Another batch of hoverflies are being raised in the laboratory, with the larvae becoming pupae. I was curious to see how larvae transform into adults via metamorphosis and did a little reading about it.
Metamorphosis is not unique to insects, but it is one of the most recognisable organisms to undergo this process and is learnt about at an early age (such as the children’s storybook, A Very Hungry Caterpillar).
There are three major types of metamorphosis in insects, described below:
- Ametabolous metamorphosis, where the juvenile is essentially a miniature version of the adult, which occurs in primitive insects such as silverfish. They continue moulting (shedding their hard exoskeletons) as they grow larger until they reach adulthood (upon which they no longer moult).
- Hemimetabolous (or gradual) metamorphosis, where insects grow through 3 life stages: egg -> nymph -> adult. Nymphs gradually develop wings and genitalia as they grow with each moult before becoming adults.
- Homometabolous, or ‘complete’ metamorphosis, has four stages: egg -> larva -> pupa -> adult. Flies undergo Homometabolous metamorphosis. Larva are mobile and feed constantly, whereas pupae are usually immobile and do not feed, and is when adult body features begin to develop. 1
Metamorphosis is a very successful growth strategy, as it is estimated up to 60% of animal species on the planet undergo Holometabolous metamorphosis.1 As juveniles and adults have different behaviours and diets, they do not compete for the same resources in the same environment, unlike other animal species.1
So, what happens when insects enter their pupal stage? The larva contains two types of cell – the larval cells, that make up the structure of the larva (such as muscles and organs), and another cell type called imaginal discs, which are undifferentiated, like stem cells, and are involved in the development of adult structures (such as wings, genitals and antennae).2 The imaginal disc cells use the proteins of larval cells, which are degraded by enzymes in pupa to build these structures. 2 Essentially, the pupa rebuilds itself from the inside out, transitioning from a vulnerable larva to an adaptable adult.
The control of metamorphosis and moulting occurs by two insect hormones - Juvenile Hormone and 20-hydroxyecdysome respectively.2 Whilst 20-hydroxyecdysome activates moulting and gene expression needed for metamorphosis, Juvenile Hormone does the opposite, suppressing metamorphic gene suppression to prevent the insect becoming an adult prematurely.2 Juvenile Hormone thus assists the larvae to grow before becoming a non-feeding, immobile pupa.
In the final larval moult, Juvenile Hormone generation is decreased whilst its degradation is increased. This allows 20-hydroxyecdysome to increase pupal specific mRNAs that inhibit transcription of larval genes.2 A second spike of 20-hydroxyecdysome triggers the imaginal disc cells to differentiate/specialise into the cells needed for the final moult to become the adult insect.2
- Jabr F (2012) How did insect metamorphosis evolve? Scientific American, viewed 2nd march 2019, available from: https://www.scientificamerican.com/article/insect-metamorphosis-evolution/
- Gilbert SF (2000) Metamorphosis: The Hormonal Reactivation of Development in Developmental Biology, 6th Edition, Sutherland (MA); Sinauer Associates. Accessed 2nd March 2019, available from: https://www.ncbi.nlm.nih.gov/books/NBK9986/
Lowe T, Garwood RJ, Simonsen TJ, Bradley RS and Withers PJ. (2013) Metamorphosis revealed: time-lapse three-dimensional imaging inside a living chrysalis. Journal of the Royal Society Interface, 10(84); available from: https://royalsocietypublishing.org/doi/full/10.1098/rsif.2013.0304 doi: https://doi.org/10.1098/rsif.2013.0304
Researchers used x-rays to examine the development of organs inside Painted Lady butterflies. Advantage of using fewer animals (and no harm, unlike dissection, the traditional method to view the insides of pupae) and creating 3D models from several images. Disadvantage of low resolution so could not observe muscular or neural changes.
A short 1-minute time lapse video of a large Hercules beetle during its various life stages.