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Investigation Of Pit Size Versus Larval Size In An Antlion

.. ore, 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.

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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.

Results 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. Table 1: Regression Results Scorpionfly Site Quail Site Diameter 71% 74% Slope 44.8% 62% Volume 49.8% 67.2% Discussion 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% Larger sample size 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 collapse – 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 on it by friction of air – 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 Antlions may function in same way 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? Do late instars stop making pits? What is the average weight at the time of pupation? Like the orb-web spider, do antlions balance energy cost and success rate? Medicine.


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