Of Butterflies and Hilltops

Swallowtails and other families

Most of our largest butterflies are Swallowtails (Papilionidae), with the Cairns Birdwing (female wingspan to 150mm) and Ulysses (108mm) notable amongst them, but we also have smaller Swallowtails such as the Blue, Pale and Green-spotted Triangles (Graphium spp.) between 57 and 65mm. Most of the Nymphs (Nymphalidae) – Crows, Soldiers, Tigers, etc – are about this size, with wingspans between 50 and 65mm. Many of the Whites and Yellows (Pieridae) – Migrants, Jezebels and Albatrosses, for instance – are in the same range, too, while the others are all smaller and Skippers (Hesperiidae) and Blues (Lycaenidae) are smaller still. (Links on Latin names take you to collections of my photos on flickr.)

Swallowtails are named for the ‘tails’ which extend from their hind wings but not all Swallowtails have tails: Ulysses and Fuscous do, while others have mere tokenistic points instead of proper tails and the Chequered, Clearwing, Dainty and the Cairns Birdwing manage without any at all. On the other hand, many non-Swallowtails, especially Blues, do have tails.

What, then, are we to make of this handsome butterfly, with its 85mm wingspan and not one but two tails on each hind wing?

Tailed Emperor
Swallowtail or Nymph?

It is the Tailed Emperor, Polyura sempronius, one of the largest Nymphs in the Townsville region. Only the Monarch (not a native, although long naturalised) and the female Varied Eggfly are as big. It is not widespread, and not particularly common even where it is found. In fact, the only place I’m sure I have seen it is on the top of Castle Hill, where I took this photo in early March.

Hilltopping

On that occasion I walked up from the carpark to the viewing point overlooking the harbour and Cleveland Bay but was stopped on the way by a cloud of very actively hilltopping Glasswings (Acraea andromacha). I don’t know whether the half-dozen Tailed Emperors amongst them were also hilltopping or just caught up in the melee.

Hilltopping? Some species of butterfly have a habit of congregating on hilltops to meet, and compete for, mates. Males perch or patrol endlessly, asserting their fitness and desirability by controlling the highest zone they can; females approach them, mate, and leave to lay eggs on food plants which may be some distance away.

The fullest description I know is from South Africa – this pdf. Here are its key points:

Hill-topping is defined as a behaviour pattern of certain insects in which males fly to the summits of hills and when there remain on the summit and show perching (territorial) or patrolling behaviour, resulting in an unexpected abundance of males on hill-tops (Scott, 1968). Hill-topping is an important part of the mate-locating behaviour found in low density species. The reason the males go to the hill-tops is to be in a conspicuous spot for the newly hatched females to find them, otherwise the females may fly kilometres without finding a mate if they tend to be sparsely scattered. …

The males may either perch on a shrub, tree or patch of ground … or may patrol back and forth on the summit … The behaviour of hill-topping species is not fundamentally different from other species; hill-topping behaviour occurs when these activities are tranferred to a hill-top. …

It appears that hill-topping behaviour can be effective only for low density species, because at high densities on hill-tops interference between males prevents mating with females and the number of hill-tops is limited. If a species is common, only a small proportion of the males can occupy a hill- top, so that most males will be forced into non hill-top situations. As population density rises, the probability that a female will meet a male before reaching a hill-top therefore increases, so that hill-topping is less important for commoner species [and] selection should eliminate the hill-topping response. …

Hill-topping species are in general large, fast-flying, solitary species with more widely scattered and less abundant foodplants than non hill-topping species …

(Wikipedia’s article on hilltopping is more general and quite short.)

None of the Glasswings settled for a photo so I will include an older one, from Magnetic Island.

Glasswing
Glasswing

Glasswing and Clearwing

To bring my little ramble full circle, the Glasswing is a medium-sized Nymph at 53-55mm but has a larger lookalike amongst the Swallowtails, the Clearwing Swallowtail (Cressida cressida, 70-80mm). Both species have translucent forewings and black-and-white hindwings.

Clearwing Swallowtail
Clearwing Swallowtail

Alternate common names for the two of them are unflattering but do reflect the similarity: they are known, respectively, as the Little Greasy and the Big Greasy.

• This article appeared in a shorter form in Kapok, newsletter of
Coastal Dry Tropics Landcare Inc. (FB page), in April 2017.

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