Bug of the Week Vol.8- Spicebush Swallowtail Caterpillar

29 August 2024

Creepy or cute? It’s hard to decide. With a striking resemblance to the Caterpie Pokémon, this caterpillar scientifically known as Papilio troilus is in it’s fifth developmental stage of life. Ordinarily it may be a questionable choice to focus on a critter that isn’t even fully grown, but in this case I would argue that these caterpillars are even more interesting than their full grown form as a butterfly.

Physical Description

Spicebush swallowtail caterpillar’s grow up to 5.5cm in length, with a vibrant green exterior and impressively large “eyes”. These eyespots are actually false and a pattern used to deter predators, as cute as they are. They sure fooled me! Along the back of the abdominal segments are six blue rings. Osmeterium is a defence organ that is only visible when the caterpillar is threatened, which resembles a snake tongue or horns to most. Located in their forelegs are chemoreceptors, which enable them to identify the leaf they are standing on and whether it is a good spot to lay eggs on. Their vibrant green skin transforms to yellow or orange before pupation.

Life Cycle

Spherical eggs are laid individually on the bottom of host leaves, which larvae then eat the shell of shortly after hatching, along with their moulted skin. How delectable. These first instar (first developmental stage) larvae are in their sh*ttiest phase, being a dark brown colour with white markings that literally mimics bird droppings.

See what I mean?

Feeding on leaves becomes the most important activity for a young larvae. It then grows into its prepupae phase where it stops eating, and prepares to become a chrysalis. After approximately 15 days, the adult spicebush swallowtail butterfly emerges, an entrance grander than any.

Diet

Spicebush swallowtail caterpillars have been documented as only feeding and living on Lauraceae plants. Think you know picky eaters? Think again, as based on laboratory feeding tests, a spicebush swallowtail caterpillar starved to death rather than initiating feeding on non-lauraceous hosts. I have never understood picky eaters, and honestly perhaps they deserve this same fate for not appreciating the vast cuisines and ingredients on offer in this world.

Location

Unfortunately, the spicebush swallowtail is only found across the east of the United States and southern Canada. I would encourage them to move to Australia so I can meet them, but I can’t blame them for not being so enthusiastic about this considering the current cost of living crisis.

Defence Mechanisms

These critters have a plethora of such interesting defence mechanisms which I felt deserved a subsection of its own. I honestly feel a bit bad for how much they have to go through just to reach adulthood, but at least they’ll never experience being stopped by transport officers as a broke uni student for not tapping on their Opal card.

Osmeterium

Osmeterium are bright yellow organs which protrude from behind the head, emitting a chemical repellant to fend off predators. The liquid repellant is made up of different acids, which actually changes over time in order to deter different types of predators which are more likely to be an issue in different instar phases.

Caterpie the adorable Pokemon provides a great example of osmeterium being used to repel predators, as he is seen emitting an awful stench to scare away enemies.

Leaf Shelters

As young larvae, leaf shelters are made for protection by folding leaves and using silk to hold the folded shelter together. Their resemblance to bird droppings further helps deter predators from even inspecting them on leaves.

Fake Eyespots

The bulging, anime resembling eye patterns that these caterpillars have all the way from their first to last instar are thought to be used to mimic green tree frogs and snakes. Although this would have the opposite affect on me, who would want to see them up close, it sure is an affective deterrent for birds who would rather not be eaten themselves by a snake!