1 Changes in wormlion Vermilionidae pit construction in response to energy inputs and disturbances Luke Hillman Department of Biology, University of Wisconsin Madison ABSTRACT Wormlions are insects that, like antlions, are sit and wait predators in thei r larval stages, for whom optimizing energy expenditures is crucial because of potentially long periods of starvation. Initial pit depth and pit growth rates of antlions have been shown to decrease in response to limited food resources. Due to the similar foraging technique of worm and antlions, wormlions were hypothesized to show similar responses. Thirty two wormlions were randomly divided into four treatments: fed disturbed FD, fed undisturbed FU, unfed disturbed UD or unfed undisturbed UU. An ap hid was dropped into wormlion pits of the FU and FD treatments, and pits in disturbed treatments were overturned daily. Pit volume was calculated daily from pit measurements, plotted against time and analyzed with regression equations. No statistical dif ference in pit growth rates t 14 = 0.939, p = .364 or elevation t 15 = .0487, p = 0.962 was reported between fed and unfed treatments. By extrapolating pit volumes of the FU and UU treatments, the optimum pit volume was estimated to be 5.92 cm 3 . Althoug h not significant, a decrease in initial pit volume appeared to exist for unfed wormlions t 17 = 1.848, p = 0.082 . The data suggest that wormlions have more efficient pit construction adaptations in response to shorter larval development and limited preda tion opportunities. RESUMEN Los gusanos leÃ³n son insectos que, como las hormigas leÃ³n, son depredadores sit and wait en sus etapas larvales, para las cuales es crucial optimizar gastos energÃ©ticos debido a potenciales largos periodos del hambre. La p rofundidad del hoyo y las tasas de crecimiento iniciales del hoyo de las hormigas leÃ³n se ha demostrado presentan una disminuciÃ³n en respuesta a limitados recursos alimenticios. Debido a la tÃ©cnica similar del forraje del gusano y de las hormigas leÃ³n, se presume que los gusanos leÃ³n presentan respuestas similares. Treinta y dos gusanos leÃ³n fueron divididos aleatoriamente en cuatro tratamientos: alimentado disturbado FD, alimentado imperturbado FU, hambriento disturbado UD o hambriento imperturbado UU. Un Ã¡fido fue colocado en los hoyos de los gusanos leÃ³n de los tratamientos de FU y del FD, y los hoyos en tratamientos disturbados fueron volcados diariamente. El volumen del hoyo era calculado diariamente, trazado contra tiempo y analizado con ecua ciones de regresiÃ³n. No se encontrÃ³ ninguna diferencia estadÃstica en tasas de crecimiento del hoyo t 14 = 0.939, p = 0.364 o la elevaciÃ³n t 15 = 0.0487, p = 0.962 entre los tratamientos alimentados y hambrientos. Extrapolando los volÃºmenes del hoyo de los tratamientos de FU y del UU, el volumen Ã³ptimo del hoyo estimado es de 5.92 cm 3 . Aunque no sea significativo, una disminuciÃ³n del volumen inicial del hoyo parece existir para los gusanos leÃ³n hambrientos t 17 = 1.848, p = 0.082. Los datos sugieren que los gusanos leÃ³n tienen adaptaciones mÃ¡s eficientes para la construcciÃ³n del hoyo en respuesta a un desarrollo larval mÃ¡s corto y a oportunidades limitadas de la depredaciÃ³n.
2 INTRODUCTION Relatively uncommon and unstudied, the only known populations of wormlions F: Vermilionidae are found in the United States and Costa Rica Petersen and Baker 2006; Zumbado 2006. Both wormlions Vermilio spp. and antlions Myrmelion spp. exhibit sit and wait predation in their larval stages McClure 1983; Zumbado 2006. These invertebrates build conical pit traps in dry, fine soil beneath overhang objects in order to trap terrestrial invertebrates. Due to this apparent convergent evolution of predation strategy, the very well studied ecology of antlions was used t o construct my hypotheses. Optimizing energy expenditures is crucial for organisms without steady inputs of energy. Sit and wait predators can be subject to long periods of starvation, and therefore, must optimize foraging techniques to conserve energy. Pr evious studies have identified an inverse correlation between food availability and pit depth in antlions Griffiths 1986. When pit traps of unfed antlions were disturbed daily, initial trap depths decreased over time suggesting that energy costs and gai ns determine trap depth Griffiths 1986. Furthermore, Hauber 1999 reported similar data; however, it was concluded that the decrease in initial trap depth was not solely a consequence of food limitation but also a result of the energy expenditures neces sary for building a trap de novo. Trap measurements also revealed a decrease in pit growth rate in response to starvation Griffiths 1986. The data further support the idea that energy inputs and expenditures determine pit size. Therefore, due to similari ties in predation strategies, food limitations and energy expenditures were hypothesized to have similar effects on initial pit volume and pit growth rate of wormlions. Here I provide a baseline study of wormlion pit characteristics and theories for change s in wormlion pit construction in response to limited energy inputs. METHODS To test this hypothesis, 32 wormlion larvae from the Monteverde cloud forest were gathered and placed in individual containers, 13 cm in diameter and filled with four centimeter s of fine grain soil as described by McCarthy 2007. The soil was gathered from the sites where the wormlions were collected. Wormlions were randomly placed into one of four treatments: fed disturbed FD, fed undisturbed FU, unfed disturbed UD or unf ed undisturbed UU with eight wormlions per treatment. Each morning, one aphid family Aphididae was dropped into each pit of the FD and FU treatments. Disturbed wormlion pits were destroyed daily using a spoon fifteen minutes after the insects were fe d. Prior to feeding, the depth and width of each pit was measured with a caliper. Assuming a conical shape, pit volume was calculated from pit depth and width measurements. Pit depth was plotted against pit width and correlation analysis was performed. D aily pit volume averages for all treatments were graphed against time. FU, UU and UD data were analyzed with a logarithmic regression analysis and the FD with a linear regression. To prevent bias in pit growth rates, day one pit volume of the FU treatment was removed to equilibrate initial average pit volumes between FU and UU treatments. FU data collected on the second day was considered day one data with respect to the UU
3 treatment. In addition, due an accidental disturbance of a wormlion pit, the final v alue of the FU treatment was omitted. Inadequate sample size as a result of wormlion deaths limited analysis of the UD treatment to seven days. Modified t tests were used to compare corrected pit growth rate and elevation of the FU and UU treatments Zar 1 984. The same tests were also performed on the raw data. To extrapolate the maximum pit volume, mean daily pit volumes of FU and UU treatments were compiled, plotted on a double reciprocal plot and analyzed with a linear regression equation. To compare av erage daily initial pit volumes of the FD and UD treatments, a student t test was performed between the FD and UD treatments. RESULTS The pit width increased when the pit depth increased r = 0.879 , n = 283, p < 0.05; Fig. 1. The pit volume increased l ogarithmically with time for the FU FU, F 1, 6 = 128.7, p < 0.0001 and UU treatments F 1, 8 = 106.9, p < 0.0001; Fig 2A . There was no difference found between pit growth rates t 14 = 0.939, p = 0.364 or elevation t 15 = .0487, p = 0.962 of FU and UU t reatments. Even when comparing raw data, no significant differences exist between pit growth rate t 16 = 1.293, p = 0.215 and elevation t 17 = 1.848, p = 0.082 Fig. 2B. Using a linear regression equation F 1, 16 = 2264.542, p < 0.001, maximum pit vol ume was estimated to be 5.92 cm 3 Fig. 3. Initial pit volume in the FD treatment increased over time F 1, 8 = 9.033, p = 0.017 , whereas there was a trend for a decrease in initial pit volume of the UD treatment F 1, 5 = 4.166, p = 0.097 Fig. 4. Not e that after eight days, three wormlions in the UD treatment failed to rebuild a trap, so the final sample size is five. Average daily initial pit volume of the FD treatment 1.23 cm 3 was significantly higher than for the UD treatment 0.85 cm 3 t 6 = 2 .691, p = 0.036. DISCUSSION Energy conservation is critical for survival of sit and wait predators such as wormlions. The data presented here provide a baseline ecological study and cost benefit analysis of wormlion pit dynamics. Wormlion pit biology i s apparently very similar to that of antlions. For both insects, a strong correlation exists between pit width and depth. Logarithmic pit growth is also a shared c haracteristic Griffiths 1986. Wormlion pit volume approaches a maximum of 5.92 cm 3 . Accord ing to the optimum pit size theory proposed by Griffiths 1986, this pit size maximizes net energy return. The data presented here suggest that wormlions respond differently to restricted energy inputs than do antlions. Increased food consumption increas es development rate of antlions Arnett and Gotelli 1999. Wormlions have a shorter larval development 11 months than antlions one to two years and therefore, should require more constant energy inputs making them more susceptible to starvation Zumbad o 2006; Gotelli 1997. If this were the case, a greater decrease in pit growth rate should be observed, but this was not the case.
4 Instead, an increased efficiency in trap building may explain the observed trend. If minimal energy is required for pit cons truction, pit growth will not be affected by starvation. Development of energy efficient adaptations would be strongly selected for in response to shorter larval development and limited predation opportunities. The confinement of wormlions to sheltered mic rohabitats may be another adaptation to avoid unnecessary energy expenditures. In the protected microhabitats, pit maintenance is kept to a minimum. Although there was no significant decrease in initial pit volume in the UD treatment, a negative trend was found. The failure of three wormlions to rebuild a pit after starvation further supports this trend. These data would most likely be more conclusive with a larger sample size and longer study period. Additionally, there was a decrease in daily mean initia l pit volume for the UD treatment. The decrease may be attributed to a combination of starvation and unnatural pit disturbances. The slight increase in initial pit volume of the FD treatment is most likely due to an increase in worm lion size. Increased f ood consumption in antlions increases insect growth Arnett and Gotelli 1999. Constantly fed wormlions are likely to increase in size as well. As a result, pit size would increase because pit size is proportional to body size McCarthy 2007. Pre and pos t experimental measurements of wormlion length and initial pit size are needed to support this claim. Although the foraging techniques of wormlions and antlions are similar, the data presented here suggest wormlions respond differently to restricted energy inputs. Adaptations to prolonged malnourishment may have arisen due to wormlions longer development. The same experiment with a longer study time and increased sample size should be conducted. Also, comparing initial pit sizes of unfed disturbed wormlion s and ones prevented from pit construction as done by Hauber 1999 will give a greater insight into energy costs of pit construction. This study provides a strong base of preliminary data on wormlions and possibilities for future studies. ACKNOWLEDGMENTS Thanks to my supervisor Tania ChavarrÃa for providing insight and direction for this project. I would also like to thank JosÃ© Carlos Calderon, Karen Masters and Pablo Allen Monge for providing me with ideas, background information, literature sources and help with statistical analysis. Thanks to Jenny Gaynor for help in data collection. Finally, I would like to thank Ryan Brower for editing my paper. REFERENCES Arnett, A.E. and N.J. Gotelli, 1999. Geographic variation in life history traits of the ant lion, Myrmeleon immaculatus : evolutionary implications of Bergmann s rule. Evolution 53: 1180 1188. Gotelli, N.J. 1997. Competition and coexistence of larval ant lions. Ecology 78: 1761 1773. Griffiths, D. 1986. Pit construction by ant lion larvae: a co st benefit analysis. Journal of Animal Ecology 55: 37 59.
5 Hauber, M.E. 1999. Variation in pit size of antlion Myrmeleon carolinus larvae: the importance of pit construction. Physiological Entomology 241: 37 40. McCarthy, A. 2007. Selection for surviv al: soil grain size preference and body size in wormlion Vermilio sp. larvae affect pit trap construction. CIEE Tropical Ecology and Conservation, Fall 2007: 100 106. McClure, M.S. 1983. Myrmeleon Hormiga Leon, Ant Lions. In: Costa Rican Natural Histo ry. D. H. Janzen ed. The University of Chicago Press, Chicago, IL, pp. 742. Petersen, W.H. and C.W. Baker. 2006. First record of wormlions Diptera: Vermilionidae in Idaho. Journal of the Idaho Academy of Science, on line article. Zar, J.H. 1984. Bios tatistical Analysis. Prentice Hall Press, Englewood Cliffs, NJ. Zumbado, M. A. 2006. Diptera of Costa Rica and the New World tropics. Instituto Nacional de Biodeversidad. Vermileondidae. Santo Domingo de Heredia, Costa Rica, pp. 92 93.
6 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 0 0.5 1 1.5 2 2.5 3 Pit Depth Pit Width Figure 1 . Correlation between pit width and pit depth of wormlions r = 0.879 , n = 283, p < 0.05 .
7 V = 1.2475Lnt + 2.1242 R 2 = 0.9767 V = 1.2962Lnt + 1.9716 R 2 = 0.9894 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 0 1 2 3 4 5 6 7 8 9 10 Day Pit Volume cm^3 Fed Unfed V = 1.2475Lnt + 2.1242 R 2 = 0.9767 V = 1.5269Lnt + 1.177 R 2 = 0.9477 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 0 1 2 3 4 5 6 7 8 9 10 Day Pit Volume cm^3 Fed Unfed Figure 2. Mean pit volume growth of fed and unfed wormlions over a 10 day period: A Corrected data fed, F 1, 6 = 128.7, p < 0.0001; unfed, F 1, 8 = 106.9, p < 0.0001 , B Raw data. A B
8 y = 0.3535x + 0.1688 R 2 = 0.993 0 0.1 0.2 0.3 0.4 0.5 0.6 0 0.25 0.5 0.75 1 1/Time 1/Volume Figure 3. Double reciprocal plot of mean daily pit vol ume and time used to extrapolate maximum pit volume F 1, 16 = 2264.542, n=18, p < 0.001 . Maximum pit volume 5.92 cm 3 is equal to the inverse of the y intercept. V = -0.1485Lnt + 1.0293 R 2 = 0.4936 V = 0.0557t + 0.9214 R 2 = 0.5303 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 0 1 2 3 4 5 6 7 8 9 10 Day Pit Volume cm^3 Fed Unfed Figure 4. Mean initial pit volume of fed F 1, 8 = 9.033, p = 0. 017 and unfed F 1, 5 = 4.166, p = 0.097 wormlions over a 10 day period in which wormlion holes were disturbed daily.
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Cambios en la construccin de hoyos por los gusanos len (Vermilionidae) en respuesta a los insumos de energa y alteraciones.
Changes in wormlion (Vermilionidae) pit construction in response to energy inputs and disturbances
Wormlions are insects that, like antlions, are sit and wait predators in their larval stages, for whom optimizing energy expenditures is crucial because of potentially long periods of starvation. Initial pit depth and pit growth rates of antlions have been shown to decrease in response to limited food resources. Due to
the similar foraging technique of worm and antlions, wormlions were hypothesized to show similar responses. Thirty-two wormlions were randomly divided into four treatments: fed-disturbed (FD), fed-undisturbed
(FU), unfed-disturbed (UD) or unfed-undisturbed (UU). An aphid was dropped into wormlion pits of the FU and FD treatments, and pits in disturbed treatments were overturned daily. Pit volume was calculated daily from pit measurements, plotted against time and analyzed with regression equations. No statistical difference in pit growth rates (t14 = 0.939, p = .364) or elevation (t 15 = .0487, p = 0.962) was reported between fed and unfed treatments. By extrapolating pit volumes of the FU and UU treatments, the optimum pit volume was estimated to be 5.92 cm3. Although not significant, a decrease in initial pit volume
appeared to exist for unfed wormlions (t17 = 1.848, p = 0.082). The data suggest that wormlions have more efficient pit construction adaptations in response to shorter larval development and limited predation opportunities.
Los gusanos len son insectos que al igual que las hormigas len, son depredadores que se sientan y esperan en sus etapas larvales, para las cuales es crucial optimizar gastos energticos debido a los periodos potencialmente largos de hambre. Se ha demostrado que la profundidad del hoyo inicial y las tasas de crecimiento de las hormigas len disminuyen en respuesta a los limitados recursos alimenticios. Debido a la tcnica similar de alimentacin del gusano y las hormigas len, se presume que los gusanos len muestran respuestas similares.
Text in English.
Insects--Behavior--Costa Rica--Puntarenas--Monteverde Zone
Insectos--Comportamiento--Costa Rica--Puntarenas--Zona de Monteverde
Tropical Ecology 2008
Wormlions pit construction
Ecologa Tropical 2008
Construccin de hoyos por los gusanos len
t Monteverde Institute : Tropical Ecology