Transformers - the inevitability of change
By James Duncan
Communities & Wildlife Officer
Like death and taxes, there's another certainty in life - change. It's fundamental, dictated by the onward march of time itself. From societal changes to the weather and seasons, it's a constant for every single species on planet earth. But of course, some species are subject to just that little bit more change than others. The best known for this are surely the insects, the largest group within the kingdom (well, technically Phylum) of the arthropods. Though the overwhelming majority of insects hatch from eggs, many undergo profound physical change known as metamorphosis, a process that offers such distinct advantages that it has led to the insects being the most successful organisms on the planet. There are three categories of metamorphic change - ametaboly (no change), hemimetaboly (partial change) or full-blown holometaboly (complete change). Whilst ametaboly is known typically as 'direct development', both partial and complete metamorphosis are quite simply a marvel of the natural world, perfect metaphors for transformation and improvement in human life.
In simplistic terms, each life stage within the process is dedicated to one specific task. In complete metamorphosis, there are four - egg, larva, pupa and adult. Larvae are eating machines pure and simple, their one mission being to grow, and grow quickly. Adults however, are tasked with reproduction and dispersal. Larvae don't usually possess a mechanism for reproduction and adults of many species may lack a digestive system, meaning they're on a slow 'fat reserve' road to starvation. It may appear that eggs and pupa are relatively inert, but they are in fact engines of radical transformation to the next stage. This methodology allows any given species to effectively partition resources and thereby avoid intraspecific competition. For example, larvae may require protein-rich food to allow for rapid bodily development while adults feed on carbohydrates to provide fuel for flight and reproduction. It's also relatively common for species to occupy entirely different habitats from one life stage to the next, eg. from aquatic to terrestrial.
Ultimately, no matter how small a larva is, even when first hatched, all its minuscule cells are primed and destined to become the prominent features visible in adult form, such as wings, legs and antennae. A specific hormone is constantly secreted, imaginatively named 'juvenile hormone' to prevent imaginal discs from developing any further than is strictly necessary. A larvae's gut, muscles and organs will grow as food is ruthlessly devoured but adult cells remain suppressed. Once a critical size is reached, a burst of hormone known as ecdysone, is released to allow an insect to moult its exoskeleton and transition to become a new instar (larval stage). Juvenile hormone continues to prevent onward development until an eventual ecdysone surge stimulates the development of a chrysalis. Finally the imaginal discs are released from their dozy slumber and their development accelerates unhindered as they fold into concave shapes, the central part of each cell destined to become an extremity on the adult insect. Were you to peek inside the body at this point, it'd appear little more then a gloopy soup, nutrient rich, an entire reconstitution of ingredients for the creation of a wonderful winged adult (in most cases). A last blast of ecdysone includes no juvenile hormone and triggers the emergence of an adult, ready to mate and lay eggs and re-start the process.
Of course, metamorphosis is not exclusively the preserve of insects and is commonly employed by fish, and more famously, amphibians. In these veterbrates the process is more typically dictated by the thyroid. Eels for example undergoe a number of metamorphic stages during transition from larva, to glass eel, to elver and eventually to their migratory stage. A good number of flatfish begin their life in a more regular 'bilaterally symmetrical' fashion with an eye on either side of the body, but one eye migrates across to the other side of the fish – which will become the upper side in the adult. Frogs and toads exhibit extremely rapid bodily changes, whereby their spiral shaped mouth and gut are reabsorbed into the body, along with the water-breathing gills. Entire neural networks are re-routed, allowing for things like stereoscopic vision. All of this can happen in as little as a day, a transformation so radical that it's hard not to marvel with wonder how it came into being at all.