It's may be develop defense mechanism when it's touch

I think the yellow horn is to have a defense function when the larval rears up from the rear to a predator who believes that the tail end of the larva is the head end. The tail end when reared often appears like a face or an alarm to the predator and horn may be part of this appearance.

I think it should be a visual deterrent, like the harmless pincers of earwigs (Dermaptera)....althuogh it could be camouflage, to make the larvae look like a twig, as it seems to be the case for the "thorns" of Membracidae.

The case of the larvae of Trilocha should be convergent evolution...two unrelated organisms showing similar adaptations.

From my observations in French Guiana on the first instar larvae of Isognathus caricae, I think this horn can be used to stabilize young larvae because it is often long in them and very mobile when the caterpillars move.

So in other words, the caudal horn in the larvae is multifunctional!

@Trevor-

It could be, but to confirm that it would be good if some references can be shared.

@Sergio Sanchez-

Visual deterrence seems to be more likely than it helping in camouflage.

The point about convergent evolution seems interesting, thanks!

Hi

Study this link

https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/sphingidae

@Samaneh-

Thank you for the link, but it does not answer my question fully. The function of the horn is not given anywhere.

horn is to have a defense function

The horns are with multi functional jobs as mentioned by the above scientists.An addition one not mentioned before is a taxonomic morphological feature to differentiate this larva from others which lead to its identification .Salam and best regards.

My guess is either visual deterrent against predators, or a weapon against other larvae. While it is not rigid enough to sting a person, a soft-bodied caterpillar may be an easier victim. However, I have never knowingly seen it used.