Larva and Caterpillar
Egg | Larva/Caterpillar | Pupa |
Adult
The word larva refers to the growth stage of all insects
with complete metamorphosis. Caterpillar refers only
to a butterfly or moth in this stage. Either word is correct, but most scientists
say larva. It is during this stage that monarchs do all of their growing; in fact
this is just about all that they do. These "eating machines" take few
breaks even for resting.
When the caterpillar becomes too large for its skin,
it molts, or sheds its skin. The head capsule is the
first part of the old skin to come off during the molting process. Then the old
skin peels back from the front of the caterpillar. At first, the new skin is very
soft, and provides little support or protection. This skin soon hardens and molds
itself to the caterpillar.
The shed skin is often eaten before the caterpillar ingests more plant food! The
intervals between molts are called instars.
Monarchs go through five instars. The
best way to tell the difference between the different instars is to compare the
size of their heads — the rest of the body grows within each instar, but the
head size stays constant. The entire larval stage in monarchs lasts from nine to
fourteen days under normal summer temperatures. If you'd like to distinguish between
the different larval instars, we have developed A Field Guide to Monarch Caterpillars.
You can order this field guide from our store or use these illustrations
by Kristen Kuda:
Egg
| First
instar |
Second instar |
Third instar |
Fourth instar |
Fifth instar
Color picture of all five instars together
Larvae have three distinct body parts. They have a head,
and a body with a thorax and an
abdomen. The head has a pair of short antennae,
mouthparts (upper lip, mandibles, and lower lip), and
six pairs of simple eyes, called ocelli. Even with
all of these eyes, the caterpillar’s vision is poor. The antennae help to
guide the weak-eyed caterpillar, and the maxillary palps
(sensory organs), help direct food into its jaws.
Each thoracic segment has a pair of jointed,
or true legs, while some of the abdominal segments have false legs, or
prolegs. Monarchs have five pairs of prolegs. The prolegs have tiny hooks
on them that hold the larva onto its silk mat or leaf. The fleshy tentacles at the
front and rear ends of monarch larvae are not antennae, but they do function as
sense organs.
Like other insects, monarchs obtain oxygen through holes in the sides of their thorax
and abdomen called spiracles. The spiracles are connected
to a network of long airtubes called tracheae, which
carry oxygen throughout the body.