DOI: 10.5061/dryad.1zcrjdg5h
title: Hunting positions in a sit-and-wait predator

description: Modifying decisions depends on the cost of change, especially in
 sit-and-wait predators. Once situated, they should remain immobile, as
 position changes can disturb their traps or alert prey and predators.
 Antlion larvae (Myrmeleon spp.) dig conical pits in the soil to capture
 walking insects. The larva then ejects the prey carcass using strong,
 backward, pendular mandible movements. Therefore, their position on the
 pit influences the direction to expel carcasses and debris. We tested the
 hypothesis that antlion larvae orient at the bottom of their pits to avoid
 future movements, and that they decide their position based on
 environmental features such as the proximity of vertical obstacles and the
 ground’s slope. In the field, we dropped debris (rice grains) into pits.
 The expelled debris direction was consistent as larvae maintained their
 position at the bottom of their pit throughout time, and their position
 was independent of the pit's distance from a wall. In the lab, we
 induced larvae to build pits in narrow and sloped terraria. Larvae
 oriented themselves in positions that avoid expelling objects toward a
 wall or uphill, preventing them from bouncing off or rolling back into the
 pit. These results indicate that larvae use information about their
 surroundings to determine how to position themselves at the pit to avoid
 having to move if the pit gets compromised.
description: We conducted two studies: one field experiment to evaluate the
 distance and consistency of debris expulsion direction, and a combined
 field and laboratory experiment to examine larvae's orientation
 concerning environmental features. I. In the field, in
 Yucantán, México, we experimentally determined the relationship between
 the expelling distance and the size of the larvae, and explored their
 consistency in the expelling direction. At the end of the dry season (May
 2023), we randomly selected 45 different-sized antlion larvae pits. We
 measured their pit ratio with a caliper; an established proxy of larva
 size. Then, we carefully dropped half of a rice grain into each pit center
 from 10 cm above the pit, simulating the fall of debris and stimulating
 larvae to clean it. To measure the consistency of the
 expelling direction, we repeated rice grain trials between 3 and 7 times
 in each pit. The number of repetitions depended on the pit status and
 larvae' behavior. The time between trials was approximately 5
 minutes. We stopped the experiment when pits showed signs of landslides
 and disturbance, and when larvae did not respond for 90 s to our
 treatment. We then calculated a Consistency Index of Expelling Direction
 (CIED). We divided the area around the pit into 8 possible orientations of
 identical areas in function of the cardinal directions (i.e., N, NE, E,
 SE, S, SW, W, and NW). CIED was estimated as the number of the most
 frequent cardinal direction in which the rice was expelled, divided by the
 total number of repetitions. The CIED index takes a maximum value of 1
 when all the expelling events are in the same cardinal direction (i.e.,
 maximum consistency). We then divided the CIEDs into three categories of
 equal range (low, medium, and high consistency). <br><br>II.
 In Sarapiquí, Costa Rica, we performed field and lab experiments, to
 evaluate whether the position of the larvae at the bottom of their pits
 was related to two environmental features (the proximity of walls and the
 slope of the ground). First, we search for pits around
 buildings. We sorted pits as near a wall when the distance between the pit
 border and the wall was less than the pit ratio. At this distance, any
 object (debris or prey carcasses) expelled towards the wall direction
 might bounce off the walls and fall back into the pit. On the other hand,
 we sorted pits as far away from the wall when the distance between its
 border and the wall was larger than the pit diameter. At this distance,
 any object expelled out of the pit towards the wall is unlikely to bounce
 off the walls and fall back into the pit. We located 80 pits, measured
 their diameter, and their distance to the wall. Forty-six pits were
 categorized as near and 34 far away from the walls. After that, we
 carefully dropped half of a rice grain into each pit center as described
 above. The experiment simulated the fall of debris and stimulated cleaning
 by the antlion. Then, we characterized the expelling direction to the wall
 location using 4 subdivisions of equal area (toward the wall, opposite to
 the wall, right side, and left side of the wall). The orientation of the
 larva (i.e., where the larva's jaws were pointing) was considered the
 opposite of the expelling direction, given that larvae used their
 mandibles as catapults to throw out object out of their pits.
 Second, in the lab, we evaluated whether larvae orient themselves
 to avoid throwing debris and/or carcasses toward nearby walls or uphill,
 preventing these objects from bouncing off or rolling back into their
 pits. We extracted 40 larvae by gently scooping the soil in the pit ~8 cm
 deep with a spoon. Larvae were randomly selected from those measured in
 the field experiment described above. We collected an equal number of
 larvae found near and far away from walls (n = 20 of each category).
 Subsequently, we transferred each larva to the laboratory and placed it at
 the center of an individual narrow container (rectangular boxes of 6 cm
 wide, 20 cm long, and 15 deep), filled with &lt;2 mm fine-grain soil
 particles obtained from commercial sand separated with screen sieves. All
 containers had a 30-degree inclination. This design provided the larvae
 with only one position to avoid bouncing off or rolling back expelled
 objects: with its jaws oriented uphill. Objects expelled to the right and
 left sides of the container will likely bounce off and return to the pit
 due to the short distance between the pit and the container's border
 and those expelled uphill will probably roll back into the pit. Therefore,
 the best position to expel objects is if the larva positioned itself with
 its jaws uphill to throw out objects downhill. After 3 days, we measured
 pit diameter with a caliper and confirmed the larvae's position at
 the bottom of their pits. The larva's position was determined by
 dropping half of a rice grain into each pit center and determining its
 expelled direction in four equal-area options: right, left, uphill, and
 downhill. The position of the larvae was characterized as the opposite of
 the expelling direction, as previously described. 
description: # Hunting positions in a sit-and-wait predator Dataset DOI:
 [10.5061/dryad.1zcrjdg5h](10.5061/dryad.1zcrjdg5h) ## Description of the
 data and file structure ### FILE "consistency_field_data.csv"
 OVERALL DESCRIPTION: This file contains the data for a field experiment in
 which we determined the relationship between the expelling distance and
 the size of the larvae, and explored their consistency in the expelling
 direction. COLUMN EXPLANATION * Pit_ID: Individual identification of each
 Myrmelion immaculatus antlion larva that we studied in the field in
 Yucatan, Mexico. * Pit_number: Consecutive number of each larva we
 studied. Note: By definition, each larva had only one value for this
 value. Hence, "NA"s are included for many rows. * Pit_Ratio:
 ratio of the pit, in cm. Note: By definition, each larva had only one
 value for this value. Hence, "NA"s are included for many rows. *
 Trial_number: repetition for each of the trials conducted with a given
 larva. * Expelling_distance_(max_in_bold): Linear distance (in cm) of the
 distance in which each larva expelled the half rice grain we dropped in
 the center of its pit. * Expelling_orientation: orientation with respect
 to cardinal orientation. *
 Consistency_Index_of_Expelling_Direction_(CIED): CIED was estimated as the
 number of the most frequent cardinal direction in which the rice was
 expelled, divided by the total number of repetitions. The index takes a
 maximum value of 1 when all the expelling events are in the same cardinal
 direction (i.e., maximum consistency). Note: By definition, each larva had
 only one value for the index. Hence, "NA"s are included for many
 rows. ### FILE "orientation_field_data.csv" OVERALL DESCRIPTION:
 This file contains the data for the field exploration to evaluate whether
 the position of the larvae at the bottom of their pits was related to two
 environmental features (the proximity of walls and the slope of the
 ground) in Costa Rica. COLUMN EXPLANATION * Pit_ID: Individual
 identification of each Myrmelion immaculatus antlion larva that we
 studied. * Pit_Diameter: diameter of the pit, in cm, measured with a
 caliper. * Pit_ratio: ratio of the pit, in cm. *
 Location_regarding_the_wall: Binary category for the pits depending on
 their location regarding the nearest wall or vertical substrate. Far=when
 the distance between its border and the wall was larger than the pit
 diameter; Near=when the distance between the pit border and the wall was
 less than the pit ratio. * Antlion_position_regarding_the_wall: Direction
 at which the antlion larvae expelled the half rice grain we dropped onto
 the larva pit trap. The direction was relative to the wall, divided using
 4 subdivisions of equal area (toward the wall, opposite to the wall, right
 side, and left side of the wall). ### FILE
 "orientation_lab_data.csv" OVERALL DESCRIPTION: This file
 contains the data for the lab experiment designed to evaluate whether the
 position of the larvae at the bottom of their pits was related to two
 environmental features (the proximity of walls and the slope of the
 ground) in Costa Rica. COLUMN EXPLANATION * Pit_ID: Individual
 identification of each Myrmelion immaculatus antlion larva that we
 studied. * Pit_Diameter: diameter of the pit, in cm. * Pit_ratio: ratio of
 the pit, in cm. * Larva_position: larva's position determined by
 dropping half of a rice grain into each pit center and determining its
 expelled direction in four equal-area options: right, left, uphill, and
 downhill.