Fly hawkmoth, fly! (2015.09.11 [Fri.])


 The nights have been getting cooler of late, and just the other evening I heard a scale cricket (Ornebius kanetataki) chirping quietly, somewhere in my room. Perhaps it was drawn by the light. These insects start turning up from August, and I couldn’t helo but think of it as a harbinger of autumn. I finally found it on a screen window. It stopped calling when I approached and shifted off about 20 cm before it resumed its song...

 In this month’s essay, however, I’d like to continue talking about hawkmoths.

 Lepidopteran caterpillars have more legs than the average animal. The thorax is equipped with three pairs of appendages it uses to hold down leaves while eating; that is, these insects take their food ‘in hand’ while chewing. The larvae on the other hand display five pairs of abdominal parapodia, which are large and clumsy compared to the thoracic appendages, but show remarkable adhesive ability needed to support the body. All the crawling, hanging and even dangling upside down we observe is thanks to these parapodia. These caterpillars thus have three sets of ‘arms’ and five pairs of ‘legs,’ 16 limbs in all, allowing them to crawl anywhere on a plant, to achieve all sorts of acrobatic feats even while eating, sometimes stretching to extraordinary lengths, and of course later to metamorphose into a chrysalis. They’re not known for speed, but I suppose it takes some preparation to transform into a swift hawkmoth, which can travel at speeds of up to 60 km/h. But that slowness makes them vulnerable to becoming snack food for birds if they make even a single wrong move.

 So what became of the final instar larva of the death’s head hawkmoth? Well, it got quite big – not just plump, but outright blobby, an actually outrageous size. In swallowtails, 80% of the final larval weight is said to be supported by the food consumed in the last instar; for hawkmoths, I guess this may be 90%. I hope it pupates soon, if only to leave some leaves on my veranda for its brothers and sisters.

 The biggest mobility issue for these caterpillars is actually in the head; the jaw needs to become large enough to consume food efficiently. Remodeling of the head and thorax is the main event in larval life, and the head swells even before the limbs begin to shed. The form of the head begins to appear, seeming to melt into shape from without, after which the larva goes quiet and after a full day begins to molt. But even then, the thin under layer of skin is the same color as the thicker one being shed, which can make it difficult at first to tell what is happening. But as it begins to transform its posterior half, the old skin wrinkles up and attaches to the body. It feels good to see the tail wriggle free of its old casing at last,


* * *

 The senior-most of my larvae (which completed the ecdysis before the last instar on September 5) started showing signs of pupation after 11 days. The back browned and the entire body seemed to blister. It crawled around the floor of its cage for a day, bumping combatively into its siblings, a behavior known as ‘wandering’ that lets caterpillars begin their pupation distant from plants, is a major nuisance to neighbors, but one that is forgivable I suppose, considering the defenselessness of the pupa. For the large caterpillar of the hawkmoth, this is a delicate process, as they need soft soil in which to burrow before pupating.

 Emergence is also a remarkable undertaking. Newborn butterflies need to hurry after leaving the chrysalis, lest they become prey for ants. They burst forth like terrestrial beetles, seeking the surface at tremendous speeds. They only spread their wings on finding a safe shelter. Unique among the bombycoids in having abandoned the cocoon, their struggle for survival is fierce. The images below show a time series of the emergence event with its butterfly-like extension of wings. As you can see, these creatures have butterfly wings for a moment before assuming their familiar hawkmoth-ly forms. The body is decorated in a Southeast Asian motif...we certainly didn’t see anything of its kind in the old days.