Extreme sex marks – scent scales and antennae

Extreme sex patches – androconia and antennae

Something that no one had obviously noted, before I observed it with the utmost astonishment, after an inquiry from my Japanese friend Hiroshi Hara, are the very strange scales along the front edge of the hindwings. Hiroshi Hara, in our correspondence about the species, asked if there were any "sex patches" ie. "sex spots" on the Colias ponteni butterflies? Sex patches or perhaps are better described as "scent scales" or androconial patches, i.e. “specialized scales in the male that carry chemical messages to females”. These occur in many different Colias species, but not in all of them. Special scent scales for this purpose, i.e. getting the female interested and receptive to the male's approaches occur in a variety of species of butterflies and can sometimes take bizarre forms such as large hairtufts in, for example, the very beautiful Agrias-butterflies that patrol the Amazon rainforest canopy.

With the naked eye, of course, there were, as everyone easily can see, no clear normal "sex spots" as, for example in our Clouded Yellow, Colias croceus where these are about 1.5 x 3 mm in size and vary in color from completely yellow to light brown. The scented scales of our Colias butterflies are also not particularly strong but are obviously a prerequisite for a successful mating in these species. With my microscope, I started to look for the type of special androconia that are so typical of many Colias butterflies. This also applies to the species in the related genera Zerena and Nathalis.

It is also interesting to compare these two genera with the Colias genus for other reasons as well. Both Zerene and Nathalis. These have their main distribution in Central America from Southern California to Colombia. Characteristic of these genera is that they exhibit similar type of "sex spots" with very special scented wing scales that also occur in many species within the genus Colias, though not in the majority of Colias species in the USA and Canada. On the other hand, it should be noted that small androconial patches also are found on Colias vauthierii, which flies in southern South America both on the mainland as well as on Tierra del Fuego and thus also at Port Famine on the Strait of Magellan. It is well known that these scented scales on the male's hindwings are of great importance for the act of mating in these species. The foreplay of the male of Nathalis iole before mating as it approaches a female in the ground vegetation has been studied closely. Males patrol the ground in search of females. If he then finds a female but is rejected, he will probably be further involved by showing off with open vibrating wings and spreading his scent, pheromones, from the scent scales of the hindwings in the hope of attracting the female. I can very well imagine that the males of Colias ponteni behaved in a similar way when they got sight of a female. Unfortunately, we will never be able to experience this spectacle of this likely extinct butterfly. That these widespread scents also result in the oversized antennal clubs is also not a daring guess.

When the male of the syntype of Colias ponteni, which through Bert Gustafsson at the Swedish Museum of Natural History in Stockholm I graciously borrowed, had been gently softened it was possible to bring forward the forewing so that I could clearly see the front edge of the hindwing, I

immediately discovered that a lot of dark brown scales spread out over a large part of the front part of the hindwing. This part of the hindwing is completely hidden by the forewing with conventional butterfly preparation, as well as when the butterfly is in rest mode. Upon closer inspection of the wing scales in this field, I was struck directly by the similarity of these very large and clear wing-scales with those of some other Colias species´ androconial patches!

The scent scales “area” of the syntype Colias ponteni in greater enlargement. Photo Göran Sjöberg.

Obviously, the tremendous amount of scent scales in Colias ponteni certainly stands in clear correlation with the oversized antennal clubs of this particular Colias species! This also suggests that the sense of smell has been something that developed early on in this genus. Nor should it be overly daring to guess that the sense of smell in this species, based on the oversized antennal clubs and the enormous area of scent scales, has been a more primitive, or more original type, than the more "developed" scent scales in the remaining Colias genus.

There are many indications that this species among others, based on the genitalia, belong to a relatively "primitive" species within the genus Colias. Lucien Berger, who has extensively studied all the genitalia of the Colias butterflies, has just attributed Colias ponteni to a special sub-genus Palaeocolias. A highly developed sense of smell is, moreover, typical of a number of "primitive" insect species, while the eyesight through evolution seems to have evolved increasingly with "later" species.

The front edge of the hindwing of the male of Colias ponteni. Virtually the whole dark area is made up of large brown, strong scent scales or androconial “area”. Photo Göran Sjöberg

One can imagine that nature itself develops its technology in the same way that we humans develop our technology with increasingly refined structures. Just compare Bell's telephone with today's mobiles. Perhaps one might ask why the female must have such large antennal clubs if the male now produces so much fragrance (pheromones) as one can assume he does with the large quantity of scent scales. However, nothing says anything about the quality of this scent scales.

The antennal clubs of the syntype of Colias ponteni. Photo Göran Sjöberg.

I will gladly return here to my example about the telephone. When the telephone was a big piece of equipment on the wall or on a table, the headset, i.e. the receiver, was gigantic in comparison to today's small earphones. Perhaps the same was true of this unique butterfly with regard to its antennae and especially the club itself where the sense organs are located? However, a trend towards today's small scent spots in the majority of "modern" Colias species should require both competition and a fairly large population.

A small population without competition often allows more bizarre forms to survive and specialize in ways that, unfortunately, also make them extremely sensitive if the conditions suddenly change. This is something that has often happened on solitary islands. We just need to think about the Dodo, Raphus cucullatus, in Mauritius. Even in Sweden we often use the saying “Dead as a Dodo” when we want to denote something that is really extinct. Colias ponteni could well be a similar example. Or rather an example of both of these scenarios where increasingly extreme attraction structures of the male are the prerequisite for female compliance, As is found in the development in males of the Birds of Paradise found in New Guinea, in the form of bizarre feathers instead of extreme scents. In addition, these very extreme Birds of Paradise species in New Guinea also seem to be linked to small isolated populations on the main island or its neighboring islands. We can, on the other hand, on this lovely island for us butterfly-lovers find at least as exclusive delightful butterflies as the Birds of Paradise. But like the Dodo once was, these birds are also extremely vulnerable to the worst predator on our planet, ourselves. Likewise, Colias ponteni has also come to meet the same fate as the Dodo, though not in our species' meat pots but forest- and land-destruction by the “development” of the islands.

These scent scales along with the extremely simple genitalia indicate that Colias ponteni would probably be, or at least belong to, the most original species within the genus Colias. or unfortunately perhaps have survived in our time. For this, the extremely large morphological difference between the sexes, i.e. the difference in the appearance of the male and female, also speaks of this particular species. I do not think about the basic colour that often clearly distinguishes the female from the male in that the females are usually yellowish-white while the males of a lot of species are darker red-brown. No, what I think of here is the marked difference in the wing patterns. Should one see a male and a female for the first time, it would be difficult, apart from the remarkable antennal clubs, to connect the male and female to the same species. These big differences, but especially the differences between this species and all the other ninety or so species of the genus Colias, would probably also be the cause of Lucien Berger's list in 1968 of the Colias genera where he attributed Colias ponteni to the subgroup "Palaeocolias". This marked difference between the sexes may also explain why in the 1970’s, in the collection drawer at the Swedish Museum of Natural History, there was a name label above the female named "imperialis" that actually aroused my special interest for this particular species!

The primitive genitalia of the species, based on Berger's description, has over the years been commented on by a number of experts, for example Shapiro and Lamas. None seem to have examined the butterfly more closely since no one, as far as I could find, mentions anything about the species’ strange scent scales and only in some cases in passing a comment on the species' fantastic antennal clubs!

Already in my earlier article on Colias ponteni, in The Entomological Society of Gästrikland´s magazine Insectifera in 1997, I was surprised that Butler, as I mentioned above, did not write anything about the extremely thick antennal clubs which is what you first notice when you see the type specimens of Colias ponteni. I had unfortunately at that time, not fully understood that the "type specimens" of Colias imperialis already at Butler's time apparently lacked complete antennae. This can of course explain Butler's lack of comments on the thick antennal clubs, but hardly his comment on the antennae. His opinion: "the brown antennae with thin club" is strange. Of the three butterflies Butler is said to have found among various plants from King's expedition just over 40 years earlier,

i.e. in 1826 - 1827, all complete antennae are missing today. Only short fragments of the antennas remain on two of the specimens while one of the males lacks any antennae.

Which butterfly Butler refers to when he describes the antenna as "brown with thin club" I do not know. The only specimen in the British Museum's collection, which, based on available photos, seems to have retained somewhat thinner antennae, would be the female found in the Rothschild's private collection. However, the third male of BMNH has retained both antennae in their entirety, as well as a thick antennal club. However, this male, as far as I can understand, Butler had no access to in his description of Colias imperialis in 1871. This particular male originates as far as we can understand from the label, from Elwes´ collection which he donated to BMNH, i.e. the predecessor of Lord Rothschild's own museum. However, we have no further information about where, when or from whom Henry John Elwes received these two specimens.

If I have something further to comment on with reference to Butler's description, it is that it does contain a large number of words in the description of, for example, the colors of the male and female, but without giving a clear picture of how the butterflies actually look. I actually think that Wallengren's more scant description is somewhat more distinct and clear. If it were not for Wallengren's local statement, Oahu, Honolulu, Butler should definitely have reacted to Wallengren's description. The big advantage of Butler's description, however, is the image of the male. Here you really get evidence of the old saying that a picture says more than a thousand words! However, something I lack in Butler's description is a picture of the strange female. Something else that you can wonder about is that there is not a single picture in the great expensive Swedish work of all newly described insects found during Eugenie's world sailing. It should not have been particularly difficult, at least in the case of the collected butterflies, to illustrate these with some simple but descriptive drawings.