Many wonder at the mystifying process of how a caterpillar turns into a butterfly, but have you ever wondered why? Why do so many insects go through metamorphosis, and how did this process originate?
Evolution by definition will favor the traits that allow an organism to most successfully survive and reproduce. Therefore, discussing the advantages that the process of metamorphosis provides is useful in understanding why holometabolous insects evolved. One advantage is that juveniles and adults can inhabit different niches. This reduces competition among members of the same species. Additionally, metamorphosis allows for insects at different life stages to be specialized to perform different functions. For example, a larvae’s main task is to consume food and gain as much energy as they can in as short a period of time as possible. In contrast, an adult holometabolous insect’s main task is to track down a mate and reproduce. The existence of different morphological life stages allows the insect to have specialized body parts and processes that make completing these tasks as efficient and successful as possible. These clear advantages provide insight into why a metamorphic life cycle is so prevalent in insects today.
Holometabolous insects didn’t suddenly evolve to have three different life stages, but went through an intermediate form as insects that undergo incomplete metamorphosis, called hemimetabolous insects. These insects undergo large morphological changes from juvenile to adult, but lack the pupal stage that is present in more recently evolved insects.
The first step into the eventual evolution of metamorphosis began with insect’s evolution of flight. Flying was intensely evolutionarily advantageous, but posed efficiency problems concerning insect development. Insects started to evolve to have smaller wings as juveniles, which would later develop as the insect matured into larger wings capable of flight. Over the evolutionary timeline, these infant-wings became smaller and smaller, and eventually ceased being wings at all.
Another challenge insects were faced with due to their wings was that wings are energetically expensive to molt, and wings that are capable of molting forces them to be heavier and therefore less effective at flight. This type of molting is still seen today in mayflies, which are older insects on the phylogenetic tree. Evolution’s solution to this problem was to cease molting after an insect has become a mature adult. This allowed wings to be lighter, bigger, and more flexible.
With the establishment of a final molt, a clear distinction between molting juveniles and non-molting adults was formed. The differences between primitive hemimetabolous juveniles and adults were present but not as extensive as the differences seen in holometabolous insects throughout their life cycle. For example, present day insects belonging to the hemimetabolous order odonata begin life as nymphs, whose bodies somewhat resemble the bodies they will eventually inhabit as adults, but have many key morphological differences. Over time, the small differences between juvenile and adult became larger, and the adult insect more different.
How hemimetabolous insects made the jump to be holometabolous is a topic of debate among the scientific community. The main disagreement concerns whether the nymph stage is analogous to the pupa stage or the larval stage. One theory is that the larval stage is an extension of the embryonic stage, in which the pupae is compared to the nymph stage of the hemimetabolous life cycle. Another theory compares the nymph stage to the larval stage, suggesting that the final molt into the adult form that nymphs perform actually takes place as two molts in holometabolous insects, with each molt producing a new form that is unique in both morphology and specialization.
The mechanics of the metamorphosis is one that will continue to baffle scientists and laymen alike with its wonderful mysteries. Without a doubt, this process proves an excellent example of the extremely detailed, complex, and perplexing nature of God’s creation.
References
Truman, J. W. (2019). The evolution of Insect metamorphosis. Current Biology, 29(23). https://doi.org/10.1016/j.cub.2019.10.009
ten Brink, H., de Roos, A. M., & Dieckmann, U. (2019). The evolutionary ecology of Metamorphosis. The American Naturalist, 193(5). https://doi.org/10.1086/701779
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