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1Changes in wormlion (Vermilionidae) pit construction in response to energy inputs and disturbances Luke Hillman Department of Biology, University of Wisconsin-Madi son ABSTRACT Wormlions are insects that, like antlions, are sit and wait predators in their larval stages, for whom optimizing energy expenditures is crucial because o f potentially long periods of starvation. Initial p it depth and pit growth rates of antlions have been shown to decrease in response to limited food resources. Du e to the similar foraging technique of worm and antlions wormlions were hypothesized to show similar responses. Thirty-two wormlions were randomly divid ed into four treatments: fed-disturbed (FD), fedundisturbed (FU), unfed-disturbed (UD) or unfed-und isturbed (UU). An aphid was dropped into wormlion pits of the FU and FD treatments, and pits in dist urbed treatments were overturned daily. Pit volume was calculated daily from pit measurements, plotted aga inst time and analyzed with regression equations. N o 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 extra polating pit volumes of the FU and UU treatments, t he optimum pit volume was estimated to be 5.92 cm3. Although not significant, a decrease in initial p it volume appeared to exist for unfed wormlions (t17 = 1.848, p = 0.082). The data suggest that wormlio ns have more efficient pit construction adaptations in response to shorter larval development and limited predation opportunities. RESUMEN Los gusanos len son insectos que, como las hormiga s len, son depredadores sit and wait en sus eta pas larvales, para las cuales es crucial optimizar gast os energticos debido a potenciales largos periodos del hambre. La profundidad del hoyo y las tasas de crec imiento iniciales del hoyo de las hormigas len se ha demostrado presentan una disminucin en respuesta a limitados recursos alimenticios. Debido a la tcni ca similar del forraje del gusano y de las hormigas le n, se presume que los gusanos len presentan respu estas similares. Treinta y dos gusanos len fueron dividi dos aleatoriamente en cuatro tratamientos: alimenta dodisturbado (FD), alimentado-imperturbado (FU), hamb riento-disturbado (UD) o hambriento-imperturbado (UU). Un fido fue colocado en los hoyos de los gus anos len 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 ecuaciones de regresin. No se encontr ninguna diferencia estadstica en tasas de crecimiento del hoyo (t14 = 0.939, p = 0.364) o la elevacin (t 15 = 0.0487, p = 0.962) entre los tratamientos alimentados y ham brientos. Extrapolando los volmenes del hoyo de lo s tratamientos de FU y del UU, el volumen ptimo del hoyo estimado es de 5.92 cm3. Aunque no sea significativo, una disminucin del volumen inicial del hoyo parece existir para los gusanos len hambrientos (t17 = 1.848, p = 0.082). Los datos sugieren que los gu sanos len tienen adaptaciones ms eficientes para la construccin del hoyo en respues ta a un desarrollo larval ms corto y a oportunidad es limitadas de la depredacin. INTRODUCTION
2 Relatively uncommon and unstudied, the only known p opulations of wormlions (F: Vermilionidae) are found in the United States and C osta Rica (Petersen and Baker 2006; Zumbado 2006). Both wormlions ( Vermilio spp.) and antlions ( Myrmelion spp.) exhibit sit and wait predation in their larval stages (McCl ure 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 apparen t convergent evolution of predation strategy, the very well studied ecology of antlions was used to construct my hypotheses. Optimizing energy expenditures is crucial for organ isms without steady inputs of energy. Sit and wait predators can be subject to lo ng periods of starvation, and therefore, must optimize foraging techniques to conserve energ y. Previous studies have identified an inverse correlation between food availability an d pit depth in antlions (Griffiths 1986). When pit traps of unfed antlions were disturbed dai ly, initial trap depths decreased over time suggesting that energy costs and gains determi ne trap depth (Griffiths 1986). Furthermore, Hauber (1999) reported similar data; h owever, it was concluded that the decrease in initial trap depth was not solely a con sequence of food limitation but also a result of the energy expenditures necessary for bui lding 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 similarities in predation strateg ies, food limitations and energy expenditures were hypothesized to have similar effe cts on initial pit volume and pit growth rate of wormlions. Here I provide a baseline study of wormlion pit characteristics and theories for changes in wormlion pit constructi on in response to limited energy inputs. METHODS To test this hypothesis, 32 wormlion larvae from th e Monteverde cloud forest were gathered and placed in individual containers, 13 cm in diameter and filled with four centimeters of fine grain soil as described by McCa rthy (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), fe d-undisturbed (FU), unfed-disturbed (UD) or unfed-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 usin g a spoon fifteen minutes after the insects were fed. Prior to feeding, the depth and w idth of each pit was measured with a caliper. Assuming a conical shape, pit volume was c alculated from pit depth and width measurements. Pit depth was plotted against pit width and correla tion analysis was performed. Daily pit volume averages for all treatments were g raphed against time. FU, UU and UD data were analyzed with a logarithmic regression an alysis and the FD with a linear regression. To prevent bias in pit growth rates, da y one pit volume of the FU treatment was removed to equilibrate initial average pit volu mes between FU and UU treatments. FU data collected on the second day was considered day one data with respect to the UU treatment. In addition, due an accidental disturban ce of a wormlion pit, the final value of
3 the FU treatment was omitted. Inadequate sample siz e 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 1984). The same tests were also performed on the raw data. To extrapolate the maximum pit volume, mean d aily pit volumes of FU and UU treatments were compiled, plotted on a double recip rocal plot and analyzed with a linear regression equation. To compare average daily initi al 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 increase d (r = 0.879, n = 283, p < 0.05; Fig. 1). The pit volume increased logarithmically with t ime for the FU (FU, F1, 6 = 128.7, p < 0.0001) and UU treatments (F1, 8 = 106.9, p < 0.0001; Fig 2A). There was no differe nce found between pit growth rates (t14 = 0.939, p = 0.364) or elevation (t 15 = .0487, p = 0.962) of FU and UU treatments. Even when comparing raw data, no significant differences exist between pit growth rate (t16 = 1.293, p = 0.215) and elevation (t17 = 1.848, p = 0.082) (Fig. 2B). Using a linear regress ion equation (F1, 16 = 2264.542, p < 0.001), maximum pit volume was estimated to be 5.92 cm3 (Fig. 3). Initial pit volume in the FD treatment increased ov er time (F1, 8 = 9.033, p = 0.017), whereas there was a trend for a decrease in initial pit volume of the UD treatment (F1, 5 = 4.166, p = 0.097) (Fig. 4). (Note that after ei ght days, three wormlions in the UD treatment failed to rebuild a trap, so the final sa mple size is five.) Average daily initial pit volume of the FD treatment (1.23 cm3) was significantly higher than for the UD treatment (0.85 cm3) (t6 = 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 ecologic al study and cost-benefit analysis of wormlion pit dynamics. Wormlion pit biology is apparently very similar to that of antlions. For both insects, a strong correlation exists between pit wi dth and depth. Logarithmic pit growth is also a shared characteristic (Griffiths 1986). Worm lion pit volume approaches a maximum of 5.92 cm3. According to the optimum pit size theory propose d by Griffiths (1986), this pit size maximizes net energy return. The data presented here suggest that wormlions resp ond differently to restricted energy inputs than do antlions. Increased food cons umption increases development rate of antlions (Arnett and Gotelli 1999). Wormlions ha ve a shorter larval development (11 months) than antlions (one to two years) and theref ore, should require more constant energy inputs making them more susceptible to starv ation (Zumbado 2006; Gotelli 1997). If this were the case, a greater decrease in pit gr owth rate should be observed, but this was not the case. Instead, an increased efficiency in trap building m ay explain the observed trend. If minimal energy is required for pit construction, pi t growth will not be affected by
4 starvation. Development of energy efficient adaptat ions would be strongly selected for in response to shorter larval development and limited predation opportunities. The confinement of wormlions to sheltered microhabitats may be another adaptation to avoid unnecessary energy expenditures. In the protected m icrohabitats, pit maintenance is kept to a minimum. Although there was no significant decrease in initi al 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 stu dy period. Additionally, there was a decrease in daily mean initial pit volume for the U D treatment. The decrease may be attributed to a combination of starvation and unnat ural 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 food consu mption in antlions increases insect growth (Arnett and Gotelli 1999). Constantly fed wo rmlions are likely to increase in size as well. As a result, pit size would increase becau se pit size is proportional to body size (McCarthy 2007). Preand post-experimental measure ments of wormlion length and initial pit size are needed to support this claim. Although the foraging techniques of wormlions and a ntlions are similar, the data presented here suggest wormlions respond differentl y to restricted energy inputs. Adaptations to prolonged malnourishment may have ar isen due to wormlions longer development. The same experiment with a longer stud y time and increased sample size should be conducted. Also, comparing initial pit si zes of unfed-disturbed wormlions and ones prevented from pit construction as done by Hau ber (1999) will give a greater insight into energy costs of pit construction. This study p rovides a strong base of preliminary data on wormlions and possibilities for future stud ies. ACKNOWLEDGMENTS Thanks to my supervisor Tania Chavarra for providi ng insight and direction for this project. I would also like to thank Jos Carlos Calderon, Karen Masters a nd 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 t hank Ryan Brower for editing my paper. REFERENCES Arnett, A.E. and N.J. Gotelli, 1999. Geographic var iation in life-history traits of the ant lion, Myrmeleon immaculatus : evolutionary implications of Bergmanns rule. Evolution 53: 1180-1188. Gotelli, N.J. 1997. Competition and coexistence of larval ant lions. Ecology 78: 17611773. Griffiths, D. 1986. Pit construction by ant-lion la rvae: a cost-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 Entom ology 24(1): 37-40. McCarthy, A. 2007. Selection for survival: soil gra in size preference and body size in wormlion ( Vermilio sp.) larvae affect pit trap construction. CIEE Trop ical Ecology and Conservation, Fall 2007: 100-106. McClure, M.S. 1983. Myrmeleon (Hormiga Leon, Ant Lions). In: Costa Rican Natural History. D. H. Janzen (ed). The University of Chica go Press, Chicago, IL, pp. 742. Petersen, W.H. and C.W. Baker. 2006. First record o f wormlions (Diptera: Vermilionidae) in Idaho. Journal of the Idaho Acade my of Science, on-line article. Zar, J.H. 1984. Biostatistical Analysis. Prentice-H all Press, Englewood Cliffs, NJ. Zumbado, M. A. 2006. Diptera of Costa Rica and the New World tropics. Instituto Nacional de Biodeversidad. Vermileondidae. Santo Do mingo de Heredia, Costa Rica, pp. 92-93.
6 Figure 1. Correlation between pit width and pit dep th of wormlions (r = 0.879, n = 283, p < 0.05).
7 nr n nr n n r nr n nr n n r Figure 2. Mean pit volume growth of fed and unfed w ormlions over a 10-day period: (A) Corrected data (fed, F1, 6 = 128.7, p < 0.0001; unfed, F1, 8 = 106.9, p < 0.0001), (B) Raw data.
8 n n r Figure 3. Double reciprocal plot of mean daily pit volume and time used to extrapolate maximum pit volume (F1, 16 = 2264.542, n=18, p < 0.001). Maximum pit volume ( 5.92 cm3) is equal to the inverse of the y-intercept. nr n n n n r Figure 4. Mean initial pit volume of fed (F1, 8 = 9.033, p = 0.017) and unfed (F1, 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