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What is that weird thing?!

I remember the first time I saw a swallowtail was startling, and strangely disturbing. There is an alien quality about the shape, and perhaps resonance with memories of pictures of mummies or quadruple amputeeism. Also perhaps a hint of claustrophobia--it's easy to imagine being locked inside, utterly helpless and vulnerable.

The conversion of a J-shaped suspended caterpillar to a pupa is pretty fast, lasting only a few minutes. It starts when the caterpillar begins to wriggle and writhe, doing just the sort of movement you'd expect would be needed in order to wriggle out of a whole-body leotard without any helping hands. The initial split in the skin occurs at the top, and reveals the green-white pupa within:

More writhing, and the split spreads downward. The skin is so thin that its edge is virtually only know it's there because of the colors.

More writhing. This is an unusual pupa, with what looks like a "face" composed of darker pigment.

The skin looks somewhat baggy as it pulls away from the pupa. In the photo below, you can barely make out the silken "harness" loop crossing the triangle-shaped black section of skin on the left.

The skin is not attached to the harness loop, and as the split reaches the loop, the skin slips easily out from underneath the loop.


I've no time to stop...

Almost done...

The skin is bunching up at the base.

Now the skin is a small bunched-up wad, and what will be the "horns" at the top are soft and somewhat crumpled.

From the other side...about to nudge the skin free... it.

The writhing continues for a few minutes even though the skin is entirely free of the pupa.

The photo below shows the pupa about 10 minutes after shedding...the "horns" at the top are starting to be more prominent.

Here's the side view:

After an hour or so, the "horns" are well-defined, and the undulating segments are starting to flatten as the pupa dries and hardens.

This pupa apparently couldn't decide whether to be green or tan...

The rest of the images on this page were taken with a digital camera built in to a dissecting microscope.

Below is a close-up image of a different pupa showing the harness thread. It sinks slightly into the soft pupa and usually does no damage, but occasionally the thread can slice into the pupa and irreparably damage it.

Harness thread

Spiracles, the oval structures shown in the three images below, are valves in the sides of caterpillars and all other insects that admit air and thus provide oxygen to the body. They also can be closed to prevent loss of water. The images show spiracles at successively higher magnifications. These spiracles measure about 1mm in the long dimension.




The image below of a single spiracle from a brown pupa better shows the "toothed" opening. These are tiny hairs thought to filter out dust or other particles


Surprise at the rear attachment point! The image below shows a pupa's unexpectedly bizarre and elaborate point of attachment to a fennel stem. The silk-covered fennel stem enters the image from the left side and goes out of focus in the background to the right. The tail-end of the pupa (on the right, terminating in the middle) has, emerging from it, reddish-brown stalks capped with darker blobs, and there is a tangled clump of silk seemingly intermingled with the capped stalks. This is reminiscent of the hooks and loops of Velcro!

rear attachment

What are these capped stalks? Are they remnant bristles of the caterpillar's hind-most "pseudopodia" (fake feet)? The bristles are what enable a caterpillar to "stick" to almost any surface, including slick surfaces such as glass and even "non-stick" surfaces such as Teflon.

But looking at a J-shaped caterpillar that has not yet pupated, they appear to not be bristles from pseudopodia. Here's an image of the rear end of the caterpillar with silk somewhat tenuously attached to the rear-most black area:

Rear silk attachment

And at higher mag, there are what appear to be capped stalks:

Rear silk attachment

When the above caterpillar started to shed its skin to pupate, the tail-silk detached from the caterpillar. The cat was then hanging loosely only by its harness-thingie and lacking the tail anchor made shedding the skin more difficult. That's the price paid by a caterpillar for a wimpy attachment job!

Below are two more pupas showing abundant silk intermingled with the stalks:

Rear attachment

And another (light-colored pupa at left, mass of silk, then fennel stem):

Rear silk attachment  
Whatever these structures are, they are abandoned during the development of the butterfly and remain behind when the butterfly emerges from the pupa. Below is the rear attachment point of an empty pupa and the dried-out fennel stem to which it was attached via a mass of silk:

Rear silk attachment in empty pupa

The above empty pupa was then snipped off as close as possible to the rear attachment point so that the inside surface could be seen as shown below.

Each stalk has a pop-rivet-like base and "collar" presumably to anchor it in the wall of the pupa.

Inside of empty pupa at attachment point

The image below shows a side view of the pupal wall with stalks on the outside and stalk bases on the inside surface.

Inside of empty pupa at attachment point