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Propagating Punctuation: Netting Commas Midst the Nettles

(c) copyright Christopher Majka
reprinted from The New Brunswick Naturalist
On a warm day in early spring when the last of the snows have melted and the first flowers are pushing their heads from beneath the cover of last year's leaves, even the most urgent of obligations cannot restrain me from a walk in the forest. It is so satisfying and refreshing to smell the aroma of spring in the flowing sap, the bursting buds and the trickling stream. Being an avid lepidopterist I keep my eyes peeled for the first signs of our scale-winged friends, the butterflies. I am frequently not disappointed when in a woodland glade I see the sharp, erratic flight and angular dark surfaces of what can only be a Comma. No, not an errant point of punctuation taken wing but a member of the genus Polygonia, otherwise known as the Anglewings.

Friends who know me are acquainted with the fact that such an observation inevitably induces equally erratic "stalking behavior" through which means I attempt to get close enough to the ever wary butterfly to distinguish just which of the six possible species found in this region it might be. This is a trickier task than one might think for the Question Mark (Polygonia interrogationis), Comma ( Polygonia comma), Satyr Angle Wing ( Polygonia satyrus), Green Comma ( Polygonia faunus), Gray Comma (P. progne) and Hoary Comma ( Polygonia gracilis) are remarkably similar in appearance.

They are unique in our region for along with their close relatives in the Nymphalidae (the Brush-footed butterflies) the Mourning Cloak and the Tortoise Shells (genus Nymphalis) and the Thistle Butterflies (genus Vanessa) they are the only butterflies to hibernate as adults through the winter. Also, at least four of our species, the Question Mark, Comma, Satyr Angle Wing, and Gray Comma have two broods every year (bivoltine, in ecological parlance). Overwintering adults emerge early in the spring and lay eggs which mature rapidly to produce a "summer" generation of adults which are typically darker in colour. These in turn lay eggs which mature even more quickly to produce the lighter-coloured adults which emerge in the fall and, crawling beneath a piece of bark, hibernate to re-emerge as the "spring" adults. In warmer climes at least the first three species will try and squeeze a third, or perhaps even a fourth, generation into the summer cycle. The other two species in our area, the Green and Gray Commas, have only one brood a year (and are thus called univoltine).

There are even more wrinkles to this unusual pattern. Not all the offspring of a "spring" female in a bivoltine species develop into "summer" adults. Some will skip the double-brooded cycle entirely and emerge as "spring/fall" adults ready to go into hibernation. It turns out that this is entirely an environmental variable and the ratio of spring/summer morphs is regulated by the amount of sunlight (photoperiod) and the temperature 1. Thus if it's too cold and there is too little sunshine a larger proportion of the butterflies will opt for a single brood -- a seemingly sensible response to inclement weather conditions or more northerly latitudes.

Now all these reproductive peculiarities are of considerable interest to ecologists and a paper by Swedish biologist Soren Nylin in the Danish journal Oikos reveals some interesting facts about these lovely butterflies. Entitled "Host plant specialization and seasonality in a polyphagous butterfly, Polygonia c-album (Nymphalidae)", 2 it is the focus of this issue's article. Polygonia c-album, in Britain also called the "Comma" is a common European butterfly. Since this species, like most other commas, has been reported to feed on a large number of host plants, Mr. Nylin was interested in seeing if different plants might be selected for the purposes of egg laying by the spring as opposed to the summer females. In particular since the eggs laid by the summer females have a much shorter time to mature would these females preferentially select plants on which the caterpillars would grow more quickly?

To test his hypothesis he examined butterflies from both England (where some 30-40% of the butterflies are of the summer morph) and Sweden (where the summer morph is rare and found only in certain years). He tested their preferences by placing females in cages which had two species of plants within. One was always Nettle (Urtica dioica), one of the Comma's favourite plants, and the other was one of Mountain Current (Ribes alpinum), Silver Birch (Betula verrucosa), White Birch (Betula pubescens), Hazel Nut (Corylus avellana), Goat Willow (Salix caprea), Wych Elm (Ulmus glabra) or Hops (Humulus lupulus), all known to be food plants of the Comma. By counting the number of eggs on the Nettle as compared to the other plant he was able to calculate an "index of preference." In addition he took the eggs and reared the caterpillars on all of these plant species measuring how long it took for the larvae to pupate.

The results of his experiments showed that the caterpillars grew best on Nettle, Hops and Wych Elm and most poorly on Silver Birch and Hazel Nut. Interestingly he was also able to show that, given the choice, the females much preferred to lay their eggs on those same plants on which the caterpillars grow best and were least likely to lay them on the plants on which the caterpillars did poorly. But most interestingly of all his results also showed that the summer morph females were even more strongly inclined to lay their eggs on these "good" plants than were their spring sisters. This is in keeping with the prediction that for the second summer generation it is important that the caterpillars mature quickly to avoid potential frosts and inclement weather. When one considers that on the "good" plants caterpillars matured in 21-23 days and 89-100% survived whereas on the "poor" ones it took 31-42 days and between 0-60% survived, the differences are appreciable.

Bear in mind that all this research is on the European Comma but since our four species of Angle Wings which are bivoltine are also known to feed on Hops, Nettle, Elm, Current, Birch and Willow it might be reasonable to suppose that a similar process is occurring here.

Mr. Nylin also speculates that these food preferences might be indicative of the evolution and phylogeny of the Nymphalinae, the subfamily which includes the Angle Wings, Tortoise Shells, Thistle Butterflies, Peacocks and Banded Reds as well as the closely related Map Butterflies. Many of the butterflies in this group feed principally on Hemps and Nettles or other herbaceous plants. The Tortoise Shells (genus Nymphalis), however, are curious in that they feed exclusively on trees of the Elm, Willow and Hazel families. The Angle Wings, which according to systematic studies are most closely related to the Tortoise Shells 3, are intermediate in that some of their members feed on Hops and Nettles while others feed on Elms, Willows and Hazels. On the following page is a chart which I have compiled which illustrates these feeding preferences.

Mr. Nylin points out that, "In the absence of a reliable phylogeny for ... the Nymphalinae this can however at present be only speculation." Nevertheless this is an interesting instance where the food plants of butterflies not only have interesting ecological repercussions but also may be indicative of the pathways of evolution.

Literature Cited

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