Investigation Of Pit Size Versus Larval Size In An Antlion

Investigation Of Pit Size Versus Larval Size In An Antlion Senior Seminar 20 April 2001 Abstract 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. Introduction 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.

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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. Materials and Methods 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. Measuring Antlion 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; furtherm …


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