Is it dead?? Pupal life status...
When anise swallowtail caterpillars pupate, they enter a stage of life that may appear to us as something less than life: no motion, little or no response to external stimuli, and a sort of, well, dead appearance. When this stage goes on for weeks, we are tempted to ask whether there really is anything happening inside that husk, or if perhaps the critter has sneakily abandoned the plans to emerge.
It's unfortunate but true that some pupas that are kept indoors do not emerge after over-wintering, and for reasons unknown to us simply dry up and that's it. We have thought that maybe they need a little misting or even watering from time to time to simulate fog or rain, but since relatively few (maybe 5-10%) of our un-watered pupas die while over-wintering indoors, we expect that this is not the main reason.
Can't usually tell by looking: in our experience, a dead pupa looks rather similar to a living pupa. The green ones don't seem to fade much, and the tan/brown ones don't have much to fade. On occasion we have found a pupa with (DON'T READ THIS IF YOU ARE SQEAMISH!) a tiny hole in it, in and out of which scurry tiny bugs---evidently having made both a home and a series of meals out of the pupa's innards. These pupas are, perhaps needless to say, done for.
One way is to weigh: Given the right tool for the job, it is easy to tell when a pupa is flat-out deceased, or at least has been for some period of time: all you have to do is weigh it. The kicker is that since these guys weigh only about a gram each, the handy-dandy bathroom scale ain't gonna cut it----you need a fairly sensitive balance. We have an electronic balance (an Acculab GS200) that reads to the nearest 1/10 gram, and while it'd be preferable to weigh to the nearest 1/100g, this one gets us into the ballpark.
And since you can't weigh an attached pupa, whether to detach it for purposes of weighing is another issue to grapple with. Detachment, btw, is not fatal to the critter.
Here are some weight data to consider (all weights are in grams):
| Weights of presumed living pupas | Weights of presumed dead pupas | |||||||||||||||||||||||||||||
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Most of these weights have a single decimal place because the balance weighs to 1/10g. The two weights with 2 decimal places were obtained when the balance couldn't make up its mind about a weight, and flipped back and forth between the two adjacent weights (for example 0.9 and 1.0).
As you can see, while there is a fair bit of variation (almost 2-fold) in the living pupas, they all substantially outweigh the dead pupas. And interestingly, while we have a smaller collection of dead ones, they are rather consistent in their weights. Well, keep in mind that since our not-exactly-research-quality balance reports all weights from 0.26 to 0.34 as 0.3 grams, we're not seeing variation that is actually there.
So on average, it appears that dead pupas have lost about 2/3 of their original live weight, and it is presumably mostly water that is lost. This substantial loss of weight makes weight a pretty good indicator of whether a pupa is alive or dead.
BTW---butterfly weights: Since the balance is out and handy and it's summertime and it seems like butterflies are emerging almost hourly, why not weigh a few butterflies, too? It turns out they will obligingly sit on the balance, and the three weighed so far all registered as 0.6 g. So, if the pupa weighs about 1 g on average and the butterfly weighs about 0.6 g, the rest of the weight (about 0.4 g) is probably mostly accounted for by the watery liquid that is left behind in the pupa when the butterfly emerges and the additional fluid which is usually squirted out by a newly-emerged butterfly. We haven't weighed a freshly-abandoned pupa yet, but the dried remnant of the pupa doesn't register on this balance at all, meaning it is less than 0.5 g.
Weighing by hand: Actually, before we ever measured pupal weights with the balance, we realized that a dead pupa was noticeably lighter than a new healthy one just by holding one in one hand (open palm) and the other in the other hand and bouncing them up and down a little. You can feel the slight difference in weight.
The Float-in-Water Test: Doesn't get any easier than this! Dead pupas float, live ones sink --- it's the LAW! (or so it seems after testing quite a few). Be nice, though, and use a napkin or paper towel or other absorbent stuff to pat-dry the recently-sunk live ones, OK?