A few days ago I found this weird thing on my Caligo memnon L4 caterpillars. When I touch them they lift their head up and project a small "inflatable" horn-like organ. It is similar to an osmeterium in papilionid caterpillars, but there is no smell. I also found the small slit on my L3 caterpillars, but there isn't any horn-like organ. Could anyone explain what this is?
Felipe Gil-T. I've got an answer on another platform from Bart Coppens: "I used GC-MS on this organ for my research in Norwhich Research Park, on caterpillars of the related butterfly Eryphanis sp. I found no significant compounds, strangely, except for palmitic acid. But that is a very common compound, I wonder if it is from the protrusion itself or contamination of the samples.. However, it has been proven that some beetle larvae use palmitic acid against ants, it is found in the fecal shields of the Chrysomelidae larvae.
There is a smell by the way but it is not very strong, it smells "dusty", like a dusty office.. More research is needed I think. I've been told the hydrolyzation of the palmitinic acid is also important to look at, but that is beyond my current level of understanding of chemistry. Maybe I will continue if I am better educated in this subject.. "
The site where this organ is means that it is called adenosma. Such glands, obviously developed mmore than once during Lepidopteran evolution, are found, at least in Yponomeutoidea, Drepanoidea, Pieridae and some other butterflies. Sometimes it it is formed as one lobe, sometimes a branched lobe, sometimes as two separate lobes. There is something on the descriptive anatomy (not chemical) - if nothing else, on the distributio of this trait among Lepidoptera in the following publication and its appendices, if this is of interest: Heikkilä et al. BMC Evolutionary Biology (2015) 15:260
DOI 10.1186/s12862-015-0520-0
Yes, most larvae of Brassolinae have an eversible prothoracic neck gland that is used for defense against ants and parasitoids.
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