Effect of rain on trap building by Myrmeleon brasiliensis in a riparian woodland from the Cerrado biome in Brazil

In areas of the Cerrado biome (Brazilian savanna) with well‐defined rainy and dry seasons, changes in climate and landscape affect the abundance and seasonality of insects. Larvae of the antlion Myrmeleon brasiliensis (Návas) (Neuroptera: Myrmeleontidae) are predators that build traps in dry sandy soil to capture prey. Here we determined the effect of rain on trap building by M. brasiliensis larvae in a riparian forest in the Cerrado biome. Differences in population size and developmental stage were found between the rainy and dry seasons. In the laboratory, the effect of rain‐soaked soil on trap building was evaluated. Fewer antlion larvae were found in the rainy season. Moreover, a greater abundance of larvae in the final stage of development (third instar) was found in the dry season, whereas more first instars were found in the rainy season. The latter revealed that wet soil affects the trap building of the larvae, as smaller traps were built in the treatment with soaked soil (simulated rain), whereas the larvae in the control treatment (no rain) continued building traps with no change in size. The findings indicate that seasonality in the Cerrado biome exerts a short‐term influence on M. brasiliensis larvae (larvae build fewer traps) as well as a long‐term influence (association with life cycle).     

Microhabitat Selection by Antlion Larvae, Myrmeleon Crudelis: Effect of Soil Particle Size on Pit-Trap Design and Prey Capture

Selective pressure for choosing an adequate habitat should be strong in semisedentary animals because they have limited mobility once established. I examined microhabitat preferences and the adaptive value of these preferences in the antlion larva Myrmeloen crudelis, a semisedentary insect that digs pit traps in soils to capture small arthropods. I tested the habitat preferences of M. crudelis between two soil types in a tropical dry forest of Costa Rica. Specifically, I compared the soil particle composition size within and outside antlion aggregations and manipulated the availability of fine- and coarse-grained soil to assess how differences in soil grain size affect the design and performance of larval traps. Adjacent to antlion pits the soil was composed of a greater proportion of fine-grained particles (2 mm) than soil 1 m away from the pits. A set of experiments demonstrated that (1) in the presence of equal availability of fine- and coarse-grained soils, all larvae built their pits in fine-grained soil; (2) the larvae required less time to start and finish traps in fine-grained soil; (3) the larvae constructed larger and deeper pits in fine-grained soil; and (4) prey capture increased greatly in fine-grained traps compared with coarse-grained traps. Antlion larvae respond to variations in the proportion of fine particles in the soil, suggesting that antlion aggregations result from an active microhabitat selection. The preference for fine-grained soils is adaptive since pits constructed in such substrate are functional for longer periods and much more successful in trapping prey than pits in coarse-grained soil. Sit-and-wait predators that use sessile traps are spatially constrained to track prey abundance. Therefore, the ability to detect and select microhabitats with better conditions that enhance capture success may be under strong selection for this type of organism.     

Why are pitfall traps so rare in the natural world?

Prey capture by trapping is uncommon taxonomically, and generally requires highly evolved cognitive powers (humans) or specialist self-secreted materials (for example, spiders and caddisfly larvae). The most notable exception to this is the conical traps dug by antlion larvae. The relative uncommonness (taxonomically and ecologically) of such pitfall traps has been described as an unexplained mystery in recent publications. Here we suggest some potential routes that might lead to resolution to this mystery. We argue that although such pitfall traps have numerous benefits and are relatively cheap and easy to construct, they may suffer two significant disadvantages relative to, for example, spiders’ webs. First, pitfall traps may require a quite specialist microhabitat. Second, antlion pitfall traps may only work to retain all but the smallest prey if the antlion is present at the bottom of the pit. Thus, antlion may be more functionally tied to their trap than spiders and (since traps are much more visually conspicuous than their owners) this may make them vulnerable to predators and parasitoids that cue on the traps. Both these hypothesised drawbacks are speculative in the absence of a strong body of data and so we discuss how both potential costs could be explored empirically.     

Disadvantages of living in a populous neighborhood for sit-and-wait predators: Competition for space reduces pit-trap size in antlion larvae

The study of how trap design responds to biotic and abiotic conditions can help to understand the selective forces affecting the foraging of trap-building organisms. We experimentally tested whether pit design can be modified by intraspecific competition for space in larvae of Myrmeleon crudelis, a common sit-and-wait predator that digs conical pit traps in the soil to capture walking arthropods. In a tropical forest in Costa Rica, we measured pit dimensions, larval body size, and the level of competition (i.e., density of neighboring traps) in 40 antlion larvae. These larvae were then taken to the laboratory and allowed to build new traps in individual containers. We measured within-individual changes in the size of traps in the field and in the laboratory, and related these to the level of competition experienced in the field. Larvae with relatively high levels of competition in the field showed a greater increase in the size of their pits in the laboratory. This change was independent of larval size. Larvae with none or few neighbors in the field showed little change in their pit sizes, whereas those with higher competition levels increased their diameter and depth up to 1,400% and 1,000%, respectively. Our results demonstrate that, at least in high-density aggregations, pit design is restricted by competition in addition to the constraints imposed by body size. This work suggests that biotic interactions can play a role in the design of extended phenotypes in sit-and-wait predators that live in dense aggregations.     

Substrate particle size affects pit building decision and pit size in the antlion larvae Euroleon nostras (Neuroptera: Myrmeleontidae)

Abstract. The larvae of the antlion Euroleon nostras are pit-builders, constructing pitfall traps in loose sand. The number of pits and the pit diameter are recorded when larvae are kept in substrates with different particle sizes. The most convenient pit-building sand fractions are two fractions with fine sand (≤ 0.23 mm; 0.23–0.54 mm). The largest pits are constructed in sand with a particle size of 0.23–0.54 mm. In this sand fraction, larvae of all three instars most readily build pits. No pits are constructed in sand with a particle size greater than 1.54 mm. First- and second-instar larvae avoid building pits in substrates of particle size 1–1.54 mm, but third-instar larvae construct pits in this sand fraction. It is assumed that the antlion is capable of distinguishing between substrate types and this hypothesis is tested by giving larvae the choice of building a pit in one of four particle-size fractions. Larvae of all three instars prefer to build pits in the fraction with a particle size of 0.23–0.54 mm. Only third-instar larvae build pits in all four fractions, but only occasionally in the coarser fraction.     

Phenotypic plasticity and variation in morphological and life-history traits of antlion adults across a climatic gradient

We report here on two complementary experiments examining the effect of climate on morphological and life-history traits of antlion adults. We first examined whether body size and wing loading of emerging adults are plastic by raising larvae, collected from five antlion populations along Israel's sharp climatic gradient, in two environmental chambers simulating temperature and humidity of desert and Mediterranean climates. The variance in adult morphology was mostly related to body size, with adults of Mediterranean populations being larger than those of desert populations. Wing-to-thorax ratio was negatively correlated with temperature, compensating for the decrease in wing-beat frequency in colder environments. Differences between climatic treatments were significant for body size but not for the wing-to-thorax ratio, suggesting that body size is more plastic than the ratio between different body components. We next investigated how the exposure of antlion pupae to different climatic conditions influences the emerging adults. Adult body mass increased with final larval body mass at a faster rate when exposed to Mediterranean rather than desert conditions. Duration of the pupa stage was positively correlated with final larval mass, but only under Mediterranean conditions. Adult survival increased with initial mass (after eclosion), but was lower under desert conditions. Similarly, adults lost mass at a faster rate when exposed to desert conditions. Notably, the exposure of the pupae to varying climatic conditions had no effect on adult morphology. Climate is a major factor affecting insect life span and body size. Since body size is strongly linked to fecundity and survival, climate thus has a twofold effect on fitness: directly, and indirectly through body size.     

Variation in pit size of antlion (Myrmeleon carolinus) larvae: the importance of pit construction

Abstract.Larvae of several antlions build pits that vary in size across and within species. The influence of food limitation and pit building experience on variation in pit size of the larvae of Myrmeleon carolinus was investigated in the laboratory. Unfed larvae that were allowed to build pits had smaller pit diameters than fed larvae. However, fed antlions that had been previously prevented from pit building, initially did not build larger pits than unfed antlion larvae that, too, had been prevented from pit building. Therefore, physiological constraints associated with food limitation alone are not sufficient to explain the reduction in pit size of food limited antlions of this species.     

Utilisation of prey by antlion larvae (Neuroptera: Myrmeleontidae) in terms of energy and nutrients

Prey utilisation at low prey densities was determined for third instar Cueta sp., Furgella intermedia (Markl) and Palpares annulatus (Stitz) larvae in terms of wet weight, dry weight, energy and nutrients. Prey utilisation was similar to other insects on a wet weight (42-47%), dry weight (46-49%), energetic (40-58%) and nutritive basis (62-79%). Lipids (33-36%) provided energetically the highest contribution of the nutrients ingested. The quantities of water, proteins, nucleic acids, lipids and carbohydrates extracted by the antlion larvae were in proportion to their availability in their prey, the Hodotermes mossambicus larvae. The quantities of nutrients extracted by the antlion larvae at low prey densities were not significantly influenced by the differences in mandible size, antlion body weight or the trapping method (building a pit or not) of the antlion species. It is proposed that a low metabolic rate and the accumulation of fat reserves, and not the extent of prey utilisation, enable P. annulatus larvae to tolerate a 123-d starvation period in which 22.3% of their body weight is lost.     

Low Density of Ant Lion Larva (Myrmeleon crudelis) in Ant‐Acacia Clearings: High Predation Risk or Inadequate Substrate?1

We experimentally determined whether the low density of antlion larvae found in ant‐acacia clearings is due to high levels of ant predation or an inadequate substrate for pit‐trap construction. Pit establishment was more affected by soil improvement than by the exclusion of ants, suggesting that soil condition is the main factor that affect antlions’density. The low density of this ant‐predator in ant‐acacia clearings may be considered as an indirect advantage of the ant's pruning behavior.     

Different predation efficiencies of trap‐building larvae of sympatric antlions and wormlions from the rainforest of Borneo

1. This study reports the discovery of sympatric populations of antlions (Neuroptera: Myrmeleontidae) and wormlions (Diptera: Vermileonidae) in a unique system of sandy microhabitats in the lowland rainforest of Borneo. The two species convergently evolved sit‐and‐wait predatory larvae, which construct pitfall traps to hunt insects. Despite similar specialised foraging strategies, the two species coexist in the competitive environment of small, isolated sandy patches in the rainforest, which begs the question: what biological characteristics allow their coexistence? 2. Based on larval morphology alone, it was predicted that antlions would build larger traps, which would allow them to efficiently hunt larger prey. Addressing this hypothesis, this study compared the volumes of traps constructed by the two species under field and laboratory conditions. A laboratory experiment compared their efficiency of capture of three ant species that differed in body size. 3. The results show that antlions constructed larger traps and captured prey more efficiently. The difference between the species could not be explained by trap size alone. The findings demonstrate that the overlap in resource use in these two species was low, and it is suggested that there is a separation in prey utility between them, allowing their coexistence in the space‐limited habitat of the tropical rainforest.     

Pitfall vs fence traps in feeding efficiency of antlion larvae

Larvae of pit-building antlions are expected to be more efficient at capturing prey than those of non-pit-builders. To test this prediction, feeding behaviors were compared in the same experimental conditions among pit-building Baliga micans and Myrmeleon bore, and non-pit-building Distoleon contubernalis. The number of prey eaten did not differ between species. D. contubernalis larvae used the walls of the experimental chamber as fence traps to capture prey. In the field, they were also found near edges of natural barriers, such as rocks, stones, tree roots, and plant stems. Artificial pitfall traps captured more arthropods near the edges of fences than farther from them. Larvae of the two pit-building species were located in the central part of the experimental chamber. In their natural habitats, the number of arthropods captured by artificial pitfall traps increased with pit size; thus, larger pits may be more efficient for capturing prey. In conclusion, pit-building and non-pit-building antlion larvae are both efficient hunters; the former hunt efficiently by making larger pitfall traps, and the latter do so by waiting for prey at the edge of the natural fences along which arthropods walk.     

The influence of host fruit and temperature on the body size of adult Ceratitis capitata (Diptera: Tephritidae) under laboratory and field conditions

The adult body size of the Mediterranean fruit fly, Ceratitis capitata (Wiedemann) (Diptera: Tephritidae), varies in natural conditions. Body size is an important fitness indicator in the Mediterranean fruit fly; larger individuals are more competitive at mating and have a greater dispersion capacity and fertility. Both temperature during larval development and host fruit quality have been cited as possible causes for this variation. We studied the influence of host fruit and temperature during larval development on adult body size (wing area) in the laboratory, and determined body size variation in field populations of the Mediterannean fruit fly in eastern Spain. Field flies measured had two origins: 1) flies periodically collected throughout the year in field traps from 32 citrus groves, during the period 2003-2007; and 2) flies evolved from different fruit species collected between June and December in 2003 and 2004. In the lab, wing area of male and female adults varied significantly with temperature during larval development, being larger at the lowest temperature. Adult size also was significantly different depending on the host fruit in which larvae developed. The size of the flies captured at the field, either from traps or from fruits, varied seasonally showing a gradual pattern of change along the year. The largest individuals were obtained during winter and early spring and the smallest during late summer. In field conditions, the size of the adult Mediterannean fruit fly seems apparently more related with air temperature than with host fruit. The implications of this adult size pattern on the biology of C. capitata and on the application of the sterile insect technique are discussed.     

Substrate selection for pit making and oviposition in an antlion, Myrmeleon bore Tjeder, in terms of sand particle size

The larvae of the pit‐making antlion Myrmeleon bore Tjeder live in open sand in riverbeds with a substratum consisting of various particle sizes. We analyzed the spatial distribution of their pits in a sandy floodplain to determine their larval and adult responses to the heterogeneous substrate. The spatial distribution pattern of their pits had an aggregated distribution, and there was a significant positive correlation between pit density and the ratio of medium‐size sand particles to total weight of sand. We examined the size of sand particles selected in the larval pit‐building behavior and the oviposition behavior of the adult. Both larvae and adults selected medium‐size sand particles. The larvae of M. bore are relatively sedentary predators and rarely move great distances. Thus, the present results suggest that habitat selection by adult females is a major factor causing the aggregative distribution of the pits.     

Conspicuous colouration attracts prey to a stationary predator

Abstract 1. Conspicuous body colouration is counter‐intuitive in stationary predators because sit‐and‐wait tactics frequently rely on concealed traps to capture prey. Consequently, bright colours and contrasting patterns should be rare in predators using traps as they may alert potential prey. Yet, some orb‐weaving spiders are brightly coloured and contrastingly patterned. How can conspicuousness of trap‐building sit‐and‐wait predators be favoured by natural selection? 2. Observations of spiny spiders Gasteracantha fornicata in north‐eastern Australia showed that the size of spiders relative to their orb webs correlated positively with relative prey numbers already captured in their webs. A possible explanation is that the relatively larger appearance of the yellow–black striped dorsal surface of this spider attracts more visually oriented prey items. Prey attracted to webs may get trapped, thereby increasing the spiders' foraging success. 3. To test this hypothesis for the function of conspicuous body colouration, a field experiment was conducted that documented the prey capture rates of spiny spiders after manipulating or sham‐manipulating their appearance. 4. As predicted, spiders that were dyed black on their striped dorsal surface caught relatively fewer prey items than did control spiders. Thus, conspicuous dorsal body colouration may be adaptive in spiny spiders because it increases foraging success and, presumably, survival rates and reproductive outputs. Overall, these data support the colour‐as‐prey‐attractant hypothesis in a stationary, trap‐building predator.     

Optimal range of prey size for antlions

1. Antlions are opportunistic trap building predators that cannot control prey encounter. Their trap should ideally retain a great diversity of prey. However, building a single trap that captures many prey with varying characteristics can be challenging. 2. A series of five different ant species ranging from thin to large, of sizes ranging from 2.75 to 6.5 mm, and a mean weight ranging from 0.54 to 6.00 mg were offered in a random succession to antlions. The state of satiation of the antlions was controlled, and their mass and the depth of their pit were recorded. The reaction of antlion to the prey, the probability of capture as well as the time to escape were recorded. 3. The probability of an antlion reaction is an increasing function of the pit depth and a decreasing function of antlion mass. The probability of capture is highest for intermediate prey mass and is an increasing function of pit depth. The time to escape is a declining function of prey mass and an increasing function of pit depth. 4. There is an upper limit to prey mass given that large prey escape out of the pit. There is a lower limit to prey mass given the difficulty to apprehend the smallest, thin species. Consequently, there is a range of prey mass, corresponding to a medium‐sized ant of 2 mg, for which the pit functions best. The physics of insect locomotion on sandy slopes was identified as the key to understanding the functioning of antlion pits.     

Optimal pit site selection in antlion larvae: the relationship between prey availability and pit maintenance costs

Journal of Ethology - Traps are an efficient method of capturing prey for ambush predators, but trap building and maintenance are costly. We describe suitable hunting sites for pit-building antlion...     

四种昆虫饵料对穴蚁蛉幼虫生长发育及消化利用的影响

选择4种可规模化饲养的昆虫——米蛾Corcyra cephalonica、家蝇Musca domestica、斜纹夜蛾Spodoptera litura、甜菜夜蛾Spodoptera exigu幼虫来饲养穴蚁蛉Myrmeleon sagax(Walker)幼虫(俗称蚁狮),研究这4种饵料对蚁狮生长发育及消化利用的影响。结果显示,用家蝇幼虫饲养的蚁狮,其体重增长、相对生长率、化蛹率、蛹重均显著高于用斜纹夜蛾和甜菜夜蛾幼虫饲养的蚁狮,而幼虫历期则比斜纹夜蛾和甜菜夜蛾幼虫饲养的短;用米蛾幼虫饲养,虽然蚁狮体重增长、化蛹率和蛹重与用家蝇幼虫饲养的差异不显著,但其相对生长率却显著低于用家蝇幼虫饲养的蚁狮,幼虫历期也比用家蝇幼虫饲养的明显延长。同时食物消化利用的结果显示,用4种饵料饲养蚁狮,它们的近似消化率差异不显著,但食物利用率和食物转化率均以家蝇幼虫饲养的蚁狮最高。经分析比较,在4种饵料昆虫中,以用家蝇幼虫每4d喂蚁狮1次的饲养效果最佳。     

Predator size divergence depends on community context

Body size is a multi‐functional trait related to various fitness components, but the relative importance of different selection pressures is seldom resolved. In Carabus japonicus beetles, of which the larvae exclusively prey on earthworms, adult body size is related to the presence/absence of a larger congener and habitat temperature. In sympatry, C. japonicus consistently exhibits smaller body size which is effective for avoiding interspecific mating, but in allopatry, it shows size variation unrelated to temperature. Here, we show that this predator–size variation is attributed to prey–size variation, associated with high phylogenetic diversity in earthworm communities. In allopatry, the predator size was larger where larger prey occurred. Larger adult size may have been selected because larger females produce larger larvae, which can subdue larger prey. Thus, in the absence of a larger congener, variation in prey body size had a pronounced effect on geographic body size divergence in C. japonicus.     

Effects of pitfall trap size and sampling duration on collection of ground beetles (Coleoptera: Carabidae) in temperate forests

Pitfall trapping is a standard sampling method to compare the abundance or community structure of ground beetles. However, effects of sampling duration on biodiversity estimation of ground beetles according to different trap sizes have not been experimentally evaluated in temperate forests in Korea. Therefore, the objective of this study was to determine the interaction between trap sizes (SB, small‐sized bottle; MB, medium‐sized bottle; PC, plastic cup; PJ, plastic jar; PT, perforated type trap; FT, funnel‐type trap) and sampling duration (T1, 2 weeks × 2 sessions, 28 days; T2, 2 weeks × 4 sessions, 56 days; T3, 4 weeks × 2 sessions, 56 days) on estimation of ground beetle assemblages in Naejangsan National Park, a temperate forest in Korea. Funnel type and larger pitfall traps collected higher numbers of individuals and species than other trap sizes. Species composition of ground beetles was different by size of traps (SB, MB, and PC vs. PJ, PT, and FT). In particular, ground beetle composition in larger traps (PJ, PT, and FT) appeared to be influenced by environmental characteristics according to localities (e.g., soil characters and dominant tree species). These findings from our study support that pitfall trapping of ground beetles can be influenced by trap sizes per se as well as sampling durations and environmental characteristics. Thus, biodiversity monitoring in temperate forests should be conducted with long sampling duration (at least 28 days) using large‐sized traps (> 7.5 cm in diameter of trap mouthpart) considering expenses and study aims.     

Prey detection in antlions: propagation of vibrational signals deep into the sand

Trap‐building antlion larvae detect their prey according to the substrate vibrations produced during movement of the prey on the sand surface. Although most studies are devoted to surface vibrational waves, in the present study, we determine the role of vibrations travelling through deeper sand layers. A behavioural experiment confirms that vibrational stimuli from prey insects on the surface of the sand stimulate the antlions buried in deeper sand layers to move towards the surface. Sand depth and particle size both have a strong effect on signal transmission. The damping coefficient (α₁₀) varies from 0.49 dB to 3.30 dB cm^?1 and depends on frequency (in the range from 100 to 300 Hz), particle size (from finest to coarse sand) and distance from the source of the vibrations. The deeper the sand, the narrower the frequency range of the signal becomes. Sand is a filter for higher frequencies. The smaller the sand particles, the more intense the filtering becomes. Fine sand with a mean sand particle size of 360 μm is a more efficient filter than coarse sand; consequently, high frequencies (> 2.5 kHz) are eliminated at a depth of 3 cm. Mean frequency depends on both depth and particle size. However, low frequency signals still propagate at a certain distance, which is biologically important in prey detection. Although the most efficient signal propagation appears to occur in coarse sand, it contains overly large particles that are inconvenient for relatively small antlion larvae. Predators seek a compromise between fine and coarse sand choosing medium sand.