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Efectos de la saciedad sobre la estructura de las trampas de pozo en la larva de hormiga len Myrmeleon crudelis (Neuroptera Myrmeleontidae)
Effects of satiation on pit-trap structure in the larval antlion Myrmeleon crudelis (Neuroptera Myrmeleontidae)
Sit-and-wait predators have many strategies to deal with food scarcity. Some predators become more active, while others conserve energy by becoming more sedentary. I decided to test the effects that starvation has on the larval antlion Myrmeleon crudelis. Myrmeleon crudelis build pit-traps, conical structures built in sand, to capture prey.
These pit-traps vary in diameter, depth, and angle for each larva. Seven individuals in one group were fed every other day, while seven other individuals were starved for all ten days this study took place. The pit-traps of both groups were then compared to each other. The results show that as larval size and feeding frequency increase so do the diameter, depth, and angle of pit-traps. These results suggest that Myrmeleon crudelis chooses to become less active in the presence of food scarcity.
Los depredadores que se sientan y esperan tienen muchas estrategias para lidiar con la escasez de alimento. Algunos depredadores se vuelven ms activos mientras los otros conservan su energa volvindose sedentarios. Yo decid probar el efecto que tiene el hambre en la larva de la hormiga len Myrmeleon crudelis. Myrmeleon crudelis construye trampas de pozo que son estructuras cnicas en la arena para capturar a sus presas. Estas trampas varan en dimetro, profundidad y ngulo entre los individuos. Siete individuos en un grupo fueron alimentados cada dos das, mientras que los otros siete individuos estuvieron en ayunas durante los diez das que se llev a cabo este estudio. Las trampas fueron comparadas entre los grupos. Los resultados demuestran que a medida que incrementa el tamao de las larvas y aumenta la frecuencia de la alimentacin, tambin incrementa el dimetro, la profundidad y el ngulo de las trampas de pozo. Estos resultados sugieren que Myrmeleon crudelis escoge volverse menos activo durante la escasez de alimento.
Text in English.
Santa Rosa National Park (Costa Rica)
Parque Nacional Santa Rosa (Costa Rica)
Tropical Ecology Fall 2009
Ecologa Tropical Otoo 2009
t Monteverde Institute : Tropical Ecology
Effects of satiation on pit trap structure in the larval antlion Myrmeleon crudelis (Neuroptera: Myrmeleontidae) Zack Newman Department of Ecology and Evolutionary Biology, University of Colorado ABSTRACT Sit and wait predators have many strategies to deal with food scarcity. Some predat ors become more active, while others conserve energy by becoming more sedentary. I decided to test the effects that starvation has on the larval antlion Myrmeleon crudelis . Myrmeleon crudelis build p it traps , conical structures built in sand, to capture prey. These pit traps vary in diameter, depth, and angle for each larva. Seven individuals in one group were fed every other day, while seven other individuals were starved for all ten days this study took place. The pi t traps of both groups were then compared to each other. The results show that as larval size and feeding frequency increase so do the diameter, depth, and angle of pit traps. These results suggest that Myrmeleon crudelis chooses to become less active in t he presence of food scarcity. RESUMEN depredadores se vuelven mas activos mientras otros conservan su energia volviendose sedentarios. Yo dec idÃ probar el efecto de la privacion de alimento tiene en la larva de la hormiga leon Myrmeleon crudelis . Forma trampas que son estructuras cÃ³nicas en arena para capturar sus presas. Estas trampas varian en diÃ¡metro profundidad y Ã¡ngulo entre individuos. S iete individuos en un grupo fueron alimentados de dÃa por medio, mientras que otros siete fueron privados de alimento por los diez dias del estudio. Las trampas fueron comparadas entre grupos. Los resultados demuestran que conforme el tamaÃ±o de la larva y la frecuencia de alimentacion incrementan tambien lo hacen el diÃ¡metro, la profundidad y el angulo de la trampa. Estos resultados sugieren que My r meleon crudelis escoge volverse menos activo durante la escasez de alimento. INTRODUCTION When confronted with food scarcity, many predators modify their behavior to in order to increase their prey capture efficiency. Some s it and wait predators like the wolf spider, Hogna helluo (Walker et al . 1999) , and four epigean crustaceans (Hervant et al . 1997) become m ore mobile in the search for food. Other sit and wait predators modify the s tructure of their tra ps to cope with low food availabi li ty . For both the desert burrowing spider, Seothyra henscheli (Lubin & Henschel 1996 ), and the orb weaving spider , Argiope ke yserlingi ( Herberstein et al . 1998), this entails building a larger web in order to increase the chance of capturing prey . In the case of antlion larvae ( Neuroptera: Myrmeleontidae), sit and wait predators that build pit traps to capture prey, there have been conflicting observations on the strategies used to cope with starvation . Several studies have shown that when starved , antlion larvae conserve energy by building smaller pit traps than fed counterparts (Griffiths 1986, Eltz 1997 , Hauber 1999 , Scharf e t al . 2009 ). Other studies have shown the opposite : starved antlion larvae create
new larger pit traps than those larvae that were fed ; thus increasing the likelihood that a prey would fall in ( Wilson 1974, Lee 1994, Arnett & Gotelli 2001, Lomascolo & Farj i Brener 2001). Heinrich & Heinrich ( 1984 ) showed a little bit of both when t he y reported that antlion larvae faced with a short term starvation period increased pit trap diameter; however, when faced with long term starvation, the antlion larvae decreased the size of their pit traps in relation to a fed control group. Other s imilar studies have shown that large antlion larvae construct larger pit traps than smaller individuals (Griffiths 1980 , Kitching 1984 , Scharf et al . 2009 ). The results shown by Griffi ths (1980), Kitching (1984), and Scharf et al . (2009) partially agree with the studies that showed fed larvae increased pit trap size more than starved individuals (Griffiths 1986, Eltz 1997, Hauber 1999, Scharf et al . 2009) b ecause fed larvae should be ge tting larger at a higher rate than starved larvae. In this study I will examine the effects that satiation has on the structure of p it traps in the larval antlion Myrmeleon crudelis by comparing three dimensions of the pit traps : diameter, depth, and angl e between a fed and starved group . I will also examine the relationship between larval mass and the diameter of pit traps. MATERIALS AND METHODS Study Species Myrmeleon crudelis is one of several species of antlions that construct pit traps as larvae . T he a dult antlions are nocturnal winged individuals that rese mble dragonflies. After mating, adult f emales lay approximately 20 eggs in soil (Farji Brener 2003). The soil or sand needs to be in dry areas protected from both rain and wind (Farji Brener 2003) . As soon as they hatch, the larvae begin to construct their pit traps. First, they spin around in order to make a circle. They then begin to spiral down ward digging deeper into the soil and flicking out soil particles with their head creating a conical pi t trap. The pit trap is designed so that when prey fall in, they are unable to escape. It has been shown that pit traps that are steeper, larger in diameter, and made of finer grained san d are more eff ective in preventing prey from escap ing ; thus pit traps with these characteristics are preferred by larval antlions ( Loiterton & Magrath 1996, Botz et al . 2003, Farji Brener 2003 ). After they construct their pit traps, the larval antlions burrow under the sand , with only their massive mandibles visible . The an tlion larvae detect prey through vibrations in the soil particles (Devetak et al . 2007); and once the prey item is located, the larvae angle their heads to the prey and flick sand , creating an avalanche (Devetak 2005, Mencinger Vracko & Devetak 2008). Thes e mini ature landslides draw the prey into their mandibles where venom and digestive enzymes are injected, allowing the antlions to begin feeding (Napolitano 1998). Study Design Fourteen M. crudelis individuals were collected from Santa Rosa National Park , Costa Rica in September of 2009 . For the month of October , these individuals were constantly fed and taken care for in separate containers. In November seven larvae were placed into a fed group, where they would be given one ant every other day. The othe r seven individuals were placed in a starved group where they would not be given food for all ten days the study took place . Before the experiment began , all 14 larvae were fed one ant and placed in separate 345 cm 3 containers filled with fine grained sand , where they built new pit traps . Using a caliper, I measured the
diameter and depth of each pit trap c onstructed by the M. crudelis to the nearest hundredth of a millimeter . Because the pit traps were all conical in shape , I used the Pythagorean Theorem ( arctan(depth/diameter)) to calculate the angle of incidence the pit trap made with the sand surface . All fourteen larvae were fed once more in the newly constructed pit traps. Subsequently, the seven antlion larvae in the fed group were given one ant every other day; while the starved group went without food for the next ten days. This resulted in a difference in feeding regimen for only days 4 though 10. The pit trap measurements of diameter, depth, and angle were recorded every other day in sequence with the feeding trials . To account for mass differences, each individual was weighed and paired with a similar sized individual in the other group (Avg. masses: 33.14 mg (fed group) and 28.43 mg (starved group) ). Statistical Analyses I used a regression anal ysis to measure the effect that larval mass had on initial pit trap diameter. To measure the influence starvati on had on pit trap construction, I ran a two tailed paired t test comparing the average change per day both groups made in pit trap diameter, dep th, and angl e. Recall that only days 4 through 10 were included because those were the days the two groups had a difference in feeding regimen . RESULTS At the beg in nin g of the experiment, the range of mass in the larval M. crudelis was 1 55 mg with initi al pit trap diameters ranging from 22 mm to 57.9 mm. Large antlion larvae created pit traps with a larger diameter than smaller individuals (Fig. 1; F 1,12 = 5.04, R 2 = 0.296, P < 0.05). For the parameters of diameter, depth, and angle, the fed group exhib ited a significantly larger daily increase than the starved group. T he fed group increased the pit trap diameter by an average of 6.04 mm every day, while the starved group increased the pit trap diameter by only 2.08 mm every day (Fig. 2; t = 2.82, df = 2 7, P < 0.01) . The pit trap depth increased daily by 4.73 mm for the fed group , but only by 0.95 mm for the starved group (Fig. 2; t = 2.72, df = 27, P < 0.05) . In the case of pit trap angle, the fed group created steeper pit traps with an increase of 1.95 Â° every day ; while the starved group actually d ecreased by an average of 0.27Â° (t = 2.54, df = 27, P < 0.05) .
Figure 1 . The effect of larval mass on pit trap diameter in Myrmeleon crudelis . Figure 2 . T he average change per day (mean Â± SE) in the 3 pit trap dimensions for fed and starved groups of Myrmeleon crudelis . Diameter and depth we re both measured in millimeters . The angle was calculated i n degrees by taking the arctan(depth/diameter) of the angle o f incidence the pit trap made with surface of the sand.
DISCUSSION Sit and wait predators utilize a variety of strategies to deal with starvation in habitats where food is scarce. The results of this study suggest that the sit and wait predator M. crudelis chooses a strategy that conserves energy when starved. Instead of exerting more energy by creating a larger pit trap, the larva decides to conserve what little energy it has and hope that prey will fall into its smaller pit traps . These results agree with many previous studies (Griffiths 1986, Eltz 1997, Hauber 1999, Scharf et al . 2009). Alternatively, the increased diameter, depth and angle of the pit traps in the fed group may be due to the fact that fed individuals grew larger; and larger larvae require larger pit traps. My results agree with previous studies (Griffiths 1980, Kitching 1984) that show that larger antlion larvae requi re larger pit traps. Since fed individuals grew more quickly than the starved individuals, they were larger and thus require d larger pit trap s . Besides need ing more room, larger individuals may create larger pit traps because they are more efficient at building pit traps due to a larger body capable of flicking more sand out in a shorter amount of time. Another possible expl anation for why fed individuals increased the size of their pit traps more than the starved individuals is that the fed individuals had to make repairs to t heir pit traps. Because the pit trap is designed to collapse slightly when prey f all in, those indiv iduals that were fed had to repair the damages to their pit traps . This could have resulted not only in repairing the pit trap but improving it by making it larger as well. The starved individuals did not need to make such repairs because prey was never gi ven to them , and their pit traps were not damaged. My results showed that e ach fed individual larva steadily increased the dimensions of its pit trap; and while the starved larvae increased two of these dimensions as well (pit trap diameter and depth), it was not as much. D espite the agreement with many previous studies, my results contrast with some others (Wilson 1974, Lee 1994, Arnett & Gotelli 2001, Lomascolo & Farji Brener 2001). However, I consider my results to be more realistic because as Scarf et al . (2009) explained , antlions that receive no prey have no i nformation to count on and thus should invest little additional effort in their traps. In conclusion, I have shown that an increase in mass and feeding frequency of antlion larvae leads to an inc rease in the size of pit traps. This adds further evidence to previous studies that have shown the same. This means that M yrmeleon crudelis is one more example of a sit and wait predator that chooses to conserve energy when food is scarce instead of adopti ng a more active strategy. ACKNOWLEDGEMENTS I am grateful to Pablo Allen for giving me the idea for this project in a time of dire need as well as for his suggestions running statistical analyses. I thank Yimen Ar a ya as well for his aid on making some of the graphs . Thanks are also in order to the EstaciÃ³n BiolÃ³gica Monteverde , where I conducted and housed my all my research. Most importantly thanks to my tica family: Teresita, ElÃ, Tracy, and EstebÃ¡n. They provided room and board, plentiful and delicious food, and a most enjoyable experience with them during the time I conducted this study. LITERATURE CITED Arnett, A. E. and N. J. Gotelli. 2001. Pit building decisions of larval ant lions: Effects of larval stage, temperature, food, and population sourc e. Journal of Insect Behavior. 14: 89 97.
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