3rd molt completed: 4th instar caterpillar of the Cecropia moth

Overnight this caterpillar molted for the third time and is now a 4th instar larva of the Cecropia moth.

The most noticeable changes in appearance are the loss of any spots on the skin, and the variation in the color of the spicules (the spiky projections along the body). Up til now all of the caterpillar’s spicules have been a uniform shiny black. Now the two rows along the dorsal surface are yellow while the lateral spicules remain black.

The caterpillar’s skin is a beautiful shade of pastel green and blue.

Getting ready to molt: Cecropia moth caterpillar

Cecropia moth larvae (caterpillars) go through four molts as they grow. The stages between molts are called “installs”. So-called “first instar larvae” are caterpillars that have hatched from their eggs but not molted, and the caterpillars that have molted four times are called “fifth instar”. By the time they reach the fifth instar stage, they can be the size of an adult finger, and at that stage they spin a cocoon inside which they will molt again into a pupa that overwinters before emerging as an adult.

This caterpillar has molted twice, and is therefore a “third instar” caterpillar. It is about an inch long at rest:

This one is getting ready to molt. The evidence includes:

  • taut skin - the caterpillar looks like a spiky sausage,

  • lack of movement for 24 hours or more, except for an occasional twitch or body twist,

  • some cloudiness around the head capsule.

The caterpillar typically rests with its head curled under its body as seen here. As it gets closer to molting, it will start to stretch and wavelike motions will originate from the tail which the caterpillar anchors down with a bit of silk. The skin will split open just behind the head and will slide back towards the tail as the caterpillar’s body expands forward and it stretches out of the old skin.

Cecropia caterpillars grow

Caterpillar basic anatomy.

In the past week since hatching, the caterpillars of the Cecropia moth have grown to lengths of about 1 cm in their resting position. Considering that 9 days ago they were each completely enclosed in an egg of about 2 mm diameter, this is an impressive gain in size.

Caterpillars have two kinds of legs, named according to which part of the body they are attached:

  • thoracic legs: these are attached to the body segments behind the head. These segments will become part of the thorax of the adult moth, and these legs in the adult moth will be much longer and will allow it to walk and grasp, and

  • abdominal legs: five pairs of legs, called “prolegs”, one pair of which - the anal prolegs - is located near the rear end of the caterpillar, with the remaining four pairs located between the anal prolegs and the thoracic legs. These legs will no longer be present in the adult moth.

The thoracic legs are segmented with hooked end, while the prolegs are fleshly and hydraulically-powered and have a ring of hooks called “crochets” at their ends.

When feeding, the caterpillars crawl along the margin of a leaf, with their legs straddling the leaf edge.

As they move, their prolegs serve as anchors and their thoracic legs as guides. Their bodies move in a rippling motion as they collapse the length of their bodies, pulling their anal prolegs forward and then sequentially reposition their other legs forward, and then extend their fore-bodies in their direction of travel.

Caterpillar locomotion: a Cecropia moth caterpillar walks along the edge of a lilac leaf.

1st instar Cecropia moth caterpillar in resting position.

1st instar Cecropia moth caterpillar in resting position.

In the resting position, the caterpillar’s head is facing down and is even curled under the front of the body slightly. The body behind the head is arched slightly upward with the largest spicules - the stiff projections that bear hairs at this stage - protecting the back and sides.

Within the next couple of days, the caterpillars should molt and become 2nd instar larvae.

Caterpillars hatch

On the 13th day after they were laid, the eggs of the Cecropia moth hatched. I placed a few freshly-cut shoots from my lilac bush into the container with the tiny caterpillars, which already appear much larger than the eggs from which they recently emerged.

The caterpillars chew their way out of the 2mm-diameter eggs and almost immediately begin feeding on the leaves of suitable host plants including maple, birch, cherry, and lilac (shown here).

1st instar Cecropia moth caterpillars on lilac leaves. Empty egg shells at bottom of container.

Cecropia moth caterpillar, 1st instar.

1st instar Cecropia moth caterpillars are black with black hair-bearing spicules on their backs and sides.

Hatched Cecropia moth eggs.

Hatched Cecropia moth eggs.

1st instar Cecropia moth caterpillars on a lilac leaf.

1st instar Cecropia moth caterpillars on a lilac leaf.

These so-called “1st instar” larvae (caterpillars that have not performed their first molt) are entirely black with tiny black “spicules” on their backs. The spicules are hard, shiny projections and at this stage carry long hairs that extend from tiny turrets along their crowns.

This one has already begun gnawing on the edge of the lilac leaf on whose edge it clings.

For the next 3 months or so, the caterpillars will do nothing but eat leaves and grow. Their size will increase about 20 times until they are larger than an adult finger. They will shed their skins four times and emerge from each molt with a different color scheme.

The 5th instar caterpillars will then construct a durable silken cocoon which will turn to a dark brown color. After passing the winter in the cocoon they will emerge as spectacular adult moths in late Spring. During their brief life as adults - about a week - they will mate and then die, with the females producing up to about 100 eggs to carry the species forward into its next generation.