Of the 3 stages of metamorphosis, the pupal stage is most intriguing.  In the larval stage, one can see the caterpillar moving around, chomping all the leaves and excreting fraps. You can also see the caterpillar growing in size from one instar to the next.  The adult butterflies or moths are of course flying around, feeding or copulating to ensure the continuation of the species.  Once the caterpillar turned into a chrysalis or cocoon, nothing much seems to be be happening on the outside.  Does the caterpillar dissolves into a soup and transform itself into a butterfly or moth as popular myth has it?  So what is really going on inside a pupa?  To find out how a caterpillar turns into an adult, one needs to be able to peep inside the pupa without disrupting the metamorphosis process....yes you guessed it, one needs to have X-ray vision!  Since I am already familiar with the use of X-ray in my floragraphy work, I applied these techniques on a pupa of a Psilogramma menephron (special thanks to lepidopterist David Mohn who donated this pupa for the advancement of science).

Serial X-ray of P. menephron pupa at day 9,12,14,16 and adult (from left to right). Top row - anteroposterior view, bottom row - lateral views. Sorry no caterpillar soup here.  

Okay, now we can peep inside a pupa but what are those structures and what does it mean?  I didn't know so its time to find out.  I first contacted Roger, who is the moth expert in HK and he referred me to Ian, the Hawkmoth expert at the Natural History Museum in London who refer me to his colleague Thomas, who together with his colleagues, recently published the development of the Painted Lady (Vanessa cardui) butterfly using microCT. Everyone seem to say 'Well, this is very interesting but we have not seen this before'.  My colleague KH and I then hit the books and trawled the net for whatever we can find on the anatomy and development of moth pupa.  There is actually a dearth of information on moth pupa development but after studying development in other insects including cockroaches, we now have a reasonably good idea what is happening.

Not content with just plain X-ray, I  decided to a CT scan of the pupa and enlist the help of my colleague and digital artist Dr KH Fung (if you are curious about KH, click here to read a recent spread about him in Slate Magazine).  Below is a sample of our CT images:

To take this to the next level, KH constructed 4D cine fly through where the camera dives into the pupa to take a look of the developing respiratory system. Here is an example of a view from within the pupa.Fasten your seat belt before you click the links below.  Enjoy the ride...

The above is just the beginning and there is still a lot to discover.  Oh just in case you were wondering what the pupa and caterpillar look like in  the visible light spectrum: