Luis:

What goes on inside a pupa?

There are three different types of insect metamorphosis (ametabolous, hemimetabolous, and holometabolous). Insects that are holometabolouss have a complete metamorphosis and go through a pupal stage.

The pupal stage is often referred to as the inactive phase in the insect life cycle because the insect isn’t moving or eating, however, physiologically speaking there is much internal activity taking place. Most internal reconstruction occurs in this stage.

I’ll just stick to the holometabolous insects for simplicity sake. Once the larva molts to the pupa, histolysis (the breakdown of tissues) and phagocytosis (the ingestion of bacteria and foreign bodies) takes place.

During this time (when the larva molts to a pupa) the developing wings, which have been developing internally in the larva, become everted and visible. Reconstruction begins to take place after histolysis, and the adult appendages begin to develop from epidermal thickenings called imaginal buds/discs. The details of the imaginal disc development vary between insects.

The muscular system goes through a lot of modification and the different muscles’ fate can either:

1. Pass through the pupal to adult molt unchanged,

2. Be destroyed and not replaced,

3. Destroyed and replaced,

4. Existing muscles reconstructed,

5. New muscles formed that weren’t previously there (formed by free myoblasts)

The alimentary canal (which is the esophagos, forgut, midgut, and hindgut) is remodeled differently depending on whether the insect eats a different diet as an adult compared to when it was a larva. Generally the forgut and hindgut are formed from imaginal rings, the midgut is reformed by regenerative cells in the epithelium.

The trachea (breathing tubes) shows almost no change, with the exception of new branches. The circulatory system also shows little change during the larva-to-pupa-to-adult molt. The central nervous system becomes more concentrated, the nerve cells and glial cells increase in number.

The control of metamorphosis (i.e., all of this stuff I have been talking about concerning the larva to pupa molt and the molt to adult) is controlled by hormones, including juvenile hormone, and ecdysone.

To escape from the pupal cuticle, hemimeabolous insects swallow air to increase volume and split the cuticle along a line of weakness. The adult pulls itself out, often with the help of gravity, and begins to expand its wings by pumping them. For holometabolous insects, escape from the pupal cocoon is a bit trickier. Different insects have developed different ways to escape, some have mandibles to chews through the pupal case, some have spines on their abdomen that help break through the case as they moved back and forth, and some produce softening secretions that actually break down the case.

Much cooler though are of course the cyclorrhapha flies. They have a structure called a ‘ptilinum’, which is a membranous sac (or basicaly a balloon) that is everted by pumping blood. The balloon presses against the puparium wall and splits the line of weakness.

Picture courtesy of http://entomology.unl.edu

Whew! So, all of that is going on during the ‘inactive‘ stage of the insect.
If you’d like to read a more detailed description, “The insects: Structure and Function” by Reggie Chapman, is the book I used as a reference for this post.

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