https://speculativeevolution.fandom.com/wiki/Rimerunner One of the weirdest creatures of Darwin IV in the book Expedition is the Rimerunner. This rare resident of the tundra regions hops along on one leg and occasionally swallows and spits out that orange object in front of it, connected by an uncomfortably thin bit of nerve tissue. The closest the book gives for an explanation is it's a sort of sensory organs. Atrophying or evolving I don't know. Maybe it's a way to smell and collect aerial plankton. Do you have any thoughts on it? Maybe what else that one organ could be about?
The osmeterium is a defensive organ found in all papilionid larvae, in all stages.1 The organ is situated in the prothoracic segment and can be everted when the larva feels threatened. The everted organ resembles a fleshy forked tongue (not unlike a snake tongue), and this along with the large eye-like spots on the body might be used to startle birds and small reptiles. The osmeterial organ remains inside the body in the thoracic region in an inverted position and is everted when the larva is disturbed in any way emitting a foul, disagreeable odor which serves to repel ants, small spiders and mantids. To humans, this odour is rather strong but pleasant, usually smelling like a concentrated scent of the caterpillar’s food plant and pineapple.
The osmeterium is usually orange. It is "swallowed" and then "spit out" by the larva. It is meant to be showy and also smell bad. Maybe the Rimerunner is dissuading predators or parasites with that thing?