Cicada Sex Education
Background
…for both activities
Brood X is coming. This year in the eastern United States from Illinois to New York billions of cicadas are expected to emerge en masse. These insects have spent the last 17 years underground, tunneling and drinking sap from tree roots roughly eight feet beneath the surface.
Following their reproductive cycle, they will emerge from the ground to break out of their exoskeletons, take their adult form, and find mates. One of the most noticeable things about this mass insect emergence will be the loud, buzzing mating call emitted by male cicadas. Large numbers of males will often sync their calls in a chorus that is anything but subtle. Together, the volume of their mating calls can reach 96 decibels, roughly the level of noise created by a passing subway train. This impressive phenomenon won’t last long as adult cicadas will only live 4-6 weeks above ground before completing their life cycle.
This natural spectacle is completely centered around male and female cicadas emerging to mate. You can sex cicadas (tell females from males) relatively easily by turning them over to observe their undersides and looking at structures that differ between the sexes.
The terminal segment of the male cicada is dome-shaped and the tymbal organs (sometimes called drummers), which produce the mating call, are visible just beneath the wings.
The female cicada has oval-shaped reproductive structures that come to a distinct point at the end. Within this point is a hollow needle-like structure called the ovipositor, which can cut a slit into a tree or shrub to deposit her eggs in.
Hands-On Activity
(outdoor activity)
This activity will require a DataClassroom license. You can start a 90-day free trial of DataClassroom anytime to try it out. This activity is suitable if your students live in an area that will experience a large emergence of cicadas.
What is the size difference between male and female cicadas in your area?
The emergence of the cicadas presents a great opportunity to get outside and collect some data from a wild insect population. Cicadas are relatively large and easy to observe, easy to collect alive, and completely harmless to handle.
Cicadas, like a lot of other animals, have clear differences in features between males and females. The degree of difference in body size between males and females, or sexual size dimorphism, varies between cicada species and populations. In this hands-on activity, students will collect ~20 live cicadas and for each individual, record the sex as male or female and measure the head width as the distance in millimeters between the eyes.
Materials list:
10-20 Container(s) for temporarily housing cicadas. Mason jars with holes punched in the lid work well, but any similar-sized plastic container will work just as well.
Calipers for measuring in millimeters or ruler with millimeters
Datasheet to record data
Computer with an internet connection (for analysis on DataClassroom)
Procedure:
Finding Cicadas
The typical head width for Cicadas is 7-10mm
If you are in an area that has a large emergence of 17-year cicadas, they should not be difficult to find. You can look in any public greenspace that has trees. Completely wooded areas with a lot of undergrowth are not ideal because cicadas will be harder to find. Parks and other well-maintained areas with trees are more suitable (this helpful website recommends cemeteries). Areas with older trees will have more cicadas.
Look for the empty exoskeletons that larval cicadas leave behind when they eclose as adults. Larval cicadas molt soon after emerging from the ground and climbing up, so look for these empty exoskeletons mostly from just above eye level and lower on tree trunks.
You want to find cicadas just as they have finished molting. Once their wings dry completely they will be able to fly and will be difficult to catch without a butterfly net. Once the cicadas begin emerging it is best to look at times when the moisture in the air is relatively high (after rains, early morning, evening). Persistence and patience are key and searching areas where you find a lot of empty exoskeletons should have you finding cicadas.
Handling and Measuring Cicadas
When you find a cicada you should wait until it is finished molting (this can take an hour) before you collect it. Emerging adult cicadas will have a soft exoskeleton and are delicate until they dry and the new exoskeleton hardens. Handle them with care to avoid hurting them. You should set up each of your temporary housing containers with a stick for cicadas to perch on.
When picking up a cicada, grasp it between your thumb and forefinger while gently holding its wing against its body. Carefully place it in your glass or plastic container. Be sure to keep containers in the shade so as not to overheat the cicadas in your temporary care.
For each cicada, sex it as male or female (see background above) and measure its head width as the distance between its eyes in millimeters. Be sure to measure carefully with your calipers or ruler. Be as precise as possible and round to the nearest millimeter if using a ruler. Record the data for each cicada in your data sheet.
Recording your data
Example dataset in tidy format.
Create a tidy data table with your data. Each row in the data table represents a single cicada. The first row must contain your column headers with the name of each variable. You should create three columns for Cicada ID#, Sex (male or female), and Head Width. Cicada ID# is info, Sex is a categorical variable, and Head Width is a numeric variable.
Upload your data to DataClassroom
Bring your data into DataClassroom from either a Google Sheet or Excel file. You can also create a data table directly in DataClassroom. Use the DataClassroom checklists for data Preparation to prepare your data for analysis.
Analysis Questions
1) Create a graph comparing the head width of males and females.
Click the scatter plot icon to make a dot plot. Show Sex (categorical) on the X axis and Head Width (numeric) on the Y axis. Use the check box that appears at the right to add Descriptive Stats to your graph.
2 ) What is the typical size difference in head width between male and female cicadas in your area?
Use your graph and descriptive stats to support your answer.
3) Looking at your graph, why do you think scientists collect many data points (use replication) to answer a question?
4) Compare your data to the Cicada Sex Education dataset in the DataClassroom Resource Library. Does the size difference between males and females (in terms of head width) that you observed look similar to the difference for any of the three species groups in the dataset?
Online Activity
(Digital activity)
This activity can be done as a complement to the hands-on activity or as a stand-alone activity done for free completely within the DataClassroom web-app.
Which geographic factors affect the body size of male and female cicadas?
Background
Body size is known to vary across closely related species and even between different populations of the same species. This is the case with cicadas.
Seven cicada species of the genus Magicicada have been grouped into three closely related groups called the Decim, Cassini, and Decula groups.
In all these groups, females are known to be larger than males.
Dataset
To compare body sizes of the three species groups of cicadas, researchers from Japan and the United States set out to measure adult cicadas across North America. They measured 4,719 cicadas in total from different locations. For each individual, they measured the head width as the distance between the eyes. They also recorded information on the latitude, altitude, and climate where each cicada was collected in order to look for correlations with body size. Such correlations can suggest which geographic factors might be important in the evolution of a larger or smaller body size. Each row in the dataset represents the mean measurement for a single-sex (male or female) within a sampled population.
Variables
Sex- This categorical variable has two values, female or male. This is easily determined in the field by observing the underside of each cicada.
Head Width (mm)- This numeric variable is measured as the distance between the outer edges of the eyes. The researchers measured this using digital Vernier calipers. Each value in this column is a mean calculated from 2-50 same-sex individuals within a population.
Species Group- This categorical variable indicates which species group the observed cicada belongs to. It has three values, Decim, Cassini, or Decula.
Latitude (ºN)- This numeric variable is the latitude in degrees North of the sampled population.
Altitude (m)- This numeric variable is the altitude in meters above sea level of the sampled population.
Annual Mean Temp (ºC)- This numeric variable is the average annual temperature at the location of population sampling.
Activity
1) What is the range (highest and lowest values) of head width measurements for the more than 4,000 cicadas measured in this study?
To answer this question, go to the graph window and Show Head Width (mm) on the Y-axis of a dot plot. Add Descriptive stats with the check box that appears to the right of your graph. Show the box and whiskers plot next to the dot plot.
2) Which species group has the largest head width on average? Which has the smallest?
To answer this question, build on the graph you made for #1. Keeping Head Width (mm) on the Y axis, Show Species group on the X-axis. You can switch from Median Based descriptive stats to Mean based on the panel to the right of your graph to see means. The Values check box will show you the calculated means for each species group on the X-axis
3) Which species has the largest sex difference in head width?
To answer this question Show Sex on Z to color code by male and female data points. Then check the box that appears for Group by Z to split the descriptive stats out by Species Group and Sex.
4) Do any of the geographic factors in this dataset (latitude, altitude, temperature) show a relationship with head width?
To check for this you will need to make three scatter plots each with Head Width (mm) on the Y-axis. On the X-axis you will need to place Latitude, Altitude, and Annual Mean Temp in each respective graph. You can add a regression line with the check box that appears to the right of your scatter plot to help visualize any relationships.
5) How might you explain the relationships between geographic factors and cicada head width that you observed in your answer to #4?
Original study by Koyama et al. in the Journal of Evolutionary Biology:
Full dataset with additional variables. This is available from the Dryad public data repository and can be uploaded to DataClassroom for further exploration with minimal formatting.
DataClassroom raw dataset. This dataset is the example dataset to show what student data might look like.