Living With Horns

First Instar Tobacco Hornworm

Caterpillars in the family Sphingidae are noted for the horn on the abdomen. Like many other insect parts, the scale of the horn relative to the body size changes as the caterpillar develops.  In the first instar tobacco hornworm larva, the length of the horn is over 25% of the body length.  In a fully developed 5th instar caterpillar, the horn is less than 10% of the body length.  The shape and the color also change.  The first instar horn is thin and black,  The fifth instar horn is orange and thick at the abdomen and tapers at the tip.  There is much speculation about the function of the horn.  Presumably it is used in defense against parasitoids.