Well, in fairness to the search engines, this butterfly does have a confusible name. In the tradition of naming Swallowtail butterflies after literary characters, it was named after the antiheroine of a Greek legend that caught the imaginations of both Chaucer and Shakespeare. Women tend to dismiss the character of Cressida; men seem to find her fascinating. Meh. I'm a woman and I say all anybody needs to know or remember about the literary Cressida is "Oh, Cressid! false, false Cressid! false, false, false!" though, in fairness to her, the men in the story are all pretty much unmitigated bungholes too. It's not as if she had a lover to be true to.
Then there was a pop group called Cressida that recorded a song also called "Cressida." Guys, of course. They didn't sell a lot of records and, as a result, according to Google their records are now collectors' items. Here's the song, as recorded in 1970:
One has to wade through a lot of irrelevance to get any information about the butterfly Cressida cressida, although it's well known and documented. Found in Australia, it resembles some South American Swallowtails more than it does the Asian ones, but it's really what biologists call a monotypic genus, a breed of its own. Among other quirks, the Clearwing Swallowtail has no "swallow tails" on its hind wings, but what really stands out about it is the tendency for its wings to fade from white to transparent.
Photo by Jenni Thynne. Females typically start out with only a few pale tan spots on their white wings. Soon even those fade, making the butterfly almost impossible to find against a background of sand.
Though most of what's been written about these butterflies comes from Australia, the butterflies have been found in neighboring countries including Papua New Guinea, the Moluccas, and Indonesia.
All butterflies' wings are basically transparent; all that color comes from the tiny scales that normally cover the wings. Cressida cressida, sometimes nicknamed "Big Greasy," tends to lose its scales much more easily than other butterflies do. Nature compensates; the wings are thick and sturdy enough to keep the butterflies flying. The butterflies seem careless about their wings and scales; among other things, though they often mate in the air, they like to take their time about mating and have been known to lie down on their sides in order to embrace face to face, wings flopping about on the ground. They're not particularly big butterflies, by Australian standards, with wingspans typically between two and three inches. Their size varies, but several other Australian butterflies are bigger. But the wings do have a slick, "greasy" look.
Photo by Darrell Gulin, who sells large prints suitable for framing at https://www.pinterest.com/pin/56506170327270898/. This male Clearwing probably started out with white scales on his fore wings, but has already lost them.
Most Clearwing Swallowtails never show as much color as the young males portrayed on Australian postage stamps in the 1980s and 1990s.
Though the wings lack "swallow tails," they have the same structural pattern as the other butterflies in the Swallowtail group, so Cressida is classified as a swallowtail genus. The species name repeats the genus name. Three subspecies are recognized, and the "nominate" subspecies repeats the genus name again, Cressida cressida cressida, which is getting ridiculous. The other subspecies are cassandra and insularis. Insularis is described as larger than the other subspecies. Cassandra is "the northwestern race" in Australia, with smaller spots on the forewings more nearly equal in size. Older sources also list a subspecies troilus, with smaller wings and duller colors, no longer recognized as a distinct subspecies.
This is one of the butterfly species whose distinctive scent is discernible by humans. Humans describe them as smelling sweet, "musky rose" rather than fruity. Birds seem to leave them alone. A kind of spider that lurks in flowers they visit usually ignores males and young females, but recognizes older females who have passed on most of the aristolochic acid in their bodies to their eggs, and eats them.
At first glance, the males and females of this species look so different that they were thought to be two separate species. Though both sexes vary in size, males are bigger than females. The size difference may be noticeable even in the eggs, which vary in size also. Male butterflies fly at one another as a threat display. They are one of the few butterfly species that actually fight; usually the male who claimed a perch first chases other males away, but sometimes they dive to the ground and fight. The purpose of these wrestling matches is to gain status, not to harm each other, and the outcomes of fights have been described as "inconclusive."
Once ready to mate, males fly at females in what some writers describe as a forcible attack, diving on them from above. Instead of flitting about together, what Clearwings do in the way of courtship looks like a struggle with the male trying to mate at once and the female stalling for time. Force may be part of the process since smaller males sometimes give up trying to mate with bigger, stronger females. However, females approach males, and couples spend hours or days snuggling. It is probably more accurate to describe this species' mating behavior as tactile stimulation rather than rape, although that word has occurred to observers. Reports of males seeking out females are rare.
Couples can fly while embracing face to face, a remarkable feat. When the female stops flapping and lets the male do the work, they can stay in the air like this all day. Photo by Karen Doyle.
Females sometimes show color, similar to males, and this seems to have mixed effects on their ability to find a mate. On the one hand, more color makes them more noticeable and proves that they are young. Younger butterflies produce more viable sperm and ova, so they're preferred as mates. On the other hand, too much color may confuse males and turn them off...what's a girl to do? Eventually most butterflies who survive to adulthood do seem to mate, but some find mates right away and others have to wait for days.
This male doesn't seem to have been confused by the female's "andromorphic" coloration. Photo by Dhfischer.
After mating, the female butterfly displays a sphragis, sometimes crassly described as a mating plug. The sphragis is visible as she flies and tends to discourage males from trying to mate with her. It is a scablike mass of congealed body secretions, large in proportion to the butterfly, that covers much of the hind end of the butterfly (but not the oviduct from which she lays her eggs). It seems at first glance to be left behind by the male. Its size suggests that the sphragis is actually produced by a reaction between male and female secretions. What do humans know, or want to know, about these things? If really determined, a couple of butterflies can dislodge the sphragis and mate, but males usually prefer to look for a female who has not mated before. Solvents humans use have no immediate effect on the sphragis of this species, but the butterflies produce a chemical that will break it down if they try.
While males tend to keep their territories, females may travel long distances. Camouflage helps them survive on their journeys.
Can you see the butterfly in this photo by Matthew Crosson? I had to tilt the screen to find her at first. Clicking to enlarge the photo may also help.
Eggs are like the eggs of other Troidini (Aristolochia-eating Swallowtails): little round beads with a slightly ribbed texture produced by droplets of aristolochic acid down the shells. Hatchling caterpillars eat their own shells. Sometimes more than one egg is laid in the same place; when this happens the first caterpillar to hatch is likely to eat the other egg, or eggs. At no stage in their lives do these butterflies seem to show any social instincts. However, once they have hatched and started moving about, the caterpillars don't seem cannibalistic. Only eggshells are really endangered.
Photo from butterflyhouse.com.au.
Their lives are hard. Albert George Orr observed that, of the eggs he found in a patch stocked with a high concentration of food plants, about one in five survived long enough to consume the caterpillar's original food plant, but only a small fraction of a percentage survived the search for a new food plant. The caterpillar can crawl along at its comfortable pace, less than two yards per hour, for six to twelve hours. It has no more instinct to find and follow the vine, or rhizome, to the next Aristolochia plant than it has to avoid eating its siblings' eggshells, and in any case its siblings may have already eaten the next plant. This first migration, in the second or third instar of the caterpillar's life, does a lot to thin the population, but typically the caterpillars have a longer trek ahead of them as all the nearby food plants are consumed. This comes in the fourth instar, by which time they can travel at speeds up to 22 meters per hour, travelling at a sort of "scout's pace" alternating between fast jerky sprints and pauses for rest, and have a chance of finding food 30 meters away from the plants they left. So, in a large "garden" space dedicated to rearing and studying these caterpillars, Orr reported that in three years he counted 33 caterpillars reaching that second migration stage, and 15 survived it. In really "wild" conditions the rate of survival is probably even less. And that was in the area where the micro-predators that can parasitize these caterpillars were not found.
Photo by Jessica May.
Caterpillars are recognizable as part of the Troidini "tribe," variable in size and color but showing some combination of red, black, and white colors. Like other Swallowtail caterpillars, they have osmeteria that exude a sweetish odor birds seem to find unappetizing. They don't display these "stink horns" indiscriminately; when prodded, Orr reported, they tended to contract and shiver.
In nature they don't travel together like this, but when set down side by side in a researcher's garden they leave each other alone. Photo by Samvassella.
The Clearwing is yet another butterfly species that live on plants in the genus Aristolochia. The plants they favor are relatively low-growing species, vines running along the ground rather than up into trees. Though Aristolochias grow fast, caterpillars can eat their leaves even faster, so the caterpillars sometimes strip their original host plants and have to wander about looking for fresh plants to eat. The caterpillar below still had plenty of leaf to eat when it was found by some "Friends of Yarrman Creek."
Photo by The Friends of Yarraman Creek. The caterpillar had eaten part of the seed pod and is now chewing along the wide end of the leaf.
Pupae, according to Orr, are extremely well camouflaged. The pupal shell is pale-colored, usually attached to a similarly colored dead grass stem. The duration of pupation varies, probably depending on weather conditions. Laboratory specimens, exposed to a constant temperature, can eclose in less than three weeks. Outdoors, pupae typically turn into butterflies in four weeks, but may remain in the pupal shell for up to two years.
Photo by Peter Samson, who notes that this pupa was about an inch long.
Eclosion seemed to be synchronized. Males emerge from pupation first; like many Swallowtails, males fly for a few days before they are able to mate, and spend this time hanging out with other males, competing for territories. Females eclose ready and eager to mate (they absorb some nutrients from males during the mating process), and cruise past the males' territories. (Both males and females of this species are known for flying slowly, though they can put on speed if they want to.) After mating the females spend most of their time looking for suitable host plants on which to lay eggs; they fly farther than males, perhaps actively looking for fresh DNA, and tend to have shorter lives. Compared with some Swallowtails, however, Clearwings are long-livers, typically flying for a month or more.
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