Arrival of the dog-days

I’ve noticed a couple of these old, empty husks clinging to vegetation at our new home over the last week. They’re the empty exoskeletons of cicadas that have metamorphosed into adults. Cicadas are somewhat grotesque, as bugs go, with large eyeballs set wide on a broad head, a thick thorax and abdomen, and large, clear wings with strong veination. These larval exoskeletons are even more so. Despite their appearance, however, cicadas are harmless to humans, and they have no interest in crops so aren’t considered a commercial pest, either.

Mostly they’re just loud. They’re the insects that make the long, sustained buzz that we tend to associate with the hot, dry, and sunny days at the height of summer. When you’re close to the cicada while it’s making this noise the sound can be as much as 120 dB – that’s louder than a jackhammer! Only the males make the noise. Most insects, like crickets, make noise by rubbing two body parts together, often the hind legs or wings. Cicadas are different in that they use a built-in mechanism to make noise. They have a set of tympanic membranes on the sides of their abdomens called “tymbals”. The tymbals have muscles that connect to their inner surface that pull the membrane inwards when they contract. This produces a clicking sound. As the muscle is relaxed and the membrane returns to its normal position, another click is heard. By doing this extremely fast and many times over, the cicada produces the buzz we associate with them. The male’s abdomen is also mostly hollow, which acts as a resonating chamber to increase the volume of the sound. Cicadas “hear” through a membrane mechanism much like a frog’s ears, which could be damaged by the loud noise he makes with his abdominal tymbals, so the male “turns them off” while he’s calling.

There are two types of cicadas. The most famous are the periodical cicadas, the ones that are on 17- or 13-year cycles and come out en masse in huge swarms in a single summer. This strategy developed to make it difficult for a predator to specialize on the adults of the species; since both 13 and 17 are prime numbers, a predator specialist would also have to be on a 13 or 17 year cycle (if the cicadas were on a 15-year cycle, for instance, a 5-year cycle predator could sync up with them easily).

The other cicada species have much shorter life cycles, usually between two and five years, and adults are present every year, giving them the name annual cicadas. Despite that I would have to have lived through at least one periodical cicada emergence, I’ve never observed it firsthand. Annual ciciadas, on the other hand, I’ve observed regularly. As I was returning to the house from the dock this morning I happened to come across this one, still in larval stage, climbing up the side of a tarp.

He looked like he’d come straight out of the sand. Cicadas spend virtually all of their lives underground. The adult females lay their eggs in the twigs and branches of trees, and the larvae, once they hatch, drop to the ground and burrow into the soil. They feed on the sap from the roots of the trees. Their front legs are broad and strong, adapted for digging, with well-developed hooks on the ends.

After several years the larval cicada finally reaches a size where it’s ready to metamorphose. It climbs out of the dirt and up onto a plant or the trunk of a tree. In the case of the cicada I found, it had decided the tarp provided a suitable substrate. The cicada sets its hooked feet firmly, then splits open the skin on the back of the head and thorax and pushes its way out. Cicadas undergo incomplete metamorphosis, meaning that they don’t have a pupal stage the way butterflies and many other insects do. Instead, they go directly from their final larval stage to the adult, crawling out of their skin in their final adult form. It comes out pale; as the insect’s exoskeleton hardens, it will darken. The adult will eventually be olive green and black.

I figured that this one was looking to find a spot to metamorphose, and I was right. I sat down and watched most of the insect’s emergence. When it was mostly out it sat and recovered some energy for a while. It stayed in that position for at least 15 minutes; when I eventually decided that it wasn’t going to be doing anything imminently I went inside to get lunch started, as Blackburnian and I had plans to go out this afternoon. I check on it once while waiting for one side of a grilled cheese sandwich to brown, and it hadn’t moved. However, in the space of time it took to butter another sandwich and put it in the frying pan, the cicada had emerged completely and unfurled its wings.

It was an interesting process to watch, because I could see the insect, while still in its larval skin, shifting to make sure its claws were firmly dug into the substrate, and then appearing to apply pressure to its back to split the skin. Once it was starting to emerge I could see it working to pump haemolymph, the insect’s equivalent to blood, into the soft wings and body parts. It would have been neat to capture some of it on video, or to take a time-lapse series of the process.

However, I did my best to get a series of photos of the emergence of the cicada, start to finish. Photos below of the process. You can see in a few of them the long, tube-like piercer the insect uses to suck sap from trees. I believe this is a Dog-day Cicada, Tibicen canicularis.

And all that’s left is the empty shell.

So I guess if the Dog-day Cicadas are emerging, that means the dog days of summer must be officially arrived.