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|Swallowtail butterflies are large, colorful butterflies in the family Papilionidae, and include over 550 species. Though the majority are tropical, members of the family inhabit every continent except Antarctica. The family includes the largest butterflies in the world, the birdwing butterflies of the genus Ornithoptera.|
Swallowtails have a number of distinctive features; for example, the Papilionid caterpillar bears a repugnatorial organ called the osmeterium on its prothorax. The osmeterium normally remains hidden, but when threatened, the larva turns it outward through a transverse dorsal groove by inflating it with blood.
The forked appearance of the swallowtails' hind wings, which can be seen when the butterfly is resting with its wings spread, gave rise to the common name swallowtail. As for its formal name, Linnaeus chose Papilio for the type genus, as Papilio is Latin for 'butterfly'. For the specific epithets of the genus, Linnaeus applied the names of Greek heroes to the swallowtails. The type species: Papilio machaon honoured Machaon, one of the sons of Asclepius, mentioned in the Iliad.
As of 2005, 552 extant species have been identified, which are distributed across the tropical and temperate regions of all continents except Antarctica. Various species inhabit altitudes ranging from sea level to high mountains, as in the case of most species of Parnassius. The majority of swallowtail species and the greatest diversity in form and lifestyle are found in the tropics and subtropical regions between 20°N and 20°S, particularly Southeast Asia, and between 20°N and 40°N in East Asia. Only 12 species are found in Europe and only one species, Papilio machaon is found in the British Isles. North America has 40 species, including several tropical species and Parnassius.
The northernmost swallowtail is the Arctic Apollo (Parnassius arcticus), found in the Arctic Circle in northeastern Yakutia, at altitudes of 1500 meters above sea level. In the Himalayas, various Apollo species such as Parnassius epaphus, have been found at altitudes of 6,000 meters above sea level.
APPEARANCE / MORPHOLOGY
The detailed descriptions of morphological characteristics of the Papilionidae, as quoted in Bingham (1905) are as follows::1,2
Egg: "Dome-shaped, smooth or obscurely facetted, not as high as wide, somewhat leathery, opaque." (Doherty.)
Larva: Stout, smooth or with a series of fleshy tubercles on the dorsum : sometimes with a raised fleshy protuberance (the so-called hood or crest) on the fourth segment. The second segment has a transverse opening, out of which the larva protrudes at will and an erect, forked, glandular fleshy organ that emits a strong, penetrating, and somewhat pleasant odor.
Pupa: Variable in form but most often curved backwards. It is angulate, with the head truncate or rounded and the back of abdomen is smooth or tuberculate. It is attached by the tail, normally in a perpendicular position, and further secured by a silken girth round the middle. In Parnassius, the pupa is placed in a loose silken web between leaves.
Imago: Wings extraordinarily variable in shape. Hind wing very frequently has a tail, which may be slender, or broad and spatulate, but is always an extension of the termen at vein 4. In one genus, Armandia, the termen of the hind wing is prolonged into tails at the apices of veins 2 and 3 as well as at vein 4. Pore wing (except in the aberrant genera Parnassius and Hypermnestra) with all 12 veins present and in addition a short internal vein, that invariably terminates on the dorsal margin.
SPECIAL ADAPTATIONS AND DEFENSE
To defend against predators, swallowtail butterflies practice Batesian mimicry, a behavior in which the butterflies' appearance closely resemble that of distasteful species to ward off predators. Swallowtails differ from many animals that practice mimicry as certain species, such as the tiger swallowtail butterfly (Papilio glaucus), exhibit a female-limited polymorphism for Batesian mimicry and others, such as the Canadian tiger swallowtail (Papilio canadensis) do not display any form of mimicry.
Predators of the swallowtail butterfly include the Red-winged blackbird, Pennsylvania firefly, Five-lined skink, Green darner, Goldenrod spider, Chinese mantid, Fiery Searcher, and Striped Skunk.
Biological Basis for Polymorphisms in Mimicry :
In Papilio glaucus (Eastern Tiger swallowtail), Y-linkage determines whether the females are either wild-type (yellow and black) or melanic (dark melanin replaces the yellow background). This genetic difference stems from the fact that melanism is controlled by a single gene, which controls the level of dopamine in the organism. During development, the enzyme BAS, which assists dopamine in producing the yellow pigmentation, normally found on the wings' background, is suppressed. Without the pigmentation, the butterfly appears mostly black (the melanic form) and is a Batesian mimic of Battus philenor, the Pipevine swallowtail. There are also Papilio glaucus that are not wholly black; several possess an intermediate "sooty" color and are sensitive to temperature.
The different polymorphisms (wild-type, melanic, and the 'sooty' intermediate) depend upon the geographical distribution and abundance of its mimic, the Battus philenor, whose wing color varies depending on its geographical location. In order to be successfully confused for the B. philenor by predators, the Papilio glaucus's background wing color matches that of the B. philenor residing in the same regional area. Studies support this theory; in the southeastern United States, the relative abundance of melanic females has been found to geographically correlate with B. philenor.
Influence of Sexual Harassment in Wing Polymorphism:
Only certain subsets of swallowtail females practice mimicry. This polymorphism is seen in Papilo dardanus, the African swallowtail butterfly, whose females have three different morphs for wing color pattern: a black-and-white pattern for Batesian mimicry, a black and yellow pattern that resembles the males of the species, and a pattern with orange patches that resembles the elderly males of the species. Given that the males of the species, which do not have Batesian mimicry, are preyed upon much more frequently by predators than the females, scientists initially questioned why females would choose an andromorph wing pattern, which would seemingly lower their fitness compared to the mimicry form.
Several hypotheses for this phenomena were made, the two noteworthy being the pseudosexual selection hypothesis and the male avoidance hypothesis. In the pseudosexual hypothesis, male butterflies aggressively approached the 'male' looking females and then mellowed their behavior into sexual behavior when they were close enough to identify them as females. In the male avoidance hypothesis, female butterflies disguise themselves in an attempt to evade male harassment, as courtship can be harmful, time-consuming, and attract predators.
One study recorded male responses to females of each morphs and found that the males consistently favored the Batesian mimics, then the black-and-yellow, and then the morph with orange patches. The scientists concluded that frequency-dependent selection did lead to equal success for all three alternative strategies: the Batesian females suffered the least number of predators but their fitness was reduced the most by sexual harassment, while the other two faced lower sexual harassment but also lost fitness from predators' attacks.
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