Old blue eyes!

Dusky Diadem, Ethope himachala.  Arunachul Pradesh, May 2013.
Dusky Diadem, Ethope himachala. Arunachul Pradesh, May 2013.

The Dusky Diadem [Ethope himachala (Moore, 1857)] is a fairly common species in upland regions of northeastern India, for example in provinces such as Arunachul Pradesh, Meghalaya and Nagaland (1).  It has a Himalayan distribution which stretches from Sikkim, through NE India and Myanmar, down into northern Thailand.  It typically occupies evergreen forest, where it can be seen basking or flying along forest streams (2).  I can across this species whilst walking along a road near Sessa in Arunachul Pradesh, NE India.

Blue eyes of Dusky Diadem, Ethope himachala
Blue eyes of Dusky Diadem, Ethope himachala

Perhaps the most striking thing about this butterfly are its blue eyes.  Eye colour is variable in butterflies; they can have brown, black, green, yellow, red, blue and probably many more colours besides (together with eye patterns or spots).  Eye colour has been used to distinguish closely related butterfly species, but it can be misleading when examining museum specimens, since the eye colours fade with time after death (3).

Butterflies have very good eye-sight and are thought to see in colour (4).  They are so brightly coloured and patterned – iridescent in many species – they must be able to see in colour, although exactly how the perceive the world (and each other), it is not possible to know for sure.  It is difficult to say why a given species of butterfly has eyes of a certain colour; it may be related to a number of different factors.  The so-called screening pigments in the pigment cells are said to generally determine the eye colour in insects. In butterflies however, there is also a structure called a tapetum – composed of highly folded tracheoles, or breathing tubes – which acts as a sort of reflection filter, improving the sensitivity of the eye and determining the colour, or ‘eye shine’ (4).  So in nymphalid butterflies such as this, the blue colour is not due to the blue absorbing visual pigments (called opsins) in the eyes, but instead is caused by the colour of the light reflected by this structure which is at the back of the eye.  Nevertheless, the blue colour is very striking against the dark umber-brown wings of the insect, and it is hard to believe that it does not somehow play a part in the behaviour of the butterfly.  Perhaps males with the bluest eyes are chosen by the females?  A nice idea but pure speculation!

Dusky Diadem, Ethope himachala, resting on a leaf. Arunachul Pradesh, May 2013.
Dusky Diadem, Ethope himachala, resting on a leaf. Arunachul Pradesh, May 2013.

On a historic note, The Dusky Diadem was one of many Indian butterflies first described by a Victorian entomologist called Fredric Moore (Moore, 1857).  He worked for the British East India Company as an Assistant Curator, located in London at the East India Museum (housed in an extension to the East India House). Moore was largely responsible for a large, 10 volume work on the butterflies of the Indian region called Lepidoptera Indica (5).  In the preface to these volumes, he described the work as being produced by “contributions….from numerous friends and correspondents who held or are now holding positions in the Civil and Military service in various parts of India”.  In other words, he did not collect the butterflies himself, but rather described and catalogued the specimens sent in by a host of people scattered across British India at the time. The list of contributors included a remarkable cross-section of expatriate Englishmen (and they were all men, apart from one lady, a Mrs. F. A. de Roepstorff, although it seems that her husband – a superintendent of the Danish settlement at Camorta – collected the butterflies in the Andaman and Nicobar Islands ) employed in the service of the British Raj.  They are all listed in the preface, and include a number of Captains, a Major, a Colonel, a Major-General, a number of Doctors, a Reverend and Knight of the Realm (Sir Walter Elliot who provided specimens and notes from Madras).  It is fascinating, at least to me (!), to think of all these colonialists, engaged in collecting moths and butterflies as they went about their varied duties on behalf of Queen and country!

Dusky Diadem, Ethope himachala
Dusky Diadem, Ethope himachala

The males are slightly smaller than the females, with darker wings.  The two sexes can be compared on Plate 54 of Volume 1 of Lepidoptera Indica (5).  The volume has been digitized by the Internet Archive with funding from the  University of Illinois Urbana-Champaign (6).  It is wonderful that these old, highly valuable volumes are available for free online.  It is worth looking at for the plates alone, most of which were illustrated by Fredric Moore’s son, F. C. Moore, who was clearly a highly talented artist.  The Moore’s must have loved their butterflies, but I find it a bit sad that they did not see them flying around in the wild – I assume this was the case as there is no mention of them visiting India – and they based their vast study on pinned specimens sent back from India.  This may explain why there was no mention of this butterflies blue eyes!  There are also no blue eyes shown in the illustrations; just brown eyes which was presumably the colour of the faded specimens.  No-one had digital cameras either in 1857!  I am so glad to live in the present, with all the wonderful technology we have to hand.

1)  Anonymous. 2014. Ethope himachala Moore, 1857 – Dusky Diadem. In K. Kunte, S. Kalesh & U. Kodandaramaiah (eds.).Butterflies of India, v. 2.10. Indian Foundation for Butterflies. http://www.ifoundbutterflies.org/sp/537/Ethope-himachala

2)  Kunte, Krushnamegh, et al. “Butterflies of the Garo Hills of Meghalaya, northeastern India: their diversity and conservation.” Journal of Threatened Taxa 4.10 (2012): 2933-2992.

3)  Robbins, Robert K., and Jeffrey Glassberg. “A butterfly with olive green eyes discovered in the United States and the Neotropics (Lepidoptera, Lycaenidae, Eumaeini).” ZooKeys 305 (2013): 1.

4) Doekele G. Stavenga  (2002). Reflections on colourful ommatidia of butterfly eyes. The Journal of Experimental Biology 205, 1077–1085.

5) Moore, F. (1857). A Catalogue of the Lepidopterous Insects in the Museum of the Hon. East-India Company in Horsfield & Moore, Cat. lep. Ins. Mus. East India Coy 1: : 1-278.

6) https://archive.org/stream/lepidopteraindic01moor#page/n3/mode/2up

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