Ent. Meddr 73, 2 – 2005 123
Colour morphs of the Ghost Moth Hepialus humuli L.
(Lepidoptera, Hepialidae) in the Faroe Islands
Janus Hansen & Jens-Kjeld Jensen
Hansen, J. & J.-K. Jensen: Colour morphs of the Ghost Moth Hepialus humuli L.
(Lepidoptera, Hepialidae) in the Faroe Islands.
Ent. Meddr 73: 123-130. Copenhagen, Denmark, 2005. ISSN 0013-8851.
Over a period of 10 years, 2,435 male Ghost Moths Hepialus humuli thulensis
from the Faroe Islands were captured for a study of male colour morphs. The
pattern of local variation found in the Faroe Islands supports the hypothesis
that cryptic colouration is an adaptation to predation pressure from birds.
Janus Hansen, The Faroese Museum of Natural History, FO-100 Tórshavn,
Faroe Islands.
Jens-Kjeld Jensen, FO-270 Nólsoy, Faroe Islands.
Introduction
The Ghost Moth Hepialus humuli L. is a common species widely distributed throughout
Europe (Karsholt & Razowski, 1996). The Hepialidae is a primitive family of Lepidop-
tera. In mainland Europe the male upper wing has a bright glossy white colour, while
the female is of a less conspicuous yellow to brown colour. The female wingspan (60-75
mm) is larger than that of males (45-60 mm) as is common in the Lepidoptera. The
male’s hind legs are set with large bushy hairs (fig. 1), from which a pheromone can
be excreted that attracts females by a smell similar to that of Wild Carrot (Daucus carota
L.) (Hoffmeyer, 1974; Skinner, 1984). At twilight, the pale male Ghost Moths will hover
over the grass to attract females. After having mated, the female will fly over the grass
and simply drop the eggs to the ground (Langer, 1957). The name Ghost Moth refers
to this distinctive behaviour. In the Faroese language the moths are known as ‘Hulda’,
which refers to mythological creatures that usually remain unseen.
Many Lepidoptera in the Shetland Islands are known to vary from the UK mainland
forms, and are almost always darker (Ford, 1976). In the Ghost Moth subspecies of the
Shetlands and Faroe Islands H. h. thulensis Newman, the male varies in colour from bright
white to dark grey (Cockayne, 1955; Dahl, 1954; Skinner, 1984; Johnston, 1999) (fig. 2).
H .h. thulensis is not known to occur in the Orkney Islands (Ford, 1976). According to
Wolff (1970) the Faroe Ghost Moths could be classified as a separate subspecies H. h.
faeroensis Dahl, however, thulensis is generally accepted.
In the Faroe Islands Jensen (1996) noted an apparent heterogeneous distribution of
different colour morphs, and suggested an association with bird predation. Predation
of Lepidoptera by birds has been summarised by Braby (1994) and he concluded that
capture rates seemed rather low. However, the bird species he describes catching butter-
flies are mostly small passerines. Dahl (1954) describes a flock of terns in Eiði, Eysturoy,
preying on Ghost Moths. He also collected anecdotal information that this is a common
occurrence and indicated that this might have influence on the colouration of the moths.
Furthermore he separated 62 collected male Ghost Moths into six colour morphs.
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Ent. Meddr 73, 2 – 2005124
When catching moths for this study we observed that terns and gulls started hunting
the moths as soon as they made their appearance in the twilight. Ford (1976) notes that
it has been shown that some birds specifically hunt for certain forms of moths, and claims
that this is the reason for some very common species to be variable.
We hypothesize that colour polymorphism in male Ghost Moths is subject to two op-
posing selective forces. One is predation by birds resulting in a more cryptic colouration,
the other is sexual selection in mate attraction causing a more conspicuous coloration
advertising presence.
Material and Methods
The material for this study comprises 2,435 male Ghost Moths from the Faroe Islands
captured in the period from 1993 to 2003. The majority of the moths was caught with
hand-held butterfly nets of approximately 40 cm in diameter. Most were caught between
15 June and 20 July with much help from local people. Sampling was conducted during
dusk in grasslands on many different locations (table 1; fig. 3). Only the moths from
Mykines, and part of the moths on Nólsoy (163) were captured using a 250 W Mercury
Vapour light-trap.
Fig. 1. Hind legs of male (top) and female (bottom) Faroese Ghost Moths.
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Ent. Meddr 73, 2 – 2005 125
The moths were usually sexed visually while sampling and the females were released
again. Sexing was primarily by size of the large bushy hairs on the male hind legs (fig.
1). Colouration of the sexes can be very similar and is not 100% reliable. Furthermore
the colouration of females can also be quite variable (fig. 4).
The sampled males were killed by freezing or with 25% ammonia, after which they
were pinned and dried. Then they were identified and sexed once more by their bushy
back legs, and as soon as they were dried, before any discolouration, they were classified
to one of the following five colour morphs: LL (double light), L (light), LD (intermedi-
ate), D (dark) and DD (double dark). Colour morphs are illustrated in fig. 2 and were
inspired by Jensen (1996) and Van Franeker and Wattel (1982).
The Ghost Moth material from this study has been deposited in the collection of the
Museum of Natural History, Tórshavn, Faroe Islands.
Potential differences in capture rates of different morphs by light traps or by hand
were evaluated by a Chi-squared test of association using 163 light-trapped and 114
hand-caught moths caught during June and July in 2001 on Nólsoy.
Figure 2. Colour morphs of male Faroese Ghost Moths
H. h. thulensis. From top to bottom: LL, L, LD, D, DD.
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Locations close to each other were grouped to the same area, with up to 7 locations per
area (table 1; fig. 3).
A brief overview of potential predatory birds closest to the moth sampling location
was assembled. Birds considered to be predatory, based on our own observations and
anecdotal evidence from Dahl (1954) are primarily Arctic Tern Sterna paradisaea, but
also species such as Common Gull Larus canus, Black-headed Gull Larus ridibundus,
Herring Gull Larus argentatus and Lesser Black-backed Gull Larus fuscus. Predation was
classified as ‘high’ when birds from nearby colonies were known to hunt at or close by
the Ghost Moth sampling location. Where no predation was known to occur, the pre-
dation was classified as ‘low’. A Chi-squared test of association was conducted between
colour morphs and predation level. Locations with no bird predation data (table 1)
were excluded from this analysis.
Statistical tests of whether the colour of the Ghost Moths varied within or between
years were not conducted, but bar charts are available (figs 5 and 6).
Fig. 3. Ratio of Ghost Moth morphs in each area, including both light-trapped and hand-netted
moths. The numbers correspond to localities listed in table 1.
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Results
There was a highly significant difference in colour morphs of moths captured by the
net-sampling method and the light-trap method at the same location and during the
same period (Nólsoy, June-July 2001; χ2=10.468; df=4; N=277; p<0.05), with higher abun-
dance of darker morphs among the moths caught by nets (table 1). Further analyses
were therefore restricted to our large samples of netted moths and we excluded the
light-trapped moths from Mykines and Nólsoy.
With all locations pooled there was a highly significant difference in the ratio of colour
morphs between ‘high’ and ‘low’ bird predation samples (χ2=93; df=4; N=1788; p<0.001)
(fig. 7). If there are geographical variations due to factors other than bird predation,
then different sample sizes could influence this result.
There was no discernible pattern in the variation of the morph ratio between years
(fig. 5). However, there appeared to be a trend of diminishing bright morphs from week
26 to 29 and rising again in week 30 (fig. 6).
Discussion
Overall the DD (darkest) and LL (lightest) morphs are found in the Faroe Islands in a
similar proportion of the population. The LL form is more widespread, but this is not
visible in overall numbers because at some locations, most notably in the Sandoy area,
there are some relatively high frequencies of DD. Remarkably the LL and DD morphs
hardly ever occur in the same area, local populations ranging either from LL to D or
from L to DD. The most prevalent overall morph was the D type, which accounts for
41% of all caught individuals but varies in abundance between 9% to 67% depending
on the area (fig. 3).
Fig. 4. Two female Faroese Ghost Moths.
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Ent. Meddr 73, 2 – 2005128
Fig. 5. Morph ratios of nine years of sampling, summed pr. year from Nólsoy.
Fig. 6. Morph ratios of five weeks of sampling in Nólsoy in 2001.
Fig. 7. The effects of high and low predation by birds on colour morphs of Ghost Moths in the
Faroe Islands.
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When looking at the geographical distribution on the map (fig. 3), the northernmost
areas, Fugloy, Norðuroyggjar, Sundalagið and Skálafjørður (and Sumba), show a fairly
equal distribution of morph ratio distributed around LD. The more centred islands,
Vágar, Nólsoy and Sandoy, appear to have a higher frequency of DD and D morphs than
other areas. Stóra Dímun has a strikingly high frequency of L and LL morphs and has
low bird predation.
If it is assumed that light trapping results in an unbiased ratio of colour morphs in
samples, then hand catching is biased towards darker morphs, i.e. the dark individuals
are more often hand caught, than the light. This is perhaps unexpected and does not
appear to conform with the hypothesis that more conspicuous forms are caught more
often by predators. If the hypothesis is true, and birds do take more of the brighter
morphs, then this discrepancy in the ratio between hand-caught and those taken by
birds, may be due to the different viewpoint of the two ‘predators’. While most of the
Ghost Moths caught by humans are seen from the same angle as the birds do, i.e. with
the ground as background, several are seen with the sky as background. This may pro-
duce the observed bias.
Location Morphs
Area Nr. Name N LL,L,LD,D,DD Bird predation
Fugloy 1 Hattarvík 185 1, 85, 74, 25, 0 High2 Svínoy 62 5, 9, 22, 26, 0 No data
Norðuroyggjar
3 Depil 106 3, 54, 42, 7, 0 High
4 Kunoy 14 1, 2, 5, 6, 0 No data
5 Norðtoftir 55 4, 13, 22, 16, 0 High
6 Hvannasund 128 11, 63, 54, 0, 0 High
7 Fossanes 25 3, 6, 14, 2, 0 High
8 Klaksvík 17 0, 7, 9, 1, 0 Low
9 Viðareiði 19 0, 7, 7, 5, 0 High
Sundalagið 10 Nesvík 14 1, 3, 8, 2, 0 High11 Norðskála 43 0, 8, 28, 7, 0 High
Skálafjørður
12 Innan Glyvur 3 0, 0, 1, 2, 0 High
13 Lamba 39 2, 15, 13, 9, 0 High
14 Runavík 28 0, 6, 14, 8, 0 Low
Nólsoy 15 Nólsoy
668 8,74,83,476,28
Higha 163 4,32,32,84,11
b 114 0,11,18,74,11
Tórshavn
16 Hoyvík 79 2, 4, 41, 32, 0 High
17 Tórshavn 14 0, 5, 7, 2, 0 No data
18 Argir 171 2, 25, 88, 56, 0 No data
Vágar
19 Leitisvatn 8 0, 0 , 2, 5, 1 High
20 Sørvágur 98 1, 24, 23, 49, 1 Low
21 Sandavágur 48 0, 18, 14, 15, 1 High
Mykines 22 Mykines 26 0, 4, 7, 14, 1 No data
Sandoy 23 Sandur 26 0, 1, 7, 13, 5 No data24 Skálavík 39 0, 3, 4, 28, 4 No data
Stóra Dímun 25 Stóra Dímun 162 22, 106, 19, 15, 0 Low
Sumba 26 Sumba 62 2, 17, 26, 16, 1 High27 Akraberg 18 2, 7, 4, 5, 0 No data
Total 2,435
Table 1. Number of Ghost Moths and morphs caught and degree of predation in the respective
locations. The numbers for ‘Nólsoy a’ and ‘Mykines’ are light-trapping data, which are excluded
from certain analysis (see text). Data used to test light-trapped against hand-caught morphs are
‘Nólsoy a’ (light-trapped) and ‘Nólsoy b’ (hand-caught).
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The colour morph ratio differed between the areas examined. As we are looking at
small populations and island situations, differences might be related to a founder-effect.
However, considering the small distances between some of our areas, we consider founder
effects to be an unlikely explanation for our observations. So we suggest that there are
other mechanisms that drive the differentiation of colour morphs within a given area.
There is a significant difference between locations with high bird predation and low
bird predation, with respect to ratio of colour morphs (fig. 7). High predation pressure
by birds is correlated with an increased proportion of dark morphs. This is in line with
the hypothesis that birds would select the lighter morphs that are more conspicuous to
them. Such a selection pressure from birds would reduce the proportion of lighter Ghost
Moth in the population. In the absence of bird predation, and especially on smaller
isolated locations (e.g. Stóra Dímun) one would expect that the selection pressure is on
the male moths to be as conspicuous as possible for the females to see them.
It seems that the ratio of morphs varies considerably between the years (fig. 5), but
without a discernible pattern. On the other hand there seems to be a pattern in the
variation of morph ratio between the weeks (fig. 6). This requires further sampling and
should be conducted while including other factors, which may influence the morph
ratio, such as bird predation and weather conditions.
Acknowledgements
S. Andreasen, Dorete Bloch, Tóta Árnadóttir, Kinna úr Dímun, Esbern í Eyðansstovu,
Annika Gulklett, Marita Gulklett, Bergur Olsen, Aksal Poulsen, Poul Poulsen, Fríðleif
Lydersen, Petur Mortensen, Annleyg Patursson, Eyðbjørt Simonsen, Poul Johannes and
Bodil Simonsen with family, Hans Eli Sivertsen, Alf Sørensen, Jákup Sørensen and Tinna
Asaadare Tótudóttir all provided highly appreciated support during our study. Bernard
Zonfrillo, Garth N. Foster and Jan A. van Franeker kindly provided comments on the
manuscript, as did the two referees Svend Kaaber and Thomas Simonsen.
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