Saturday, April 4, 2009

Cicada nymph skin

Back in the beginning of September, S_ and Sam were out for a walk in the woods when they found this clinging to the side of a tree:

Tokyo must be destroyed!

Tokyo must be destroyed!

It’s not the whole bug, it’s just the skin. There was originally a Cicada inside. As far as I’ve been able to find out, we only have one species of cicada in this area, the Dog Day Cicada, Tibicen canicularis. These are the ones that make a screaming noise that sounds to me a lot like someone cutting sheet metal with a power saw. So, even though the nymph skins of all the cicadas look a lot alike, we can be pretty sure of the species of this one just because of when and where it was found.

Even though it’s just the skin, it is almost as good as having the whole bug. All the surface details are still there, you can clearly see the eyes and the fine whiskers on the face,

and even the antennae are still clearly visible and distinct.

As big (about an inch long) and scary-looking as this is, the nymphs are completely harmless. Those honking big, hooklike front legs are adapted for digging, not for carving out chunks of flesh. They actually lead very quiet lives. The nymphs hatch out from eggs laid under the bark of tree branches, and drop to the ground where they dig until they find a tree root. Once they find one, they latch onto it and start sucking on the juices.

They keep this up for a long time. Years, in fact. The dog-day cicadas stay underground for about three years, but some other species of cicada (the periodic cicadas that live further south) stay down there for seventeen years. It takes them a long time to grow this way, because tree sap is not all that nourishing, but eventually they are fat enough to dig their way to the surface and climb up the trunk of their host tree. Then they crack open their backs to emerge as adults, leaving behind their old skins for the amusement of small children, and to provide homes for other small insects. Like this earwig.

House Pseudoscorpion

What timing! I’d just gotten the camera mounted to take nice, high-magnification photos of tiny things, and then S. tells me there is a pseudoscorpion in the bathtub, just *begging* to be photographed. So, here it is:
pseudoscorpion_sharpfocus588k.jpg
Then, we flipped it over to have a look at the underside:
pseudoscorpion_on_back540k.jpg
And then, finally, zoomed in on one claw:
pseudoscorpion_clawdetail148k.jpg
Considering that this little guy was only about 3 mm long, I think the photos came out pretty sharp. We figured that this should be good enough to get a full, positive ID, so I found an identification key to the pseudoscorpions of Michigan in the MTU library[1] . . . and had my illusions pretty much dashed. It turns out that you distinguish pseudoscorpion species from each other based on very fine details of the claws, and where exactly the sensory hairs (setae) are located. Unfortunately, even though the claw detail photo is pretty good, it isn’t good enough to count setae. According to the author of the ID key, even an expert might take as much as a week to prepare, dissect, and examine a pseudoscorpion to determine the species. There are supposed to be at least 29 species in Michigan, so the next ones we find might get taken in to the lab for a more thorough examination.

While a really positive ID isn’t practical from these photos, the fact that it was in the house (and, specifically, in the tub) means it was almost certainly Chelifer cancroides, also known as the “house pseudoscorpion” or “book scorpion”. This is a “cosmopolitan” species, meaning that we have carried it around the world and it pretty much lives everywhere that humans live. They are actually very common, but are rarely seen except when they, say, venture into the bathtub to get water. They spend the rest of their time eating other, even smaller arthropods (mites, clothes moth larvae, and the like), and generally get dismissed as being harmless and of no economic impact. They are reported to live for 2-3 years, which is pretty good for such tiny creatures.

Even though they look superficially like scorpions, they are in a separate order (Pseudoscorpiones), and are about as closely related to spiders as they are to scorpions. Instead of poisoned tails, they have venom glands in their claws that they use to kill prey. Of course, at less than about 3 mm long even for the largest species, they are completely incapable of breaking the skin of a human, so their venom is irrelevant to us. Pseudoscorpions frequently get around by hitching a ride on other insects, like the one in this rather startling picture.

[1] Sigurd Nelson Jr., (1975), “A Systematic Study of Michigan Pseudoscorpionida (Arachnida)”, The American Midland Naturalist, Vol. 93, No. 2, April 1975, pp. 257-301

Friday, April 3, 2009

stone centipede

If you turn over any given rock in Michigan, you are likely to find one of these:

stonecentipedefulldorsal.jpg

This is a “stone centipede”, order Lithobiomorpha, so called because that’s what they live under[1]. They run like water, flowing around obstacles and into holes in a way that’s very much like the way a stream of water flows, and are kind of hard to catch. On top of the speed, they are slippery, and their dozens of legs are very good at forcing them through crevices, or out from between your fingers. Even their antennae have a disturbingly fluid nature, flowing over surfaces and contorting in a way that is more like what you would expect from tentacles than from antennae.

That first picture was low-magnification because the whole centipede is much too long to fit into a single high-magnification image. Here’s a few showing it in more detail:

Front end

In the picture of the head end, you can see two bulges on the sides of the head. These are the “poison claws” that they use to grab their prey. Here’s a (somewhat blurry) picture showing the claws opened and looking very ominous:

stonecentipedegrabbersopen.jpg

Getting back to the body, here is the midsection:

stonecentipedemiddle.jpg

While stone centipedes do have lots of legs, they don’t have a hundred legs. This one would have had 30 legs, except that it is missing one of the rear ones. There is one pair of legs per body segment, even though some of the legs look like they are coming from the junctions between segments. This is because the body segments are wildly variable in size, with some of them being practically nonexistent. I’m not sure why that is, I would have expected all the body segments to be about the same size.

Rear end

The last few sets of legs point almost straight back, which doesn’t seem like it would be all that helpful for walking. Maybe they are used to push forward? Mostly, it just seemed to drag them around.

While stone centipedes are reputed to be able to “bite”, once again I can’t confirm that. I was picking this one up and pushing it around, and it was doubtless pretty annoyed with me, but it didn’t let me have it with the poison claws. I understand that some of the big tropical centipedes have pretty painful bites, but as I’m sure everyone has gathered by now, Upper Michigan is hardly the tropics[2].

Anyway, stone centipedes are pretty aggressive predators, preying on the other things that live under rocks like woodlice, worms, springtails, grubs, and who knows what else. They are easily the fastest-moving things you are likely to find under a rock, and their thin body shape makes it easy for them to squeeze quickly into crevices.

stonecentipedeside.jpg

They are pretty stong, too. I had this one in my photographing dish with a glass slide over it, and it was strong enough to muscle the slide aside and crawl out. Then it was off like a shot, and I had to scramble to catch it again (this was when it had a really good opportunity to bite me, but didn’t take it). I ended up having to hold the cover slide down with my finger to keep it from getting away again.

—————-
[1] I was looking at the “categories” list on the sidebar, and noticed that while I had multiple specimens for the other major classes of arthropods I’m likely to find around here (arachnids, crustaceans, and insects), for the myriapods there was only this one, lonely millipede. So, Sam and I went out flipping over rocks to find some more myriapods to fill out the list a bit.

[2] The term “subarctic” comes to mind[3], although I guess it is officially classified as a “humid continental” climate. I know a faculty member at the University who was visiting the Soviet Union (back when there was a Soviet Union), and mentioned where we were located. The Soviet professor he was talking to was convinced that our university must be the US Government’s equivalent of Siberia for exiling politically-disgraced scientists. I dunno. Maybe it is.

[3] The Keeweenaw Peninsula was just about the last place in Michigan to come out from under the ice at the end of the last ice age: the glaciers from the “Marquette Readvance” didn’t melt off until about 9,900 years ago, several thousand years after the lower peninsula of the state had melted off completely. Even then, a lot of the U. P. was periodically inundated for the next several thousand years until the lake levels stabilized. That sounds like a long time for wildlife to recolonize the area, until one considers that wingless arthropods like centipedes had to walk about a thousand miles to get up here since the ice melted. A mile every 10 years or so doesn’t sound that hard to us, but to critters less than an inch long that can’t fly, it’s a pretty fast clip. The centipedes move fairly briskly, so they probably got here comparatively quickly (maybe 5000 years ago), but things like woodlice must have had a hard slog of it (unless they hitch-hiked on birds or something). Some of the slower creatures, like earthworms, didn’t make it up here at all until they were carried up by human activities (earthworm eggs in mud stuck to tires, or abandoned fishing bait, for example). So, even though a lot of the things that live in the soil in, say, Ohio could probably live just fine up here, they just haven’t gotten this far yet.

soil centipede

Back on May 10, this is one of the things that Sam and I found under a rock. We didn’t spot it at first, because it looked a great deal like a small plant root - at least until it moved. It’s almost two centimeters long, but less than a millimeter wide.

It’s a soil centipede, order Geophilomorpha. These are actually getting close to deserving the name “centipede” (”hundred-legger”), because for this one I count 39 pairs of legs (78 legs total). Some other species of soil centipede actually do have over a hundred legs.

An interesting point about centipedes (or at least, I think it is interesting): the different types of centipedes are actually quite distantly related to each other. Notice I said it was in the Order Geophilomorpha. The Stone Centipede that I posted a while back (and that we found under the same rock) is in the order Lithobiomorpha. Since they are in different orders, they are no more closely related to each other than, say, beetles are to butterflies. And other “myriapods”, like millipedes, are about as closely related to centipedes as they are to lobsters or spiders. I understand that the biologists currently think that the “myriapods” are pretty much what the ancestral arthropods looked like. We evidently have a bunch of distantly-related groups that, even though their last common ancestor was a long time ago, didn’t happen to evolve in a way that changed their body morphology too much. As a result, they still all look generally similar to each other even though they probably became separate groups sometime before there were dinosaurs.

Anyway, soil centipedes are generally carnivorous, and run under leaf litter and down earthworm holes to eat other small things that they find underground. These are enough smaller than the stone centipedes that I can’t really see them being any harm to anyone, and I don’t see any sign of poison claws in these pictures (although, to be fair, the heads are so small that it’s hard to see details). They evidently don’t have eyes, because, well, why would they have them? What is there to see under a rock, anyway? Normally, you probably won’t even notice these until you go specifically looking for them, but as soon as you look closely they are probably all over the place anytime you start digging.