Handling the tricky stuff

Handling the tricky stuff

Wednesday, August 22, 2012

Midlife Crisis

4th instar hickory horned devil doing the limbo. Must have been the tequila.

Midlife can be… ugly.

I changed the sleeves today -- I ignored my own wisdom and made the sleeves smaller than the ones I made as a teenager, which means I have to transfer caterpillars to new branches when the leaves all get eaten. With small caterpillars, this isn’t easy, so it’s not what I recommend other people do. Use an old double bed sheet, sewed in a tube.
Still, there are advantages to moving the caterpillars – you get to see how they are all doing.  Interestingly, I’ve lost about 2/3 of my hickory horned devils, primarily, I think to disease or natural mortality when they were in the 1st instar (when they were really little). I have about 40 left, which is fine – 100 6-inch caterpillars would be pretty hard to maintain. This kind of drop-off between hatchlings and larger caterpillars is not unusual – I assume that it’s one of the reasons why the moths lay so many eggs. But the survivors seem very healthy, with “only the strong” surviving.  In contrast, the lunas have had very little drop-off in population – they are all smaller (still 2nd instar) but I have at least 100.

A variety of instars hanging out together.

What I find most interesting though is a big variation in the rate of growth among the hickory horned devils – in the sleeves there are a lot of 3rd instars (about an inch long) quite a few 4th instars ( 2 -2.5 inches – they will grow to about 4 inches before their final molt), and a few that are still 2nd instar (less than an inch). All these caterpillars hatched within five day period, so that’s quite a spread. Why some are growing much faster than others, I can’t guess. It could be genetic differences: as I said in an earlier post, caterpillar broods can have a lot of variation in appearance, for example. It could also be accident (some caterpillars found more nutritious leaves than others) or disease/injury – some got a little bit sick and it slowed their growth. Or it could be something totally different.

A shed skin. /This is something human kids don't have to do when they move to the next grade.

My guess is that this is genetic variation. The probable reason why there is so much variation in the appearance of brother/sister caterpillars within single broods (if you look at the previous post you will see photos of two Pandora Sphinx caterpillars – one green, one orange – I have seen four different color patterns within a single brood) is that trying out different looks is like giving nature a choice. Who knows which one may carry a hidden advantage in any given time and place? Similarly, caterpillars who grow rapidly may have a survival advantage over slower growing ones… or vice-versa.
…Which brings up the issue of the current appearances of the 3rd and 4th instar caterpillars. What’s up with that? As I blogged about a couple of posts ago, the mottled appearance of the 2nd instar caterpillars mimics bird droppings, giving those caterpillars a good disguise from hungry birds. But the 3rd and 4th instars are solid chocolate brown, with relatively large (almost leglike) horns. While the brown color could match the bark color of a twig, these guys seem to spend all day chewing on the green leaves, where they stand out. The hors make them stand out more, but aren’t large enough to seem credibly threatening. I’m puzzled by how this works for the bugs at this stage in their lives – if you have any ideas, let me know. In any case, they’re beginning to look really cool, in an “Alien vs. Predator” kind of way, which is good enough for me.

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