By now you’re probably familiar with the molting process: we’ve covered it in a previous issue of Notes from the Lab and during an Online Bug Encounter: so what’s the difference between molting and metamorphosis?
Molting is the actual process of shedding the old cuticle of the exoskeleton; metamorphosis describes the differences between each life stage of the insect. While all insects molt in order to grow larger, not all insects experience metamorphosis in the same way. There are three types of insect metamorphosis: ametaboly (no metamorphosis), hemimetaboly (incomplete metamorphosis), and holometaboly (complete metamorphosis).
Some of the most primitive insects are ametabolous, meaning “no metamorphosis.” While they still molt in order to grow, these insects look exactly the same in their adult stage as they do in their juvenile (or nymphal) stages, just a little larger. The only true ametabolous insect order is the wingless Zygentoma, the silverfish. Other ametabolous arthropods include springtails, diplurans and proturans, though they have been reclassified into the class Entognatha (as opposed to the class Insecta).
Some insects experience hemimetaboly, or incomplete metamorphosis. These insects experience gradual changes in size throughout their nymphal stages until they molt into their adult, winged stage. Insects like grasshoppers, cicadas, praying mantids, walking sticks, and dragonflies are all hemimetabolous.
When these insects emerge from their egg, they typically look like tiny, wingless versions of their adult form. For the most part, the nymphs occupy the same ecological niche the adults, meaning they occupy the same habitat and consume the same food. There are a few hemimetabolous insects that are an exception this rule: mayflies, dragonflies, and stoneflies are all aquatic in their nymphal stages. However, despite the difference between the aquatic nymphs (naiads) and the adults, they lack a vital component to classify them as holometabolous: a pupal stage.
The majority of insects (45% – 60%) experience complete metamorphosis, or holometaboly. Butterflies, beetles, bees, flies (to name a few); all experience complete metamorphosis. These insects have 4 distinct life stages: egg, larvae (like a caterpillar), pupae (a dormant, cocoon-like stage) and adult (the winged form).
The evolution of holometaboly is a hotly debated subject; how it evolved is still largely a mystery. Winged insects appeared in the fossil record sometime in the Paleozoic era (about 540 – 250 mya.) and recent phylogenetic analysis reveals that holometaboly evolved just once; meaning all insects that experience complete metamorphosis evolved from a single common ancestor.
Like the aquatic nymphal stages of dragonflies, mayflies and stoneflies, the larval stages and adult stages of holometabolous insects occupy different ecological niches. The ability to exploit unique resources in each stage of life likely drove the evolution and diversification of insects; holometabolous insect orders are some of the most diverse on the planet.