A cocoon is the protective casing built by a moth larva (caterpillar) for its transformation into a pupa, the non-feeding, transitional stage of metamorphosis. The appearance of these structures is incredibly varied, ranging from smooth, papery ovals to rough, camouflaged bundles of leaves and debris. Caterpillars construct these shelters to provide insulation and defense against predators and the elements. The visual characteristics of a cocoon—its size, color, shape, and texture—are directly tied to the specific species of moth that created it.
Distinguishing Cocoons from Chrysalises
A frequent point of confusion is the difference between a cocoon and a chrysalis. The term cocoon refers exclusively to the external silk covering spun by a moth caterpillar around its developing pupa. This silken case is an added layer of defense and is often supplemented with materials from the environment.
In contrast, a chrysalis is the hard, bare skin of the butterfly pupa itself, which is typically found hanging from a branch or other structure. Butterfly caterpillars do not spin an outer silk shell; instead, their final larval skin hardens into the chrysalis. If a protective casing is made of silk, leaves, or other incorporated debris, it is correctly identified as a moth’s cocoon.
The Materials and Textures of Construction
Moth cocoons are shaped by the materials the caterpillar incorporates for protection and camouflage. The foundational element of nearly every cocoon is silk, a protein fiber the larva secretes from its salivary glands through a spinneret near its mouth. This silk can form a tough, dense wall or a loose, net-like webbing, depending on the species.
Beyond the primary silk layer, many caterpillars weave in secondary materials to enhance the cocoon’s integrity and disguise. These materials often include bits of chewed wood, soil, small pebbles, or the caterpillar’s own shed larval hairs. The resulting texture can vary dramatically, creating a hardened, calcified shell in some species, or a softer, papery feel in others. Some cocoons, like those of the bagworm moth, are entirely covered in plant needles or leaf fragments, making them look like hanging plant debris.
Common Variations in Appearance and Location
The location and construction style are often species-specific, creating a wide range of appearances. Many of the large Giant Silk Moths (Saturniidae) produce visible cocoons. The Cecropia moth creates a large, bulky, baggy cocoon that is securely fastened lengthwise along a sturdy twig or branch, appearing like a tough, brown sack.
The Polyphemus moth typically spins its oval-shaped cocoon inside a single leaf, which then detaches from the tree and falls to the forest floor with the autumn litter. This strategy relies on blending in with the surrounding dead leaves for overwintering camouflage. Similarly, the Luna moth cocoon is also made within a curled leaf but is found hidden in the ground-level leaf litter.
The Promethea moth spins its elongated cocoon inside a leaf, but first uses silk to reinforce the leaf’s petiole, firmly attaching it to the host plant. This ensures the cocoon hangs securely from the branch through the winter, often appearing like the last remaining dead leaf on a deciduous tree. Other, smaller moth species burrow into the soil or under bark crevices, creating small, hardened cocoons that incorporate surrounding dirt and wood particles for concealment.