When Ecdysis in Cicadas goes wrong.
News Category: Cicadas 101
When Ecdysis in Cicadas goes wrong.
I'd like to discuss in depth about the cicada's final molting process. While not unusual, as most arthropods go through a molting process in one form or another, the mechanics of the process (also known as "ecdysis") is quite fascinating.
About cicadas and their development cycle.
Female cicadas lay eggs in thin branches of trees and bushes. The type of species usually is indicative of whether the branches are live or dead wood. Some genera - like Tibicen females - generally lay their eggs in dead branches. Some Tibicen females will even lay eggs in other wood substrates like 2 x 4's that make up your patio deck or fence posts and even furniture. They have also been documented laying eggs in thick tree bark.
Other genera, like Magicicada and Okanagana prefer live wood.
When the cicada hatches from egg it is what is known as a first instar nymph. At this stage it drops to the ground and manages to work its way beneath the surface where it attaches itself to the roots of small plants or grasses and begins to feed.
An instar is a single stage in the development cycle of arthropods such as insects and precedes an instance of ecdysis. After ecdysis is performed, the next instar stage is achieved. Cicadas will complete 5 instars during their development. Between each stage they will continue to feed on the roots of various plants and trees. During each ecdysis/instar stage, the nymph grows larger but their form and structure doesn't change much so ecdysis is not a true a metamorphosis.
Once the nymph reaches the 5th instar it is time to emerge from the ground. This usually happens from spring into summer depending upon species and soil temperatures.
The time it takes for a cicada nymph to complete all 5 instar stages is also species dependent. In the case of Magicicadas, the development is "synchronized" with siblings of the same brood. This synchronized development is so well documented that accurate predictions regarding emergences can be made and why Magicicadas are considered "periodical". In Magicicadas, this development can be set by the number of years it takes for them to emerge. In the case of southern Magicicada broods, it is 13 years and in northern broods it is 17 years.
Predictability of emergence of non-periodical cicadas (also known as "annual" cicadas), is not as reliably known. It is believed that Annual cicadas of the same species when hatched from egg in the same year actually develop underground at different rates so their development cycles are not synchronized. Unlike in Magicicadas their rates of development overlap. That is why we experience the same species of Annual cicadas year after year.
However, some locations may experience an abundance of a particular species of Annual cicada in a single year and see fewer of that same species the following year but they would still be present.
In the spring and summer of a nymph in the 5th instar stage is when it emerges from the soil and goes through one final ecdysis process. The objective here is for the cicada to reach full maturity. This also spells the "beginning of the end" for a cicada's life because when it reaches adulthood, males sing for females for the purposes of mating and continue the propagation of the species.
Technically speaking, it has been noted in some articles that the final adult form - i.e.; with wings fully formed, can be considered a "6th" instar stage. I like to think of the final 5th instar nymph to adult form as actually the point where the cicada goes through a "true" metamorphosis because behavior, physical appearance and habitat preferences change dramatically.
For example, cicadas live most of their lives underground as juveniles and move to above ground living as an adult. Males and females develop wings to fly and escape predators and males develop the ability to sing as adults.
As discussed in a previous article about cicada deformities sometimes for no apparent reason, the process of ecdysis can go horribly wrong.
With the remainder of this article we will focus on a particular Tibicen lyricen cicada nymph and demonstrate one of many scenarios where the process fails.
The rest of this article is heavily-ladened with images. In order to get the best out of this article. Please view each individual slide show in each section heading and after wards read the corresponding text paragraphs.
From the Soil to Molting - Phase 1
The above photos show that we have what appears to be a normal looking Tibicen lyricen nymph at 5th instar stage. This particular specimen was taken in July of 2011 while doing survey work on Tibicen lyricen's northern-most range in New England.
When emerging from beneath the soil, the cicada nymph will seek out a tree or other surface in which to settle down to complete its final ecdysis process. In order to better observe this process, I was able to transport this nymph home in order to take photos in a controlled environment.
Cicada Ecdysis - The Early Stages - Phase 2
Once the cicada settles down, a crack forms length-wise along the nymph's mesothorax through which it begins to emerge. The part of the thorax known as the mesonotum in the cicada adult is first to come through followed by the pronotum and then the head. This part of ecdysis is hazardous. If the head fails to come free, then ecdysis is over and the cicada will be unable to complete the process and live for several days with its head stuck in its nymphal shell.
Fortunately, in this instance you can see that freeing the head was successful and the cicada slowly begins to wriggle free of its nymphal skin.
Cicada Ecdysis - Mid-stage - Phase 3
Up to this point everything seems to be going as planned and we have what appears to be a normal ecdysis process. But as you can see from the photos above, there definitely seems to be something not quite right. A strange blue fluid begins to emerge from the tips of the two front forelegs.
Upon closer inspection we can see that the very ends of the forelegs where the tarsi followed by the claws should be are missing. The missing parts of the forelegs will prove to be detrimental to the cicada during sclerotization later on once ecdysis is complete.
In addition to problems with the front forelegs we see that the mouth parts are not fully formed. We see that the internal parts of the mouth are fully exposed along with the outer "sheath" or labrum did not emerge from the nymphal skin.
A closer comparison
When we compare our troubled specimen with that of a normal Tibicen lyricen specimen we can begin to distinguish the problems. With our normal specimen, all parts of the forelegs are present whereas our problem specimen is missing portions of its forelegs.
The two long brown processes in the problem specimen are exposed while in the normal Tibicen lyricen specimen they are fully enclosed.
The two exposed brown processes form the internal stylet which is used to penetrate the xylem of plants for feeding. Since they are not fully encased in the labrum, our specimen will not feed successfully and will more than likely starve to death.
Cicada Ecdysis - Mid-stage - Phase 4
Approximately 45 minutes to 1 hour has passed between phase 3 and phase 4. During this time, the newly emerged teneral waits for its legs to schlerotize sufficiently enough in order to pull itself from its nymphal shell and to support its own weight.
In the pictorial series above we can see that since the front forelegs are missing the crucial tarsi and claws, the cicada struggles to free itself.
We also see that with the passage of time the clear blue fluid - known as "hemolymph" - or insect blood - has begun to turn milky. This is due to exposure to atmosphere and like regular blood it starts to congeal.
Hemolymph is a fluid that fills the internal spaces of arthropods. It can be considered "blood" and like mammalian blood, it is crucial for an insect's survival. Hemolymph contains water, carbohydrates, proteins and other organic compounds. It is pumped throughout an insect's body through various valves in an open circulatory system.
Unlike mammalian blood which is contained in a closed circulatory system and is iron based, hemolymph in insects is copper-based which, when exposed to oxygen causes it to turn blue.
During ecdysis hemolymph is very important and helps the insect to schlerotize properly.
In the above photos, as our specimen emerged, some hemolymph drops spilled onto the table and over time, the hemolymph drop turned greyish-black and becomes quite viscous.
Cicada Ecdysis - Final Stage
Now that the specimen has completely freed itself from the exuvia, a great amount of time and energy is expended to allow the cicada's wings to fully expand and to take on its more familiar shape. Eventually as the wings form fully, they will fold in and be placed "roof-like" over the body. Everything starts out normally but as previously mentioned, because the tarsi and claws are missing our specimen cannot maintain a sufficient grip on the branch and slowly begins to move downward towards the base of the branch.
Note how the specimen is hanging on with the mid and hind legs. The front forelegs have proved to be useless. (See the final photo above in the series). During this teneral stage, proper wing formation is essential. The wings are as delicate as tissue paper and when the wings come into contact with a surface, they have warped out of shape. When the wings turn membranous and rigid our specimen will be condemned to a short life without flight. This will make it extremely vulnerable to predation.
So, what exactly went wrong here? We have only touched on one scenario in a possible hundred scenarios where ecdysis goes wrong. Anything from injury to disease or possibly poor nutrition can cause catastrophic results.
In a nutshell, as the cicada prepares for ecdysis it becomes inactive. During this time a digesting fluid is secreted which causes the separation of the old cuticle from its underlying new layer. Obviously, this fluid must evenly distribute between the old cuticle and the underlying layer.
With an increase in blood-pressure the crack forms along the back of the specimen. It is probably this mechanism along with the uneven distribution of the digesting fluid that - when it begins to crawl out through the crack - that caused the forelegs to "rip" out of the old nymphal shell. Also for certain outer portions of its mouth parts to be left behind.
All other malformations experienced by the specimen, i.e.; the deformations of its wings were ancillary.
I hope you enjoyed this article. It took several weeks to write. As to the specimen, as expected it really didn't last long. It stayed alive for approximately two days then died. That doesn't mean that its life was a fruitless one. It did provide valuable information as a study specimen and also provided important distribution data and is now pinned and labeled as a voucher specimen for the state of New Hampshire.
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