Investigation of Pit Size Versus Larval Size in an Antlion, Medicine, College Term Papers.com


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Investigation of Pit Size Versus Larval Size in an Antlion

Investigation of Pit Size Versus Larval Size in an Antlion The purpose of this study was to test whether there was a relationship between the size of an antlion and the size of its pit. Fieldwork was undertaken at UT's Brackenridge Field Laboratory in Austin, Texas. There we selected two aggregations of larval antlions from which to obtain the data. Pit diameter and slope were measured to obtain the pit volume. After pits were measured, larvae were collected and weighted in the lab with an analytical balance. Regressions of larval weight versus pit diameter, slope, and pit volume were obtained. All three variables showed significant coefficients of determination (R-squared * 0.7). These results are then compared to previous studies that found weaker relationships between larval size and pit size. Possible reasons for these differences are discussed. The antlion got its name from early European scholars who likened the insect to, a small creature, extremely hostile to ants. Antlion larvae are carnivorous predators that feed mainly on small mobile arthropods such as ants, as well as the occasional spider, beetle, fly, caterpillar, wasp, and mite that falls into its pit. Our research was conducted on the species Myrmeleon crudalis, which is from the order Neuoptera. This species lives preferably in sheltered, sandy regions and builds shallow, cone-shaped pits in the surrounding substrate. The larvae use these pits to capture their prey. The insect falls into the pit and is unable to climb up the loose sand on the sides of the pit. The antlion lies buried at the bottom of the pit and catches its prey with its strong, piercing mandibles. The larvae secrete digestive enzymes through the mandibular groove into the prey item. After the antlion sucks out the soft tissues of the prey, the exterior shell is discarded by actively pushing it out of the pit. Although M. crudalis is a pit-building species, not all antlion species build pits. Some species hide under debris, gravel, or wood and wait for prey to pass by in close enough range for capture. The lifecycle of the antlion involves many stages. Adults lay sticky eggs on sandy soil and the eggs become coated with fine layer of dust for camouflage. Larvae hatch from the eggs after few weeks and then undergo three instar stages before pupating. Larvae may even live several years before pupating - lack of food can lead to longer life-span whereas if there is abundant food supply, the larvae may mature much more quickly. The larvae pupate in the soil in a cocoon of sand and silky fibers. After approximately one month of pupating, the adults emerge from the cocoons. Adult antlions closely resemble dragonflies and damselflies, however, unlike the latter two, adult antlions are very weak flyers. During the day, the adults lie perched on camouflaging branches and are only really active in the evenings. The adults can reach approximately 4 centimeters in length, and they can have a wingspan of 8 centimeters.1 The life span of the adult antlion is approximately one month. The one month time span is just enough time for the adults to reproduce and begin the life cycle again. There are approximately 2000 different antlion species distributed throughout the world, however, antlions are mainly found in the warmer climates of Southern Africa, Australia, and the southern United States. The most common species are M. obsoletus in North America, and M. formicarius in Europe. M. carolinus is the most common species found in Florida and other southern states of America. Antlion larvae from the species we studied built conical shaped pits in the surrounding sandy substrate. Larvae build their pits in four steps. First, the larvae move in what appears to be very random, undirected movements just under surface of the soil. Possibly these movements serve to loosen the substrate or test the site for gravel obstructions. The antlion then proceeds to establish the circular boundaries and diameter of its pit. In the next step, the antlion spirals downward to deepen the pit. The antlion moves backward while displaces the sand by scooping it up with its head and mandibles and actively throwing it out of the developing pit. Finally, once the diameter and slope of the pit have been constructed, the larvae make the last adjustments by lacing the slope walls with the finest sand.2 Once the pit is built, the larva buries itself at the bottom of the pit completely submerged except for the protruding mandibles. This is referred to the sit-and-wait method of prey capture. The more famous example of this method of capture is the arachnid family who builds webs to capture their prey. The antlion species which we studied, Myrmeleon caudalis, was a pit-building species. The purpose of our study was to test whether there is a relationship between the size of an antlion, and the size of its pit. We hypothesized that there would be a positive correlation between the size of an antlion and the size of its pit. As the larvae grow bigger, so will there pits increase in size. We Previous studies reported that there was a weak relationship between larval size and pit size.3 We used three variables to determine the size of the pit: pit diameter, slope distance, and pit volume. We determined the size of the antlion larvae to be the weight of the larvae. Our antlion data was obtained in late October 1999 through November 1999 from the University of Texas at Austin's Brackenridge Field Laboratory in Austin Texas. We chose two study sites, the first site boarded Lake Austin Boulevard and we named it the "scorpionfly site". This site was wooded with cedar-elm trees; the substrate has been periodically disturbed over the last several years by a tractor clearing ground vegetation. This action has opened up the understory and has created some localized spots of loose sandy-loam substrate where antlions made their pits. Loose substrate was also found at the base of cedar-elms, and thus hosted antlion pits. The second site we named the "quail site" because this site was situated inside the fencing of an old quail enclosure. The substrate at the quail site was mostly sand and the tin awning above the site protected the pits. At the scorpionfly site we obtained three measurements for each of the randomly selected pits. The first measurement we obtained was the distance between the pit we selected and next nearest pit. We called this measurement the nearest neighbor measurement. We used a fifteen-centimeter ruler to measure the distance and any measurement greater than fifteen centimeters was recorded in the same group as being greater than fifteen centimeters. We began to collect data on the nearest neighbor, but it became clear that this variable had no influence on pit size; furthermore, previous studies found no relationship either between pit and nearest neighbor.3 For the second measurement we used a dial-caliper to measure the diameter of the pit. We selected the long diameter of the pit (the pits were often not perfectly symmetrical) and measured the distance to the nearest hundredth of a centimeter. At times difficulties arose in determining the actual edge of the pit. This may have introduced a small amount of error in measures of pit diameter in these cases. Measuring the slope distance required a delicate procedure to prevent disturbance of the buried antlion. We used a small stick that was placed at the bottom of the pit and was allowed to rest against the side of the pit. This distance was then transferred to the caliper to obtain a measurement to the nearest hundredth of a centimeter. Once again, the method that we used may have introduced a small amount of error in out calculations since the slightest movement caused erosion of the sides of the pits. To capture the antlion, we used a spoon to scoop out the sand at the bottom of the pit and the contents were emptied into a sieve. If we were unsuccessful in our attempt to capture the antlion, the pit size measurements for that pit were not included in our analysis. Each antlion was placed in an individual vial and taken to the laboratory for weighing. An analytical balance was used to measure to weights of the antlion larvae to the nearest ten thousandth of a gram. The remaining antlions were released back into Brackenridge at a different site. We followed the same procedure for collecting data at the Quail site. Because the quail site was sectioned off, we were able to carry out a complete data collection from all the pits at the site, as opposed to the random selection of pits at the scorpionfly site. To work out the volume of the pits, we used the formula for the volume of a cone: volume = height (1/3 p r2). Because we only measured the distance of the slope and the diameter, we found the height measurement by using the right-angle triangle theorem: height = � [(slope)2 - (1/2 diameter) 2]. We performed regression analysis on our data for both of the sites. Once we had obtained all the measurements, a few of the larger antlions were selected for rearing to adulthood in individual rearing chambers. Reared specimens were then identified by a specialist at the Texas A & M Entomology Museum. At the quail site, the R2 for the diameter versus larval size (measured as the weight of the larvae) was 74% (n=76). This was slightly higher than at the scorpionfly site which had an R2 = 71% (n=82). For the regression analysis of slope distance versus larval size, the R2 value ranged from 62% at the quail site to 44.8% at the scorpionfly site. The final regression analysis we performed was the pit volume versus the larval size. This measurement gave the overall correlation between the size of the pit and the size of the antlion that made the pit. At the quail site, the R2 = 67.2% versus the R2 = 49.8% at the scorpionfly site. The above regressions are shown in Table 1. From our regression analysis at both sites, pit diameter, slope distance, and pit volume all correlate significantly with larval weight. Pit diameter shows strongest correlation with larval size. One possible reason for this may be because the pit diameter was the easiest variable to measure. We also found that our R2 explains more of the pit diameter variation than previous studies. Our R2 = 74% at the Quail site versus only an R2 = 20-26% from other studies. Mark E. Hauber conducted study on the influence of food limitation and pit building experience on variation in pit size. Study showed unfed larvae had smaller pit diameters than fed larvae. However, fed larvae previously prevented from building pits did not build larger pits than unfed larvae under same conditions. Therefore, physiological constraints associated with food limitation not sufficient to explain difference in pit size. Regressed pit diameter versus larval size: R2 = 20-26% Greater lower range of larval size (instars) Our range of wet weights: 0.1 - 40 mg Hauber et al. range of weights: 0.5 -9.5 m Hauber probably only sampling first and second instars Our study was field study versus Hauber's laboratory study No pit diameter measured on larva greater than 40 mg Larva can be greater than 76 mg, therefore half of weight gain associated with development not regressed Could also perform multiple regression on data for diameter and slope distance to obtain greater R2 values Biophysics of pit construction by Jeffrey Lucas Studied effect of sand particle size on dynamics of pit construction Experimental organism was also M. crudalis Hypothesized pit slope is determined by angle of repose of sand and Stoke's Law drag force. Angle of Repose - Maximum angle reached before particles - Smaller particles have a higher angle of repose Stoke's Law - Trajectory of a particle with a given initial velocity is affected by drag force imparted - Smaller the particle, higher the drag force, and shorter the distance it will travel Hypothesized finer sand would produce pits with steep slopes Regression analysis showed variation in sand grain size accounted for variation in pit slope (P=0.02, N=24) However, did not explain variation in diameter (P=0.001, N=22) Proposed that benefit of increasing diameter would equal cost of reducing slope distance What implications do these two studies have? Hauber proposes food limitations have effect on size of pit Lucas proposes structure and construction of pit is constrained by properties of sand Furthermore, Lucas believes antlions regulate pit diameter and slope Therefore, study of the size of antlion versus size of pit relationship may be too simple Biomechanics of trap-building have been studied in only two groups of organisms - orb-weaving spider and antlion Spider builds its web on principle of least-weight structure This minimizes amount of material needed to catch prey - a balance of energy expenditure and success rate Spider constructs its web so that large, harmful prey fly through Lucas proposes antlions regulate pit diameter so large prey can escape Studying trap biomechanics increases understanding of advantages, disadvantages, and constraints placed on trap-building predators Also increases understanding of evolutionary adaptations these organisms display in trap-building behavior Does pit size increase with increase in larval size in late instars? What is the average weight at the time of pupation? Like the orb-web spider, do antlions balance energy cost and success rate? Bibliography:

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