Word of the day: Tubercle
(Who am I kidding, these words aren’t random. I pick words that relate to Lepidoptera – from the Torre-Bueno Glossary of Entomology)
Tubercle, a small knoblike or rounded protuberance… in caterpillars, body structures, sometimes bearing setae, e.g., pinaculum, verruca.
Bonus word! Tuberculiform, shaped like a pimple or tubercle.
Caterpillars usually aren’t just hairy all over – the primary and secondary hairs, or setae, are arranged in an organized fashion that is mostly conserved throughout Lepidoptera. Chaetotaxy, or the arrangement of setae, can be important for distinguishing between families or even species. The length of hairs and their relative positions to each other may seem subtle, but can have important phylogenetic implications.
The hairs are all named based on their position on the body; we currently use a system devised by Hinton (1946). For example there are usually two hairs toward the dorsal (upper) side of the segment, so they are called D1 and D2. These may arise from a small or unnoticeable protrusion, to a large conspicuous wart. This is where terminology gets tricky. Many scientists have their own preferred names for these structures, and their own cut-off points for when to use which word. They are usually describing the same structure, it is just a matter of how large it is. In our lab we tend to use pinaculum for a small one, tubercle for medium, and wart for large. When in doubt, you can call it a tubercle.
Here is an example of a small one, in an SEM image I took of a setae (Sympistis species).