Cheshire

Phasmids are really cool insects. You probably know them better as stick insects. They’re insects which have evolved probably the most perfect cryptic coloration in the animal kingdom-they’ve evolved to look like sticks. They’re very, very good at this because not only do they look like the last thing a predator would ever eat, many move with a rhythmic motion that makes it look like they’re being swayed by the wind. Richard Dawkins has a pretty good video about the evolution of this type of camoflauge (note: this isn’t the research I’m discussing here):

We have some here in Iowa, but I’ve been looking for them the past three years and haven’t been able to find them-maybe this year. Stickbugs aren’t things you can normally (legally) buy online. They’re considered agricultural pests by APHIS and just like butterflies and moths, there are some pretty strict requirements for their care. You basically need to keep them quarantined.

There are two kinds of insects which got me interested in entomology: parasitoids and phasmids. When I was a kid, I would raise sod webworms (Crambus), tomato hornworms (Manduca) and armyworms (Spodoptera), usually by harvesting the foliage I needed from neighbor’s yards (they hated me) and by keeping them in jars.

Unfortunately, my parents wouldn’t let me keep insects in the house and forced me to raise them in the garage because they thought it was just a stage. Since they forced me to keep my pets outside, they were exposed to whatever animals were out there. I’d clean them out every day but even when you do your best, frass (bug poo) still accumulates and attracts insects. Unfortunately for my parents, I would occasionally rear out wasps or flies out of my caterpillars. Some kids would be horrified by this, but I thought it was really neat.

Parasitoids are one insect which sealed the deal for me. A few years later, they started sending me to summer camp. I still hadn’t outgrown the bug phase, but now they were sticking me into the middle of the woods with all sorts of interesting critters that I normally associated with tropical rainforests. At this summer camp, I would find Pseudolucanus capreolus, all sorts of Saturniids (Automeris io and Actias luna) but my favorite finds were actually specimens of our native phasmids (probably Diapheromera femorata or something like that).

I’d take these guys home, rear them up until the end of summer and then they’d die. These guys were the only things which saddened me when they died-they were beautiful and graceful. Not particularly hard to find if you know what you were looking for, but still not that common. I’d read up on them, and I’d hear stories of these stick insects from all over the globe which were huge. Some from Australia could get to almost two feet in length! Some would mimic scorpions as a last line of defense, but my favorite were the ones who covered themselves in spines to make themselves a rather painful meal…like these guys:

Figure 1. Photo composition of different ‘tree lobsters’ compared with a winged, canopy-dwelling stick insect. (a) Male and (b) female of D. australis, (c) male and (d ) female of Canachus alligator, (e) male and ( f ) female of Eurycantha horrida, and (g) male of Phasma gigas.

These guys are known as tree lobsters. They all look very similar. They’re big and spiny. They’re flattened top to bottom. They hang out on the ground instead of in plants. They gather in large numbers to lay their eggs in the ground. These guys look and act almost identical.

Tree Lobsters are endemic to small islands a few hundred miles east of Australia which includes Lord Howe Island, New Caledonia and such. The characteristics I listed earlier make unique amongst phasmids which normally live in the canopy of forests. Dryococelus australis is amongst the rarest animals on the planet. They were thought to be driven to extinction by rats which were introduced to the island in the 1920s. These guys have pretty formidable defenses, but that doesn’t help when your main predators eat your nymphs before they’re big enough to defend themselves. Here’s a bigger photo of those femoral spines:

Ventral view of left hind leg of ( L ) male D. australis and ( R) male Eurycantha calcarata.

Wicked, aren’t they?

A few years back some researchers found a small group of D. australis clinging to existence on a small (~200 meter wide) outcropping by the coast of Lord Howe Island. They were captured, taken to Melbourne zoo and they’re planning to release them back into the wild as soon as they clear up that rat problem. In the meantime this gives us the opportunity to study them and learn a little bit about them. So scientists looked at the mitochondrial DNA.

Bayesian phylogenetic tree showing relationships among euphasmatodean taxa and placement of the ‘ tree lobster’ ecomorphs. Branch lengths are drawn proportional to time and values above branches are Bayesian posterior probabilities. Non-monophyletic subfamilies are indicated by coloured branches according to the inset key. The tree lobster taxa are highlighted grey. Monophyletic taxa are indicated by vertical bars.

Bigger image

For the past 150 years, people thought that the Lord Howe Tree Lobsters were closely related to the New Caledonian phasmids which looked almost identical: Similar shape, similar morphology…spines in all the right places…nearly identical behavior. However, it seems that this is an example of convergent evolution on an extreme scale. Despite being nearly identical in every way, The Lord Howe Island Tree Lobster is more related to stick insects from Australia rather than the seemingly identical species on New Caledonia or New Ginuea.

Instead of evolving from one another, it’s now thought that all these tree lobsters started out as arboreal phasmids and each lineage independently island-hopped from place to place. As they landed on their respective islands, the same traits…both physical and behavioral…developed, probably to keep them from being eaten by predators.

The tree lobsters aren’t the only phasmids which underwent this type of adaptive radiation. The phasmids found in Africa, Australia and Asia show the same type of convergent evolution as do marsupials and placental mammals. Many phasmids from across the world mimic each other’s morphology and ecological roles despite the fact they’re not only on separate sides of the world, but have drastically different ancestry.

It would be nice to have some fossil phasmids to check the molecular clock and I’m sure these will be found in the future. However, the authors of the paper made an observation I found interesting. As far as we can tell, the Lord Howe Island tree lobster diverged from it’s last common ancestor on the Australian mainland about 20 million years ago, but the island they’re found on is somewhere between 6.5 and 7 million years old. Lord Howe Island was formed as a result of volcanic activity and it’s the youngest amongst a chain of seamounts. The oldest in the chain, Nova Bank is about 23 million years old which could mean that the ancestors of this guy drifted to Nova Bank from the Australian mainland and drifted from island to island as the older islands eroded behind them.

Hat tip to bug girl for mentioning this paper on her blog.

Thomas R. Buckley, Dilini Attanayake1 and Sven Bradler (2008). Extreme convergence in stick insect evolution:
phylogenetic placement of the Lord Howe
Island tree lobster Proceedings of The Royal Society