This file is also available in Adobe Portable Document Format (PDF). 
by
Lisa Carloye
Biology Department
Elon University
This case study, based on an article by J.E. Lloyd (1965), was developed for a special topics entomology seminar for upper-level biology majors; it could also be incorporated into a general biology course as an illustration of aggressive mimicry and a lesson in scientific process and data interpretation. It would also be appropriate for an introductory ecology class or a class on animal behavior.
The case explores the intricacies of behavior in the “femme fatale” firefly—female fireflies in the genus Photuris that mimic the flash pattern of females in the genus Photinus to lure Photinus males and eat them. Students read the case, which incorporates entries in an old field notebook, and answer the questions for homework (although the journal format is fictitious, the recorded observations are genuine, and were originally made by James E. Lloyd—see References below). The instructor presents a series of handouts to students for in-class analysis and discussion in a progressive disclosure format (see: Handout 1, Handout 2, Handout 3, Handout 4, and Handout 5). The case concludes with an assignment in which students write an abstract summarizing the main points of the case (see: Homework Assignment).
Students in my class will already have been exposed to the concept of mimicry from previous coursework and I assign a relevant chapter from Waldbauer (1996) to help them prepare for this case.
This case was developed for a 90-minute class period. Students prepare by reading the case story and answering the questions for homework. When the class next meets, the instructor leads the students in a 15- to 20-minute discussion of students’ answers to the homework questions, with student-generated hypotheses written on the board.
The remaining class time is spent evaluating a series of five graphs that are handed out one at a time. Students work in groups of four to answer the questions associated with each handout, which are then discussed as a class before moving on to the next handout. I do not have students turn in answers to the handout questions.
The data presented in the five handouts and the ensuing discussion allow students to further explore the question of how this behavior evolved in female fireflies of this particular species and how the behavior might be adaptive. The graphs are taken from Eisner et al. (1997) and demonstrate that “femmes fatales” acquire not only food from their prey but also defensive compounds that protect them from predation by spiders.
After discussing the second handout, I ask students which of the student-generated hypotheses the data were gathered to test, and we then spend the rest of the class exploring the “defensive chemicals” hypothesis. I allocate about 15 to 20 minutes for the first handout and about 25 to 30 minutes total to cover the remaining four handouts.
Students often have difficulty interpreting the graphs but the group work helps them successfully work through this task. It is important to allow enough time for discussing the graphs as a class to ensure that students feel confident in their understanding of the material.
The case could be modified for a shorter class period by asking students to work through Handout 1 before coming to class and/or giving them the final graph (Handout 5) to interpret and incorporate in the abstract they write as homework (see below).
The take-home assignment, which each student completes, is to incorporate all of the data examined into a written abstract (half-page summary) for a hypothetical paper that would include all of the graphs presented in class. This abstract should emphasize the following points:
A sample abstract written by a senior undergraduate is provided in the Answer Key to this case.
Fireflies are beetles in the family Lampyridae. True to their name, most species use light to attract mates. Interestingly, the fireflies that occur in the Western United States do not have light-producing organs and, thus, most people think that fireflies only occur in the Eastern and Midwestern United States.
Firefly habitat is varied, but fireflies typically are found in grassy fields or along the brushy banks of streams and marshy areas. Fireflies undergo complete metamorphosis, and the larvae are worm-like creatures that prey on snails and scavenge dead insects. The adult diet is not fully known, but it appears that most either do not feed or sustain themselves on plant nectar.
Firefly flashing is an intricate form of communication in which the males advertise their presence to females. The females only flash when responding to the flash of a male, and they do so to indicate their location and willingness to mate. It is the males who fly around flashing their lights; females perch on a blade of grass, a twig, or low-lying branch. This flash-response system is fine-tuned and allows males and females to recognize members of their own species.
Each species has a characteristic flash pattern defined by the intensity and duration of the flash and the height and shape of the flight pattern of the males as they flash while flying. Evans (1993) includes a reproduction of a wonderful drawing showing the variety of male flash patterns seen in the genus Photinus (including the common J-shape, a rhythmic pulsing and a zig-zag pattern). This diversity and specificity of male flash patterns is suggested by the height and duration of the bumps in the graph in Handout 1. Although the females respond with a flash pattern that is different from that of the males, it is also species-specific and is characterized by the intensity and duration of the flash and the lag time before responding.
Several different species (including species in different genera) typically occur in the same habitat, and it is the distinctiveness of the flash pattern that maintains their reproductive isolation. Femme fatale fireflies are unusual in their ability to mimic the flash pattern of females of other species, and thereby trick the males.
If you have access to David Attenborough’s “Trials of Life” video series, you can show the “Finding a Mate” video, which has a segment in which Attenborough “calls” a male firefly by using a small flashlight to mimic the flash and delay of the female. I have used this visual to reinforce students’ understanding of the overall system; it’s a nice backdrop against which to discuss the unusual natural history of the femme fatale.
Students in my special topics entomology seminar felt that this case was a very useful addition to the course (rating=4.6 on a scale where 1=strongly disagree to 5=strongly agree; N=10), helped them to better understand the scientific process (rating=4), gave them valuable practice with critical thinking (rating=4.9), enhanced their ability to read graphs (rating=4.2), and enhanced their ability to interpret graphs (rating=4.1).
Answers to the questions posed in the case study are provided in a separate answer key to the case. Those answers are password-protected. To access the answers for this case, go to the key. You will be prompted for a username and password. If you have not yet registered with us, you can see whether you are eligible for an account by reviewing our password policy and then apply online or write to answerkey@sciencecases.org.
Acknowledgements: This case was developed with support from The Pew Charitable Trusts as part of a Case Studies in Science Workshop held at the University at Buffalo, State University of New York, on May 21–25, 2001.
Date Posted: 03/26/04 nas; last revised 12/26/04 nas.
This file is also available in Adobe Portable Document Format (PDF).
Originally published at http://www.sciencecases.org/fireflies/fireflies_notes.asp
Copyright © 1999–2010 by the National Center for Case Study Teaching in Science. Please see our usage guidelines, which outline our policy concerning permissible reproduction of this work.
