vegetated islands (pers. obs).
When tracked over 3-d periods, average dragonfly
abundance at Mal´
e dropped (from 100% on day 1) to
46% and then 22% on days 2 and 3 respectively. This
likely overestimates residency, since any immigration of
small numbers of dragonflies (less than or equal to the
number emigrating) would not be detected. Considering
only the four largest influxes (which accounted for 52%
of dragonfly numbers in 48 influxes, and for which any
subsequent immigration would likely be proportionately
less significant), dragonfly numbers dropped to 40% on
day 2 and just 12% on day 3. Dates of P. flavescens
occurrence in April–June for the three years with the most
complete data for those months, and dates of passage of
the Somali Jet over Mal´
e are noted in Table 2. Dragonfly
occurrence during April–June coincided with the passage
of the Somali Jet in both years during which its occurrence
was clearly indicated in the available meteorological
data.
Records of migrant birds peaked during November–
December (Figure 1c), and are correlated with dragonfly
abundance (Kendall’s rank correlation coefficient, τ=
−0.68, P <0.01).
DISCUSSION
Appearance of dragonflies in Maldives
Although surface winds are predominantly westerly
during October (Figure 1b), dates of first arrival
(Figure 2) strongly suggest that P. flavescens is migrating
from India. There is a clear progression of first arrival dates
from north to south, from south India towards Mal´
eand
on to the southern atolls. In addition, arrival dates are
slightly earlier on the east of the Maldives than the west
at the same latitude, hinting at arrival from the north-east
rather than due north.
It is already known that, starting each September and
continuing into October, P. flavescens migrate southwards
in large numbers within southern India (Fraser 1924,
1936; Larsen 1987). Pantala flavescens movements to the
west have also been observed in southern India and Sri
Lanka in September (Fraser 1954). If these dragonflies are
indeed arriving in Maldives from India (and Sri Lanka),
this would involve a sea crossing of about 600 km to
Mal´
e, and 1000 km to the southernmost islands. The first
dragonflies to arrive in Mal´
e in October invariably do so
when surface winds are still from the south-west, towards
India. It is not immediately obvious how dragonflies travel
from India, apparently against the wind.
These dragonflies are renowned for their long-distance
movements, and several have been characterized as
obligate Inter-tropical Convergence Zone (ITCZ) migrants
(Corbet 2004). In the ITCZ, winds converge, and then rise,
forming clouds. Corbet (1962), drawing on the locust
migration studies of Rainey (1951), hypothesized that
P. flavescens migrates with the ITCZ as it makes its seasonal
excursions north and south, thereby taking advantage
of converging winds which automatically carry them to
areas where rain falls (and reproduction is possible).
In October the ITCZ moves southward (Lobert & Harris
2002, Waliser & Gautier 1993), across the Maldives. The
ITCZ front is inclined towards the equator. Thus, while
surface winds at Mal´
e (Figure 1b) usually remain westerly
until the end of October, winds north of the ITCZ (and

ITCZ migrations 351
above the front) are predominantly north-easterly. The
dragonflies can thus migrate to the Maldives against the
direction of the surface winds by travelling at altitude in
the ITCZ.
These dragonflies are known to travel at altitude
(Corbet 1984, 2004). In the case of P. flavescens, radar
studies in China have detected substantial migratory
movements at heights of up to 1000 m (Feng et al.
2006). Pantala flavescens occurs at particularly high
altitudes in the Himalayan region (Corbet 2004, Vick
1989, Wojtusiak 1974), where it has been reported at
up to 6300 m, the highest record for any odonate (Corbet
2004). The altitudes at which dragonflies may fly from
India to Maldives are unknown, but suitable winds do
occur at 1000–2500 m.
In the Maldives, islanders describe seeing swarms
of P. flavescens ascending and descending as if to
and from great altitude (although these insects are
probably not visible with the naked eye at more than
about 100 m). One informant, who has many years of
experience as a meteorological observer, noticed large
numbers of P. flavescens descending onto his island
(Vilingili, adjacent to Mal´
e) at about 1600 h local
time one day in the second half of October 2008.
Looking vertically upwards with binoculars he could
make out descending dragonflies up to a height which
he estimated to be about 1000 m (Abdul Muhusin
Ramiz, Director, Maldives Meteorological Service, pers.
comm.). My own observations indicate that the first
appearance of dragonflies at Mal´
e coincides with the first
movements of ‘upper’ clouds (altitude unknown but by
my estimate higher than 1000 m) from the direction of
India, while ‘lower’ clouds continue to move from the
west or south-west. For example, at Mal´
e in 2003, both
‘lower’ and ‘upper’ clouds were moving from the south-
west on 4 October. On 5 October, while ‘lower’ clouds
continued to move from the south-west, ‘upper’ clouds
were moving from the north-east. The first dragonfly
of the season was recorded in Mal´
e on that same day.
Many more dragonflies were recorded later in October,
although surface winds remained mostly westerly for
nearly another month, with the first north-easterly winds
at sea level not being recorded until 3 November that
year. In 2006, dragonflies first appeared in numbers at
Mal´
e on 18 October. Radiosonde data from that same day
indicated the presence of ENE winds at altitudes of 1200–
2400 m and 2900–5000 m. Unfortunately, there are no
radiosonde data from previous days.
North of the ITCZ during the north-east monsoon, air
masses are transported from India to the Maldives, as
studies of atmospheric pollution have clearly documented
(Lelieveld et al. 2001, Lobert & Harris 2002). Strong sea
breezes along the west coast of India lift pollutants (and
no doubt also insects) up into an elevated land plume
at about 1000–2500 m, where they are transported
towards Maldives at speeds of about 10 m s−1(Lelieveld
et al. 2001, Raman et al. 2002). With such tailwinds,
dragonflies might make the crossing from India to Mal´
e
in24horless.
This estimate of flying time may appear at odds with
Figure 2, which might seem to imply that P. flavescens
take (for example) about 8 d on average to travel from the
southern tip of India to Mal´
e. But the timings in Figure 2
largely reflect the southward passage of the leading edge
of the ITCZ. The ITCZ may take about 8 d to migrate south
from the southern tip of India to the latitude of Mal´
e. Only
once it arrives can P. flavescens appear, but those first
individuals may have left India just 1 d earlier.
The migration continues into February, but 73% of all
dragonflies in Mal´
e were recorded between 18 October
and 18 December. This matches rather closely with
the passage of the ITCZ. Comparison of Figures 1a and
1b shows that the occurrence of dragonflies does not
correspond well with the frequency of north-east winds,
which might be expected if they were being exported
randomly from India, as is the case with atmospheric
pollution (Lobert & Harris 2002). Rather, the peak of
dragonflies occurs during the intermonsoon period, i.e.
during the passage of the ITCZ.
The appearance of the dragonflies during the
intermonsoon also accords with local knowledge. In
northern Maldives P. flavescens is known as hei nakaiy
dhooni (which roughly translates as ‘October flyer’);
this name is taken from the local calendar, hei nakaiy
being the period (18–31 October) during which these
dragonflies usually first appear in numbers each year.
Maldivians consider the annual arrival of the dragonflies
to be a harbinger of the north-east monsoon; the
Maldivian saying iruvaya dhondhooni a un translates
as ‘the north-east monsoon (iruvai) is about to arrive
when the dragonflies (dhondhooni) come’. And finally,
the popular pastime of dragonfly catching traditionally
continued until 12 December, which marked the start
of steady north-east winds, and the commencement
of the kite-flying season. (Although many Maldivians
have considerable knowledge about aspects of dragonfly
biology which they can observe for themselves, I am
unaware of any traditional knowledge regarding their
origins or reproduction.)
To summarize so far, vast numbers of dragonflies
(mainly P. flavescens) appear in the Maldives from October
onwards each year. The available evidence suggests that
they are arriving across the ocean from India by flying at
altitude within the ITCZ.
Transoceanic dragonfly migration
The numbers of P. flavescens flying out over the ocean from
India every year must be in the millions: there are 1200

352 R. CHARLES ANDERSON
islands in the Maldives; observations on several islands
show that each may hold over a thousand dragonflies
after a large influx; there are many influxes each season;
more dragonflies may pass over or north of Maldives
without stopping. This accords well with Fraser (1954)
who ‘observed for many years, the annual migration west
of myriads of ...Odonata belonging to the genera Pantala
and Tramea. During the month of September they pass out
from Ceylon [Sri Lanka] and the Western Ghats of India
in a ceaseless stream of never ending millions, and none
return. Is this annual migration not comparable to the
periodical one of the lemmings of Scandinavia? ... Such
a prodigal waste of life ...’
Those P. flavescens that arrive in Maldives do not
appear to stay long. Large influxes are often followed
by days of diminishing numbers, with counts in Mal´
e
suggesting a residence half-life of just 1 d or less before
re-emigration. The reproductive status of P. flavescens
arriving in Mal´
e is unknown, but at least some are
mature: a few were seen in tandem and attempting to
oviposit on the shiny roofs of parked cars. A mix of mature
and immature individuals has previously been reported
from both the Maldives (Olsvik & H¨
am¨
al¨
ainen 1992)
and southern India (Corbet 1988) at the same season.
Emigration may follow once the lack of suitable breeding
sites becomes apparent. Whatever the reason, they are
soon gone and this raises the question, where are they
going?
The answer appears to be Africa. The dragonflies’
behaviour, with vast numbers casting themselves out
across the ocean each year, must be maladaptive, as
assumed by Fraser (1954), unless they can reach land
again. If they are able to complete the crossing they can
exploit both the monsoon rains of India (Figure 3a), and
the ITCZ rains of eastern and southern Africa (Figures 3b
and 3c), to breed in ephemeral water bodies (free of long-
lived predators like fish, but rich in food such as mosquito
larvae). To capitalize on such transient opportunities
these dragonflies have remarkably brief larval lives, and
complete several generations each year. In the case of
P. flavescens, naiads can complete their development in
as little as 38–43 d (Kumar 1984, Suhling et al. 2004).
Larval development must be rapid because rainwater
pools might dry out at any time; it can be rapid both
because temporary tropical pools are usually warm, and
because the absence of predators frees the naiads from
the typical odonate hide-and-ambush strategy, allowing
them to hunt more actively (Johansson & Suhling 2004,
Suhling et al. 2004). Longevity of the adults is unknown
because they disappear soon after emergence, but it has
been estimated that P. flavescens might complete four or
five generations each year (Corbet 1984, 2004; Corbet
et al. 2006).
As noted above, P. flavescens migrate southwards in
large numbers within southern India each September–
October (Fraser 1924, 1936; Larsen 1987). As shown
here, dragonflies first arrive in the Maldives in October.
They appear in the granitic Seychelles (in 4◦Sand
some 2700 km from India) in November (Bowler 2003)
and Aldabra (9◦S, 3800 km from India) in December
(Campion 1913) (Figure 4). These timings reflect the
slow southward migration of the ITCZ (Waliser &
Gautier 1993), not the much faster westward passage
of individual dragonflies, which travel with the north-
east monsoon winds behind (i.e. north of) the ITCZ. As
was the case with dragonflies not reaching Mal´
e until
October, P. flavescens does not reach Aldabra in numbers
until December because the ITCZ has not migrated south
to that latitude until December. However, the dragonflies
that do arrive then may have taken only a few days to
make their crossing from India. Dragonflies from India
might reach East Africa in more northerly latitudes as
early as September.
Pantala flavescens can make flights of up to 4000 km
(Corbet 1979), which is the distance from India to Kenya.
Large numbers of P. flavescens (and of several other
dragonfly species listed in Table 1) do appear in eastern
Africa from September onwards (Corbet 1962, 1984;
Pinhey 1961). It has been assumed that all are intra-
continental migrants. The data presented here suggest
that some may be inter-continental migrants arriving
from India.
Any dragonflies flying in the elevated land plume
at about 1000–2500 m would be transported from
India not just towards Maldives, but on towards Africa
(Lelieveld et al. 2001, Raman et al. 2002). The ITCZ
is particularly complex over the Indian Ocean, but a
pronounced convergence zone, marked by a line of
clouds, develops between Maldives and Somalia each
year during November–December (Sato et al. 2007).
Dragonflies carried along this route from southern India
to southern Somalia would need to make an ocean
crossing of about 3500 km. With tailwinds of about
10 m s−1, dragonflies, perhaps combining gliding or
soaring with flight at minimum power velocity, might
make the crossing in about 4 d.
Pantala flavescens and several of the other species listed
in Table 1 are known gliders and have much enlarged
bases to their hind wings, which is a feature associated
with gliding (Corbet 1962, 2004). To make such an ocean
crossing it is likely that energy consumption must be kept
to a minimum, and that soaring (i.e. gliding in air that
is rising faster than the dragonfly’s rate of descent due
to gravity) plays an important role. It seems particularly
likely that soaring occurs within the rising air of the ITCZ
itself.
In addition to enjoying low-cost flight across the ocean,
the dragonflies might also be able to feed en route.
Micro-insects appear in abundance at Aldabra with the
dragonflies (and the rain), in December (Frith 1979).

ITCZ migrations 353
Figure 3. Seasonality of rainfall in three areas around the western Indian Ocean. Western India (a). East central Africa (b). South-eastern Africa
(c). For each area average monthly rainfall is estimated from four stations, the locations of which are marked with triangles, squares and circles
respectively in Figure 4.
They are likely carried there, presumably involuntarily,
by the same winds, and perhaps also concentrated
within the ITCZ. For the larger dragonfly species,
the smaller dragonflies themselves might provide food:
Anax ephippiger (Burmeister) is known to prey upon P.
flavescens during joint migration in the Gambia (Corbet
2004).
Dragonflies do sometimes occur far out at sea, well
beyond coastal waters (Corbet 2004), and transoceanic
dispersal may explain the biogeography of some taxa
(Dijkstra 2007). However, it has been assumed that in
most such cases the dragonflies were transported by
weather fronts, storms or other dramatic or transient
meteorological phenomena. This is the first report of what
appears to be a regular, transoceanic dragonfly migration.
Transoceanic bird migration
In contrast to the dragonflies, it has long been known
that several bird species (including those listed in Table 1)
migrate across the western Indian Ocean from India
to East Africa during the boreal autumn (Ali & Ripley
1987, Clement & Holman 2001, Moreau 1938, 1972).

354 R. CHARLES ANDERSON
Figure 4. Location map. Months indicate first arrival dates of large
numbers of Pantala flavescens at four locations across the western Indian
Ocean. The arrow gives a schematic indication of the proposed passage
of P. flavescens across the western Indian Ocean in October–November.
Rainfall stations (as used in Figure 3) are marked with triangles (western
India), squares (east central Africa) and circles (south-eastern Africa).
Scale: 10◦latitude =600 nautical miles =1110 km.
Many of these birds appear to make landfall not in
northern Somalia, the nearest point to India, but further
south in southern Somalia, Kenya or even Tanzania,
an over-water distance of about 3500–4000 km. Why
and how these birds make such extraordinary migrations
is unknown. That of the Amur falcon Falco amurensis
is of particular interest (Bildstein 2006) because its
migration is so long, arduous and counter-intuitive
(between the Russian Far East and southern Africa);
because, uniquely, the entire species makes this ocean
crossing; and because parts of its migratory circuit remain
unconfirmed.Itsmigrationis still considered ‘...oneofthe
great ornithological mysteries of the present day’ (Naoroji
2006).
Common features of these birds are that their crossings
peak in November–December (Figure 1c), and that they
travel at high altitudes (Clement & Holman 2001, Moreau
1938, 1972; Satheesan 1990). These coincidences in
timing and altitude of bird and dragonfly migration
presumably allow the birds to take advantage of the same
tail winds as the dragonflies. In addition, the birds listed in
Table1areall medium-sized speciesthatareknown to feed
on large insects, with some specializing on dragonflies.
This, I suggest, is no coincidence, with at least some
of the birds taking advantage of the dragonflies for in-
flight refuelling. Finally, all of these birds are from eastern
populationsofspecies(or in thecaseof the Amurfalcon,an
eastern sibling species, and with the apparent exception
of lesser cuckoo) that also occur more widely further west,
from where they migrate more directly to southern Africa.
One interpretation is that, historically, as the breeding
ranges of these species expanded eastwards, those eastern
birds that found themselves in India in autumn were pre-
adapted to head for southern Africa. The serendipitous
occurrence of suitable winds and food supplies allowed
them to do so directly across the ocean without making
the longer and potentially more hazardous journey via
Arabia.
Return migration
A second, small influx of dragonflies to the Maldives
occurs over just a few days each May (Figure 1a). These
dragonflies (over 99% P. flavescens) seem most likely to
have arrived from Africa. Their appearance coincided in
two out of three years (Table 2) with a period of strong
westerly winds and rain, associated with the onset of the
south-west monsoon and the westerly Somali Jet. It is
usually only during May that this low-level jet passes over
both East Africa and the Maldives. The collection of an
African black emperor dragonfly (Anax tristis Hagen) in
the Maldives in May (Blackman & Pinhey 1967) supports
the African origin of these dragonflies.
The arrival of small numbers of dragonflies in the
Maldives out of Africa in May is of interest. But more
significant is the likelihood that during June–July the
Somali Jet provides a means for ITCZ dragonflies to
return from Africa to India, and complete a multivoltine
migratory circuit. Large numbers of P. flavescens do
suddenly appear in India at this time (Corbet 1998, 2004).
Again it seems to have been assumed that these are intra-
continental migrants, but at least some are likely to be of
inter-continental origin.
Multivoltine, ITCZ-associated migrations within Africa
have already been suggested for P. flavescens (Corbet
1962, 2003, 2004). Corbet (1962) noted that
P. flavescens,andalsoTramea basilaris (Beauvais),
appeared in numbers in equatorial Uganda only twice
annually, in March or April and again in September or
October, these timings coinciding with the passage of the
ITCZ. Further south, in Tanzania and Mozambique these
Table 2. Dates of main occurrence of Pantala
flavescens in Mal´
e during April–June, and of passage
of the Somali Jet.
Pantala flavescens present Somali Jet passage
18–30 May 2003 Not clear
8–14 May 2004 2–9 May 2004
9–11 May 2007 4–12 May 2007

ITCZ migrations 355
species appeared in large numbers in December–January,
againcoincidingwiththe arrival oftheITCZ.Other reports
are compatible with the hypothesis that Pantala flavescens
is particularly abundant in different African locations
during the ITCZ rainy seasons (Gambles 1960, Pinhey
1951, 1961, 1976; Samways & Caldwell 1989).
For those P. flavescens that appear in the Maldives from
October onwards each year, the alternating monsoons,
combined with the movements of the ITCZ, provide an
opportunity for an intercontinental migratory circuit that
could potentially include breeding in: the ITCZ short
rains of equatorial East Africa in October–November;
the ITCZ summer rains of southern Africa in December–
February; and the ITCZ long rains of East Africa in
March–May; before returning to India with the ITCZ and
Somali Jet to breed in the south-west monsoon rains in
June–July (Kumar 1984). This hypothetical migratory
circuit, completed in four generations, would cover a total
distance of about 14 000–18 000 km.
More generally, as intimated by Corbet (2003), vast
populations of P. flavescens must be continually on
the move, with successive generations forever chasing
the monsoon, forever trailing the ITCZ on its endless
seasonal excursions. But the Indian subcontinent does not
extend into the southern hemisphere. And so, although
P. flavescens successfully exploits the vast expanse of
the monsoon-soaked subcontinent for reproduction, the
generation that emerges in India must emigrate to
another continental area where ITCZ rains are falling.
Some of their descendants may return with the next
south-west monsoon. In other words, the life cycle of
P. flavescens requires a transoceanic emigration from
India, and an intercontinental migratory circuit is made
possible by the region’s seasonally alternating winds and
rains.
The monarch butterfly Danaus plexippus (Linnaeus) of
eastern North America (Brower 1995, Urquart 1987)
is often cited as having the longest regularly repeated
migration of any insect. The migration of this butterfly is
indeed impressive, covering over 7000 km in an annual
circuit that stretches from Mexico to southern Canada,
and takes an average of four generations to complete.
While the dragonflies discussed here cannot match the
spectacular over-wintering aggregations of the monarch,
their migrations appear even more extraordinary. In
particular, the migratory circuit proposed here for
P. flavescens, if correct, is twice as long as that of the
monarch and includes the longest regular ocean crossing
known for any insect migrant.
Relatively few birds make the return boreal spring
migration to India across the western Indian Ocean
(Figure 1c). The winds, and associated dragonfly cargo,
do not favour a passage to India until June, which is too
late for most birds. Instead most migrate up through East
Africa and across Arabia (Ali & Ripley 1987, Moreau
1972), mainly in March and April. An exception is the
pied cuckoo Clamator jacobinus which famously arrives in
India with the rains of the south-west monsoon in June
(Ali & Ripley 1987, Whistler & Kinnear 1934), apparently
directly across the Arabian Sea from Somalia, and which,
as a brood parasite of other birds, does not need to return
earlier.
Unanswered questions
It must be emphasized that while the available evidence
does support the various hypotheses developed here,
much of that evidence is circumstantial. For example,
there is as yet no proof of Indian dragonflies arriving in
East Africa, far less of migrating birds eating dragonflies
while at altitude over the western Indian Ocean.
Practical approaches that might test some of
these hypotheses and yield further insights include:
identification of Indian dragonfly vagrants in East Africa
(and vice versa) at the appropriate times; regular
recording of migrant dragonfly arrival dates, mass
movements, numbers and breeding activity at different
locations on the proposed migration routes; and trace
element or stable isotope analysis (May & Matthews 2008,
Rubenstein & Hobson 2004). Whatever the truth of the
matter, the annual arrival of vast numbers of migratory
dragonflies in the Maldives offers the opportunity for
research into many poorly understood aspects of the
biology of animals that have previously been regarded as
difficult to study, because their appearances are usually
so irregular (Corbet 2004, Holland et al. 2006).
In addition to the lack of direct evidence, the discussion
here raises a number of questions which cannot be
answered at this stage. One argument presented in
favour of P. flavescens completing the crossing from India
to East Africa is that there should be strong selection
against genotypes that are carried out to sea in large
numbers but fail to cross. This raises questions about other
species which participate in the migration, albeit in much
smaller numbers than P. flavescens, but which may not
reach Africa. For example, Anax guttatus (Burmeister)
and Diplacodes trivialis (Rambur) are known from the
Indian subcontinent, Maldives and Seychelles (Blackman
& Pinhey 1967, Bowler 2003) but apparently not from
Africa; are those that arrive in the Maldives genuine
waifs?
It should also be mentioned that this study deals
mostly with dragonflies that appear to be emigrating
from India at the end of the south-west monsoon to
avoid the dry north-east season. However, the north-
east monsoon does bring rain in October–December to
a limited area of south-eastern India and eastern Sri
Lanka. And P. flavescens does occur there in large numbers
through into January (Corbet 1988). How these animals

356 R. CHARLES ANDERSON
fit into the migration scenario presented here is at present
uncertain.
There are questions too regarding bird migrants. In
addition to the species listed in Table 1, some smaller
bird species also appear to make the crossing from
India. The most commonly recorded are barn swallow
Hirundo rustica and tree pipit Anthus trivialis. They too
are insectivores and they too appear most frequently in
both the Seychelles and the Maldives in November. This
suggests that they too may travel with the same winds
as the larger birds and the dragonflies, although much
about their migrations remains unknown.
CONCLUSIONS
To summarize, the hypotheses developed here, with the
best-supported first and most speculative last, are: that
the dragonflies (predominantly Pantala flavescens) which
appear in Maldives every year from October onwards
arrive from India; that these dragonflies fly at altitude
following the ITCZ and using advantageous tailwinds;
that many complete the ocean crossing from India to East
Africa; that there is a return crossing (of a subsequent
generation) in small numbers to Maldives in May and in
large numbers to India in June–July; and taken together,
that, as part of a wider web of migratory movements,
there is an annual migratory circuit of P. flavescens
across the Indian subcontinent and eastern Africa,
involving perhaps four generations, requiring two ocean
crossings and covering something of the order of 14 000–
18 000 km. Also, for birds, that those species which
cross the western Indian Ocean at the same time as the
dragonflies do so at the same altitude to take advantage
of the same tailwinds and (in some cases at least) to feed
on insects en route.
Despite the uncertainties regarding these hypotheses,
the annual appearance of millions of dragonflies in the
Maldives is in itself a quite astonishing phenomenon. The
likelihood that these dragonflies are making a regular,
seasonal transoceanic migration (a feat previously
unknown for any insect), and the additional possibility
that P. flavescens is completing a regular migratory circuit
twice as long as any previously recorded for any insect,
should excite further interest in these extraordinary
animals.
ACKNOWLEDGEMENTS
I am most grateful to numerous colleagues and
friends in Maldives and India who assisted with data
collection, especially Mohamed Shiham Adam, Mohamed
Ahusan, Robert Anderson, Susan Anderson, Ahmed
Hafiz, Francy Kakkaserry, Donna Toon, Lucy Whitestone,
Zaha Waheed and others. Abdulla Algeen and Abdul
Muhusin Ramiz of the Maldives Meteorological Service,
Captain Richard Smith and Nigel Collar provided helpful
information and comments. Mohamed Shiham Adam
produced the maps. Adrian Skerrett of the Seychelles Bird
Records Committee, and Frank Ward of the Royal Naval
Birdwatching Society provided copies of their databases.
Radiosonde data were collected by the JAMSTEC MISMO
programme. K-D Dijkstra, Mike May, Edward Munn, John
Phillips and an anonymous referee reviewed drafts of this
paper and provided valuable criticism.
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