Imagining the Moth

The female regal moth I caught on July 26 was dead by last night, having laid 135 eggs (which entomologists call “ova” – scientists like to have their own jargon) . I’ve cut them out of the cardboard container I kept her in and put them in the clean glass container you see above for careful monitoring. The standard egg laying container is a paper grocery bag, but I didn’t have one, so I made due with  a stationary box, which did the job. They are the largest lepidoptera (butterfly/moth) eggs I’ve ever seen and they are a beautiful translucent green, except for a few, which are now partially brown. Those are that way because the eggs are transparent and you can actually see the embryonic caterpillars developing inside – the brown is their heads—I’m guessing some will hatch tomorrow. I have saplings of their foodplants ready in pots – when they are really little I want to grow them inside the house where I can control the environment.

Before you feel too sad for the moth who died for this , I need to say don’t fret – she accomplished her primary mission in life: laying eggs. Giant silk moths only survive a few days as adults in the wild, living entirely off fats that they accumulated as caterpillars. They have no functioning mouthparts and they don’t eat.  They have only two missions – to mate, and to lay as many eggs as possible.

I”ll talk about their mating behavior another time (because it’s an important topic), but their egg laying is also interesting. Regal moths, like many of their close kin, often fly great distances, depositing an egg or two on one tree, then flying a quarter mile to lay another egg. There are a lot of possible explanations for how this behavior evolved.(I like to call these “explanations” “hypotheses” because no one actually knows how the pressures of nature encouraged the behavior’s development, and, frankly, it’s hard to imagine an experiment that would prove the “real” cause. Honestly, what follows is all unproven speculation – intelligent guesses. In science, the next job would be to find some clever way to test these ideas – to design an experiment and then collect solid information – “data..” However, nature – the world outside is a messy, complicated place and coming up with intelligent designs for such experiments is very, very difficult. For now, I’ll stick to the fun, imaginative part – the hypotheses.

 My personal best guess  for why regal moths have evolved to fly long distances and lay single eggs is that, since these eggs grow into very big caterpillars, it would be a bad idea for too many of them to be on one tree because they are likely to attract the attention of predators (such as birds) and once a predator has found one caterpillar, it is going to be primed to see another one nearby. So, this hypothesis says I saved the female moth the effort of flying all over the forest to lay her eggs, so she probably was able to get more laid before her fat-stored energy ran out (fat is really just a chemical battery, if you think about it). She had to lay them all together, which might be disastrous in nature where there are birds, but I’ve got her back – I’ll protect her caterpillars from predators. This makes me the good guy. Really.

However there is another “hypothesis” for why this moth has evolved single egg rather than mass laying behavior and this explanation perhaps means that I’m not being so clever in “farming” these caterpillars. Some of the regal moth’s near relatives, the oakworm moths (species in the genus Anisota), do tend to lay their eggs in big masses, and the caterpillars feed together in big troops, especially when they are young. (Herding together must serve some kind of protective function for these caterpillars, like it does for bison and schooling fish and ducks and starlings and the like, but I have no idea how that works – make your own hypothesis.) Oak moth caterpillars are considerably smaller than regal moth caterpillars at maturity and they grow up faster. Importantly, this means they have less time chance to catch a disease that could spread from caterpillar to caterpillar quickly, wiping out all a moth’s brood in one rapid epidemic.

       Young oakworm moth (A. osalari) caterpillars. National Park Service photo by Sally King.

This is a familiar problem to anyone who has ever tried to raise a lot of giant silk moth caterpillars  – you have to grow them in crowded conditions (you’re really “farming” them) and various diseases can appear and destroy your project. So perhaps another explanation (hypothesis) for why the regal moth lays single eggs is that this protects long-maturing caterpillars from spreading the diseases a few are likely to catch during a long period of infancy. This could be the explanation, or both explanations could be right or… something else entirely might be the cause. Nature is complicated and it’s hard to imagine all the problems that might come up in the life of a moth.

But it’s still fun to make guesses about the evolution of unusual characteristics in a species. One other feature of regal moth egg laying behavior that I also think is a bit strange is the fact that these moths produce only one generation a year, even here in the south where summers are long. This is actually not typical of most giant silk moths. Lunas, for example, have at least two generations a summer here in North Carolina, and researchers have noticed that they have three in Texas. The first lunas hatch and lay eggs in April here, but the regal moth waits until the end of July or August to hatch, mate and start laying eggs. Why?

We can reject the explanation that the moth has only enough time for one generation a year because it is so big that it takes a long time to grow. It does take a relatively long time to grow, but not really that much longer than a luna, which goes through the whole cycle twice. So let’s add another piece of evidence: its closest relatives (other species in the sub-family Ceratocampinae) also have only one generation (one “brood” as scientists say, or “univoltine” behavior) though many are much smaller moths. The authors of The Wild Silk Moths of North America  have suggested what I think is a convincing hypothesis for this: “In the United States and Canada the greatest number of species occur in the Southeast. Midsummer flight in warm, humid weather maybe an adaptation related to the tropical affinities of the group.” In other words, the authors note that relatives of these insects are a lot more common in the American tropics, so species like the regal moth may represent relatively recent invaders from tropical climates and they may still have hold-over physical adaptations from their not-too distant past, such as favoring the time of the year when North Carolina feels like Mexico or Honduras. In the tropics, the moths lay eggs in the rainy season, since that’s when the food plants are freshest. Perhaps these tropical moths have carried the home country behaviors here.

 This is another perhaps un-provable hypothesis regarding evolution, but I like it because it again makes you expand your imagination to consider yet another dimension that can affect things: history. Consider the fact that only a little over ten thousand years ago (a blink of an eye in biological history, really) most of North America was covered by the expansion of a thick ice sheet. (Ice might have been a mile thick where I live now.) When that happened, where did all trees, bugs and other animals go? They either died out or they moved south to where there was no ice (Florida would have been ice free, but much cooler than now.) Then, when the ice receded back to the north, old species returned and some new species invaded from the south. The regal moth and its relatives may have been just such invaders – they still carry the signs of their history in the way they live.

So, in order to come to grips with how a moth works you need to imagine all kinds of things – what it’s really like out there – and what it must have been like out there a long time ago. Who knows what the most important “causes” were? There were certainly many, all working together and the result of the biological evolution they caused is the moth we have now. To really know we await clever minds, brilliant experiments and … data.