Caterpillar Use of Frass

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Unusual Use for Fecal Pellets by Butterfly Caterpillars?

Caterpillars of several species of butterflies attach frass (excrement, fecal pellets) to the midrib of the leaf they are eating. Hatchling caterpillars add these pellets to the tip of the leaf or, depending on species, from the side of the leaf.
Red-Spotted Purple butterfly
Red-Spotted Purple

frass on a leaf vein
Red-Spotted Purple
butterfly fecal pellets
on a leaf vein
fecal pellets frass on a leaf vein
Microscope view
frass - excrement
on a leaf vein

Caterpillars will actually extend a leaf vein with frass or create a line of frass from the side of a leaf.
viceroy butterfly

Viceroy butterfly

Viceroy caterpillar on a leaf vein on willow
Viceroy butterfly
caterpillar on an
extended leaf vein
viceroy caterpillar on a leaf vein on willow
Poor quality photo
of Viceroy caterpillar
on extended leaf vein

Ruddy Daggerwing butterflies lay eggs on other parts of the leaf than Viceroys. As a result, the caterpillar starts eating where-ever it hatches and creates a line of frass off the side of the leaf.
ruddy daggerwing butterfly

Ruddy Daggerwing

ruddy daggerwing caterpillar larva
Ruddy Daggerwing
caterpillar resting
on a line of frass
ruddy daggerwing caterpillar
Badly out of focus
photo of Ruddy caterpillar
on a fecal leaf extension
Other species of butterfly caterpillars will also create leaf extensions with their frass. The Florida Purplewing and Goatweed Leafwing are two other species that exhibit the same habit.

Although it is theorized that this behavior is to discourage predators, some caterpillars exhibit the opposite action. Several are noted for thier 'frass-flinging' custom. These caterpillars create nests of leaves and literally pick up and fling their frass out of their nest of leaves. It is theorized that by doing so, predators will find it more difficult to locate the caterpillar. Some lepidoptera caterpillars attach frass and leaf bits to their bodies. Others will create a nest of frass and pupate inside the frass nest.