Metamorphosis, defined as a change in form, is a hallmark of the life of insects. But metamorphosis is not limited to insects. All animals go through changes during their lifetimes. Even humans. Our family photo albums and memories of our teenage years provide irrefutable evidence of such changes.
The process of metamorphosis varies widely among various types of animals. In many it is subtle and gradual, including things such as growth, hormone balance, or hair color.
That is why grandparents who haven't seen a grandchild for some time often say things like, "My, how you've grown!" Parents, on the other hand, see their kids every day. Consequently, parents are sometimes surprised when their children's height marks on the closet door are two inches higher or new pairs of shoes are two sizes larger than just a couple of months before.
This type of metamorphosis is sometimes called gradual. Some insects develop in this way, too. The young look similar to their parents but lack the wings and size typical of their parents. Like human children, these young insects gradually become adults.
Other insects undergo an abrupt change from juvenile to adult. And the juvenile looks nothing like the adult of the species. A caterpillar becomes a butterfly or moth, a maggot becomes a fly or a grub becomes a beetle.
Such a miraculous transformation occurs in a developmental stage known as the pupa. The word pupa is the Latin word for “doll.” Apparently, an insect in this transformative stage reminded ancient people of the carved dolls prevalent at the time.
There are several types of pupae in the insect world. Or do you prefer pupas as the plural form of the word? An argument over which word is correct is similar to the Fred Astaire and Ginger Rogers argument over the pronunciation of tomato in the 1937 movie, "Shall We Dance." Our modern dictionaries list either word as correct. My first entomology teacher used the word pupae, so that's how I'll say it. You use pupas if you want!
Among insect pupae, there are those that are termed naked. These pupae, as the name suggests, are, well, naked. That means they don't possess the extra outer covering found on pupae of some insects.
Most butterflies, including the well-known, world-traveling monarch, have naked pupae. As the monarch pupa develops, the formerly translucent, cellophane-like covering becomes clear. Consequently, some of the pieces and parts of the butterfly-to-be become visible. Kind of like giving the world a sneak peak at the attire the butterfly will wear for the rest of its life!
Other insects are more modest during this time of transformation. A few conceal the naked pupa in a soil chamber constructed by the juvenile before it pupates. This is the case for that pest of the garden – the tomato hornworm. Some beetles such as Japanese and May beetles pupate in this way as well.
Many insects, including moths, create the material used to construct their pupation chamber. Think silk here. Yes, the caterpillar of a moth Bombyx mori produces the silk that we use as multi-purposed fiber. The silkworm produces the fiber in the silk glands in its head. When preparing for pupation, the silkworm will wrap itself in a continuous strand of 1,000 to 3,000 feet of silk before it settles down to become a moth.
Other moths also produce silk. Angoumois and Indian meal moths – pests of stored grain products such as flour and birdseed – leave a silky mess where they feed. Wax moth larvae, pests of stored beeswax, also spin cocoons.
So do the giant silkworm moths. These are the largest moths found in the U.S. and Canada. They include the cecropia, polyphemus, luna and promethea moths. The silk of these moths is of a grey-brown color that allows pupae to blend into their surroundings. The luna moth caterpillar also incorporates leaves into the pupal case to further its camouflage.
Insects other than moths also produce silk for use in pupation chambers. These include fleas, wasps and even an aquatic insect – the caddis fly. During the pupal stage all insects are fragile and immobile. For some insects, it appears a silk coat is good protection for pupae, or pupas, if you prefer.