Eastern Tiger Swallowtail
Caterpillars are the larval stage of butterflies and moths. It is the second part of their
four-stage life cycle (egg, larva, pupa and adult).
Caterpillars have long, worm-like bodies
with 6 true legs. They can also have a variable number of stumpy false legs (called
prolegs), which help them to move and cling to things.
Caterpillars can change dramatically from when they first hatch to when they're ready
to pupate. Some can increase their body mass 10,000-fold in just a few weeks - that's like
a baby growing to the size of a sperm whale! Many look very different as they grow.
Curious about caterpillars? Just for fun, try the:
The caterpillar (larva) hatches out of the egg within 2 to 3 days. When a caterpillar) first hatches
from its egg, it is very small! This young caterpillar is referred to as a first instar
The caterpillar eats voraciously but their skin cannot grow with them. In order for a caterpillar to
grow larger than the skin it had when it hatched, it must make a new, larger skin. The caterpillar
does this by first growing a new skin underneath the outer skin. Then, when it is ready, it "sheds"
the old skin, and the newer, larger skin underneath is exposed.
This process is called molting. After the caterpillar has molted for the first time, it is referred to
as a second instar, and it has some room to grow. The third instar caterpillar also
eats and grows until it is too big for its skin. It molts again, and the caterpillar with its new skin is
referred to as a fourth instar caterpillar.
After about 3 to 4 weeks it begins to pupate. To do this, the caterpillar attaches itself to a branch
with a button of silk, hangs upside down and its chrysalis forms after its final molt of caterpillar skin.
not a resting stage as many people think. Quite to the contrary, a lot is happening to the pupa.
The body of the caterpillar is transforming into an adult butterfly.
Wings are fully formed, the beginnings of the wings were actually forming underneath the
caterpillar's skin before its last molt) in the chrysalis. Antennae are formed and the chewing
mouthparts of the caterpillar are transformed into the sucking mouthparts of the butterfly.
After 10 days of pupation inside the chrysalis it emerges as an adult butterfly. As an adult,
it drinks nectar from flowers using its long tube-like tongue called a proboscis. From the nectar
it gets energy to fly, reproduce and lay eggs for another generation.
Caterpillars come in many colors, shapes, and sizes. Some caterpillars are quite hairy,
while others are smooth. Despite differences between species, though, all caterpillars
share certain morphological features.
Many of these caterpillars are most obvious when they're fully grown and looking for a
place to either pupate or settle down for the winter, though some are easily spotted on
their favorite food plants. Here are some of the species that you are most like to identify
The first section of the caterpillar body is the head. It includes six eyes, called 'stemmata,'
the mouthparts, the small antennae, and the 'spinnerets', from which the caterpillar produces
silk. Antennae are present on either side of the labrum but are small and relatively inconspicuous.
The labrum is like an upper lip. It is used to hold food in place while the mandibles do their
The caterpillar’s tentacles are sensory organs. Caterpillar’s eyesight is poor, and tentacles are
tactile. They aid in navigation on the front. They may also play a role in defense/predator
confusion on the rear, leading a potential predator to think that the monarch’s rear is its head.
Located in the head section, the mandibles are jaws that are used for chewing leaves.
Spiracles are external openings that allow gas exchange (respiration). The caterpillar contracts
muscles to open and close the spiracles. One spiracle pair is found on the first thoracic segment,
T1, and the other eight pairs are found on the first eight abdominal segments, A1 through A8.
Caterpillars have six pairs of simple eyes called ocelli. They are small, simple eyes that can
detect changes in light intensity, but cannot form an image. Ocelli are composed of
photoreceptors and pigments. Ocelli are usually located in two clusters of six eyes on the
sides of a larva's head
A segment is a body section of the thorax or abdomen. A caterpillar has three thoracic segments
and 10 abdominal segments.
The thorax is the second section of the caterpillar body. It consists of three segments, known
as T1, T2, and T3. The thorax contains three pairs of true legs with hooks and a dorsal plate
called the 'prothoracic' shield. The prothoracic shield is located on T1, the first segment. The
color pattern of this shield is valuable for identifying different species of caterpillars.
There are three pairs of segmented legs, also known as thoracic legs or true legs, located in
pairs on each of the three thoracic segments. Each true leg ends in a tiny claw. These are
distinct from the fleshy, false prolegs found along the abdominal cavity.
The third section of the caterpillar body is the abdomen. It is 10 segments long, classified as
A1 through A10, and includes the prolegs (false legs), most of the spiracles (breathing holes
used for respiration), and the anus (the final stop along the digestive tract)
A caterpillar's body is basically a tube for processing and storing food. A set of mouth parts
lets the caterpillar chew its food — typically leaves and other plant parts. The mouth
empties into a very long intestine with fore and hind parts.
Caterpillars are not very efficient food processors. Caterpillars often utilize less than a 1/4 of
the food they ingest. Partly this is due to a mismatch between the food content and the needs
of the caterpillar. Caterpillars typically need more protein and less sugar and starch than their
host plants provide. The 3/4 of the food that is not utilized is excreted as copious amounts
of frass (feces). Caterpillars efficiently remove and absorb the amino acids
from the food and excrete excess starch and fiber.
Prolegs are fleshy, false, unsegmented legs, usually found in pairs on the third through sixth
abdominal segments. The soft prolegs bear hooks on the ends which the caterpillar uses to
cling to foliage, bark, and silk. Experts sometimes use the arrangement and the length of these
hooks to identify caterpillars at the family level. The number and size of the prolegs can also
be identifying characteristics.
The anal prolegs are a pair of unsegmented, false legs that are located on the last abdominal
segment. The prolegs on A10 are usually well developed.
Most caterpillars are covered in hair-like bristles called setae. These tiny filaments are often
too pale to see. But a number of caterpillars are covered in thick swathes of setae that they
mean for us to see and to heed.