Form and Function

     The Papilio xuthus is a medium-sized butterfly, around four and a half inches in diameter on average. It has cream, white, and yellow forms in both sexes, and often has two black postdiscal spots on its hind wing. Often, small patches of a darker yellow can be seen near the end of the hind wings (Miller and Miller 2004). The mature caterpillar is light green in color, with small white patches on its inferior surface. In some cases, there is a black band running across the area that separates the head segment from the body (Miller and Miller 2004).
    All butterflies are endopterygotes, which means that their body structure completely changes its morphology at least once during their lifetime (Smart 1985). The butterfly life cycle is a continuous circle. Much like the age-old question, “which came first, the chicken or the egg?”, it is debatable whether the butterfly cycle begins with the egg, the caterpillar, or the adult butterfly (Schappert 2000). However, the beginning is often told as the adult female butterfly laying a fertilized egg. The egg develops into a caterpillar and the caterpillar matures into a Superior view of P. xuthus butterfllybutterfly (Schappert 2000).
    Butterflies encounter many obstacles to survival, including avoiding predation. Organisms of all kinds have evolved significantly by natural selection and developed interesting mechanisms to aid their survival (Kunte 2009). In the genus Papilio, the distribution of form and function of mimicry is incredibly complex. In some cases, closely genetically related species have very different styles of mimicry (Kunte 2009). The P. xuthus has evolved two unique mimicking traits that aid its survival. First, as young caterpillars emerge from their eggs, they appear small and black and white in color. The young caterpillar resembles bird feces that have fallen onto a leaf (Marshall 2006). This form of Batesian mimicry is also utilized by other small insects such as beetles and spiders (Preston-Mafham et  al. 1999). The organism mimics a model species and is successful because the predators that hunt them mistake them for a dangerous or unappetizing species and are more likely to leave them alone (Smart 1985). Because bird droppings are typicallyPapilio xuthus with osmeterium ejected versus not ejected relatively small in size, this strategy will only work while the caterpillar is also small in size (Preston-Mafham et al. 1999). As it grows, the caterpillar develops another unique form of mimicry to ward off predators.
    The second structure that aids in a P. xuthus’ survival is a forked structure that is ejected from a slit on the first thoracic segment behind the caterpillar’s head called an osmeterium.  It is bright orange in color and takes on the appearance of a snake’s forked tongue (Wagner 2005). It is capable of emitting a strong smelling liquid, terpenee, which is foul enough to deter predators. It may also scare them by its sudden appearance and bright color (Marshall 2006).


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