Wednesday, 14 July 2010

From Caterpillar to Pupa: An amazing nature sequence photographed in 50 images.

DSC02367

Most, if not all of you would have seen life cycle images of butterflies where the eggs, caterpillar, pupa and the adult are all shown together. However, have you ever wondered about the transformation process first from caterpillar to pupa and then from pupa to butterfly? I mean, yes, we all know that one comes out of the other, but have you ever visualised the process? Yesterday I had the unique opportunity to keep track of the caterpillar of a Grass yellow butterfly (Eurema blanda), as it readied to pupate and decided to photograph the entire sequence.

DSC02219DSC02223

This sequence begins with the caterpillar wandering about restlessly, searching for the ideal spot to pupate in.

DSC02238DSC02264

At this stage, the caterpillar is very restless and crawls up and down in search of the ideal spot.

DSC02272

In this case, it has crawled up a nylon line and seems to have decided to pupate there. However, it still hasn’t decided where exactly and it rotates its position along the line several times.

DSC02275DSC02278

The choice of the location is crucial to the survival of the pupae as once it is selected, the pupae will remain immobilised here, weathering whatever conditions may prevail in the area.

DSC02273

Once the position and location has been finalised, the caterpillar remains in position for a further 4-6 hours. Although it remains motionless, a lot of biochemical and physiological processes are occurring within its body.

DSC02298

A very important step is the mechanism for attaching itself to the location. The caterpillar initially spins a silk pad at the base. This silk pad is extremely small and consists of fine silk hair like hooks.

DSC02337

The caterpillar then attaches its anal end to this silk pad with its last pair of legs. It the proceeds to hang itself, head downwards.

DSC02340

However, in this species there is a second attachment mechanism, a suspension loop. In a similar way to the earlier button, it  attaches a fine silk line to the supporting nylon rope.

DSC02286

….first on one side, elongating  the suspension till it has the required length,

DSC02288DSC02289

…. and then switching over to the other side to accomplish the same, until it has formed a complete loop, suspending its body from its middle to the support

DSC02290DSC02291

The caterpillar now hangs, suspended by the two attachments to its support line and is ready to begin the actual pupation process.

DSC02301DSC02297

Below are 2 images, I took with the light source coming from different angles to highlight different structures on and off its body.

DSC02313DSC02318

Once so much was accomplished, the caterpillar remained suspended like this for a further 14 hours, with very little external changes being visible. In between, the sun was replaced with rain and the light turned to dark and back again to light as a new day dawned.

DSC02368DSC02369

The only visible changes to be observed was that the remnants of the anal claspers became narrower, as it became modified into a structure called as the Cremaster. The cremaster consists of hundreds of hooks which are thrust into the silk pad mentioned earlier. It functions like velcro and will sever to anchor the pupa as it forms.

DSC02374

After about 14 hours in this position, the larva finally begins to come alive and start twitching.

DSC02375

The curved position straightened itself to a more diagonal position as the larva wriggled about. Then, the tip of its head began to split open and the pupa could just about be seen pushing its way through.

DSC02376DSC02377

As the pupa began to push its way forward, the old skin of the caterpillar began to be progressively displaced along its body.

DSC02378DSC02379

When the head of the pupa was fully out, it began to raise a spiky projection on its head that until now lay firmly flattened against it. I presume, this spike houses the antennae-to-be.

DSC02380DSC02381DSC02383

It was hard to make a distinction between where the caterpillar skin ended and where the pupa began.

DSC02384DSC02385

However, it was quite clear that it is the dorsal side of the caterpillar that splits open, while the ventral side remains in one piece.

DSC02387DSC02388

The pupa was quite animated in its transformation and the wriggling movement was what was helping it emerge out of the caterpillar skin.

DSC02390DSC02392DSC02395DSC02398

Once the head emerged, the remaining movements were much quicker and the whole pupa emerged quite rapidly, pushing its earlier skin towards the base.

DSC02403DSC02404

One final shrug from the body disposed off the skin. The skin fell away to the ground, where i presume it will serve as a food source for other insects such as ants.

DSC02406

In some cases such as in the pupa shown below, the skin is never fully shed, and remains attached at the tip, giving the illusion that the tail is the head!

DSC02221DSC02409

After shedding its skin, the pupa continues to wriggle in its suspended position.

DSC02416DSC02430

This wriggling motion, aids to pump its body fluids to all the right places, making it enlarge in the right places, and gradually take on the shape that the final pupa will look like.

DSC02435DSC02440

Finally, as the pupa is fully formed, it begins to relax and there is very little movement. It is during this phase that the hardening of its soft pupal case happens, giving it a fixed shape. It then hangs like this for a couple of weeks, all the while, quietly growing and transforming itself on a cellular level into the butterfly that it will finally become.

DSC02441DSC02446

PS:

These images were shot over a 22h period, amidst sun, rain, a sleep deprived night and the resulting sleepy morning, where I had to check upon the larva every so often to catch the right moments. The caterpillar has not been captured or coerced in any way and all shots were taken in my back garden, where it chose to pupate. If you appreciate the effort behind these images, please help to spread the word to others by sharing on twitter, facebook etc. I`m keeping track of a few pupae and I hope to be able to capture their emergence into butterflies. If you are interested, keep watching this space or hit up the links on the left to subscribe to these pages.