Ecdysis: When Growing Up is More than Skin Deep
Sunday, 12 June 2022
Spring has almost arrived in the Argentinian city of La Plata. As the platenses go about their daily business a coreid bug goes through imaginal ecdysis, emerging in its adult form. By making crawling movements, shifting back and forth, the bug cracks its old shell which splits down the back and allows the animal to appear. The new cuticle darkens and hardens – the closest process we have as humans is tanning (as in the production of leather rather than the time spent on one’s back on a beach). The growth is usually achieved by transfer of body fluids from soft body parts before the cuticle hardens. Large insects with tracheal respiration will take in air and blow themselves up like an inverted balloon (but don't overdo this simile in your mind). Hardening of the cuticle depends on the species. It can take place over a matter of hours – it needs to happen as quickly as possible as the animal is more exposed to predation than usual at this point in its life.
Perhaps the best known example of ecdysis is found in the cicada which lives underground for most of its life. When they finally come to the surface (one species can take seventeen years to do so) they shed their old exoskeleton and emerge in their adult form. Close up it can look like something from a science-fiction movie.
Many people who live in areas where cicada have been breeding since time immemorial dread the days of emergence as vast numbers appear at the same time. It is believed that this is caused by something called predator satiation. If local predators cannot possibly eat all of the cicada that emerge then once they are full they will leave the rest alone to breed. That, at least, is the theory and it certainly seems to work.
Perhaps the dread could be turned in to a more positive form of anticipation. Cicada, especially the plumper females, are considered a delicacy in many parts of the world where they are found. Cicada stir-fry anyone?
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The rufus grasshopper goes through the imaginal ecdysis (or molt) after which it reaches the adult phase. Unlike the cicada, the female grasshopper will refuse the advances of any male for about four days and for the two days after that she will mate but will certainly not sing to attract a male. After a week, however, she is back on form and will actively sing to attract a partner. Once this has successfully happened she will then go back to being defensive and will refuse any advances until her eggs are laid.
Image Credit
Image Credit
Image Credit
The exuvia of a regal jumping spider. Have you ever wondered why spiders emerge from their eggs looking like adults, albeit tiny ones? This is because most species undergo ecdysis while they are still inside their egg. The spiderling that emerges is ready for anything the adult world can throw at them.
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This remarkable series of pictures taken in the UK shows a spider shedding its skin. Most spiders will do this dangling from a single piece of thread and simply leave their discarded skin behind. Generally, spiders will undergo ecdysis between five and nine times until they reach maturity and their maximum size. Others, like the long-lived tarantulas and funnel-web spiders will molt every year even once they mature.
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Hats off, though, to the male of the synema species of crab spiders. The female spider will eat the male after mating if she possibly can so he has come up with a bright idea – and considering the difference in size between the sexes something was needed to even the odds. He mates with the female while she is still callow, that is the time her exoskeleton is hardening after ecdysis and she is unable to eat him because her jaw is too soft. It is vital that he gets his timing right (and doesn’t always) but at least some males get to… fight another day.
Image Credit
The rufus grasshopper goes through the imaginal ecdysis (or molt) after which it reaches the adult phase. Unlike the cicada, the female grasshopper will refuse the advances of any male for about four days and for the two days after that she will mate but will certainly not sing to attract a male. After a week, however, she is back on form and will actively sing to attract a partner. Once this has successfully happened she will then go back to being defensive and will refuse any advances until her eggs are laid.
The exuvia of a regal jumping spider. Have you ever wondered why spiders emerge from their eggs looking like adults, albeit tiny ones? This is because most species undergo ecdysis while they are still inside their egg. The spiderling that emerges is ready for anything the adult world can throw at them.
This remarkable series of pictures taken in the UK shows a spider shedding its skin. Most spiders will do this dangling from a single piece of thread and simply leave their discarded skin behind. Generally, spiders will undergo ecdysis between five and nine times until they reach maturity and their maximum size. Others, like the long-lived tarantulas and funnel-web spiders will molt every year even once they mature.
Hats off, though, to the male of the synema species of crab spiders. The female spider will eat the male after mating if she possibly can so he has come up with a bright idea – and considering the difference in size between the sexes something was needed to even the odds. He mates with the female while she is still callow, that is the time her exoskeleton is hardening after ecdysis and she is unable to eat him because her jaw is too soft. It is vital that he gets his timing right (and doesn’t always) but at least some males get to… fight another day.
Caterpillars of butterflies and moths also undergo ecdysis. This may come as a surprise as they always looks soft and plump enough for their skin to simply expand. However, the larval stage is more often than not the chief feeding stage of their life-cycle and caterpillars are voracious. They eat so much and grow so rapidly that most larvae must molt up to five times before they can begin the pupal stage.
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They are such hungry creatures that they leave nothing to go to waste. Once their old exoskeleton is shed caterpillars will often eat it.
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From cockroaches to mayfly, from scorpions to lobsters, from centipedes to orthoptera unknown, ecdysis is everywhere.
If you have ever been a little less than credulous at claims that a friend’s stick insect’s leg has grown miraculously back – then don’t be such a doubting Thomas in the future. It is quite likely the case. In the weeks leading up to ecdysis, tissue below the limb in question (or rather where it used to be) will form a layer of progenitor cells known as a blastema. These cells will re-order themselves, during ecdysis, to create a new articulated limb. Occasionally one molt will not be enough – the limb will look shorter than the others at first. A second ecdysis and the limb will be barely recognizable from the others.
We will leave you (perhaps this might have been expected) with a look at the final radical molt of dragonflies and damselflies. You can read more about this astonishing transformation in our feature here. Yet in recent years, these cast-aside exoskeletons have been serving a new, far-reaching purpose.
Exuviae, the remnant of ecdysis (with which you are now most familiar), have, incredibly, a crucial use for scientists. As you might imagine, estimating the population of species which range over large areas is quite difficult if capture or counting are your sole methods of monitoring numbers. The indirect method of assessing populations through their discarded exoskeletons is proving to be invaluable to those involved in monitoring species such as dragonflies. Although searches at sites have to be repeated less people (and less expertise too) are needed to conduct a survey. Perhaps, after all - for dragonflies at least - being uncomfortable in your own skin, can be an advantage.
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