Predators as drivers of insect defenses

Insects have evolved various types of antipredator defenses. For example, many insects have evolved crypsis, and exhibit cryptic body colors and shapes for hiding from predators. Other insects produce toxins as a form of chemical defense against predators, and some toxic insects are aposematic, with conspicuous body colors for advertising their toxins. Insects can also develop hairs, spines or hard exoskeletons as morphological defenses to protect themselves from predation. In addition, insects can evolve behavioral defenses, in which insects exhibit autotomy or dropping, or feign death. This study investigated which predator types evoke these types of defenses, through a review of the effectiveness of antipredator defenses in insects against carnivorous animals that are commonly used as model predators in studies. These predators include other insects, spiders, fish, frogs, lizards, birds and mammals. The results provide the first step for clarifying the evolutionary drivers of antipredator defenses in insects. The following aspects should be considered for future studies: multiple predator species and sufficient replication, alternative prey and predator models, and tolerance to predators in insects.     

Warning displays in spiny animals: one (more) evolutionary route to aposematism

To date, theoretical or laboratory simulations of aposematic evolution in prey animals have focused narrowly on internally stored chemical defense as the source of unprofitability and ignore aposematic advertisement of physical defenses such as spines (and defensive hairs, claws, etc.). This has occurred even though aposematism in spiny animals has been recognized since the 19th century. In this paper we present the first detailed theoretical consideration of aposematism in spiny animals, focusing on questions of initial evolution, costs of display, and coevolution of displays with defenses. Using an individual-based evolutionary model, we found that spines (or similar physical defenses) can easily evolve without aposematism, but when spines do evolve, aposematic displays can also easily evolve if they help to make the prey animal distinctive and if they draw attention to the physical threat. When aposematic displays evolve, they cause reduced investment in costly spines, so that, in addition to signaling unprofitability, aposematic display may enhance the cost-effectiveness of antipredator defenses (one exception to this conclusion is if the display is itself as costly as the defense). For animals with stinging spines, combining physical and chemical defense, the evolution of aposematic display may lead to reduced investment in the toxin compared to the spine. This occurs because spines act as both secondary (repellent) defenses and as primary defenses (their own visible, honest advertisement), whereas internally stored toxins only (generally) act as repellent secondary defenses. We argue that conspicuous aposematism in spines functions as an attention-getting mechanism, whereas conspicuous aposematic display in purely toxic animals may be explained by signal reliability arguments. Finally, one (more) route by which aposematism may initially evolve is by spiny rather than purely chemically defended species, spreading to species with other forms of secondary defense as the signal becomes common.     

Spine and dine: A key defensive trait promotes ecological success in spiny ants

A key focus of ecologists is explaining the origin and maintenance of morphological diversity and its association with ecological success. We investigate potential benefits and costs of a common and varied morphological trait, cuticular spines, for foraging behavior, interspecific competition, and predator–prey interactions in naturally co‐occurring spiny ants (Hymenoptera: Formicidae: Polyrhachis) in an experimental setting. We expect that a defensive trait like spines might be associated with more conspicuous foraging, a greater number of workers sent out to forage, and potentially increased competitive ability. Alternatively, consistent with the ecological trade‐off hypothesis, we expect that investment in spines for antipredator defense might be negatively correlated with these other ecological traits. We find little evidence for any costs to ecological traits, instead finding that species with longer spines either outperform or do not differ from species with shorter spines for all tested metrics, including resource discovery rate and foraging effort as well as competitive ability and antipredator defense. Spines appear to confer broad antipredator benefits and serve as a form of defense with undetectable costs to key ecological abilities like resource foraging and competitive ability, providing an explanation for both the ecological success of the study genus and the large number of evolutionary origins of this trait across all ants. This study also provides a rare quantitative empirical test of ecological effects related to a morphological trait in ants.     

Potential cotton aphid,Aphis gossypii,population suppression by arthropod predators in upland cotton

The cotton aphid, Aphis gossypii Glover, predation rate of convergent lady beetle, Hippodamia convergens Guerin‐Meneville, was determined by assigning a single predator randomly to each of four prey density treatments in the laboratory. Prey densities included 25, 50, 100, and 200 aphids per Petri dish arena. Predation response was recorded at 1, 4, 8, 16, 24, and 48 h after assigning predators to their prey treatments. Rate of consumption increased through time, with all 25 aphids consumed during the first 4 h of the experiment. At the highest density, adult lady beetle consumed on average 49, 99, 131, 163, 183, and 200 aphids within 1, 4, 8, 16, 24 and 48 h, respectively. Predators showed a curvilinear feeding response in relation to total available time, indicating that convergent lady beetles have the potential to suppress larger populations of aphids through continuous feeding by regulating their predation efficiency during feeding. The analysis of age‐specific mortality in absence of prey revealed that lady beetles could survive for an extended period of time (more than 2 weeks) without prey. The ability of a predator to survive without prey delays or prevents the rebound of pest populations that is a significant factor in natural biological control. A two‐year field sampling of 10 cotton arthropod predator species showed that spiders (27%) were the most dominant foliage dwelling predators in the Texas High Plains cotton followed by convergent lady beetles (23.5%), hooded beetles (13.5%), minute pirate bugs (11%), green lacewings (9.5%), bigeyed bugs (7.5%), scymnus beetles (3%), soft‐winged flower beetles (2%), damsel bugs (1.5%), and assassin bugs (1.5%). A field cage study showed that one H. convergens adult per plant released at prey density of one aphid per leaf kept the aphid population below economic threshold for the entire growing season.     

Araneophagic behavior of Gardena brevicollis Stål (Heteroptera: Reduviidae): Foraging on pre‐dispersal spiderlings in the spider's web

Araneophagic behavior of an Emesinae assassin bug, Gardena brevicollis Stål (Hemiptera: Reduviidae) was observed in the field. The bug invaded a web of Acusilas coccineus Simon (Araneae: Araneidae) spiderlings and hunted five juvenile spiders. The bug showed stalking tactics to prey on spiders, whereas luring tactics, which have been reported in other Emesinae bugs, were not observed. To the best of my knowledge, the present study is the first report of araneophagy in the genus Gardena and the first report of araneophagic behavior in East Asian assassin bugs.     

Pricklier with the proper predator? Predator‐induced small‐scale changes of spinescence in Daphnia

Phenotypic plasticity in defensive traits is a common response of prey organisms to variable and unpredictable predation regimes and risks. Cladocerans of the genus Daphnia are keystone species in the food web of lentic freshwater bodies and are well known for their ability to express a large variety of inducible morphological defenses in response to invertebrate and vertebrate predator kairomones. The developed defenses render the daphnids less susceptible to predation. So far, primarily large‐scale morphological defenses, like helmets, crests, and tail‐spines, have been documented. However, less is known on whether the tiny spinules, rather inconspicuous traits which cover many Daphnia’s dorsal and ventral carapace margins, respond to predator kairomones, as well. For this reason, we investigated two Daphnia species (D. magna and D. longicephala) concerning their predator kairomone‐induced changes in dorsal and ventral spinules. Since these small, inconspicuous traits may only act as a defense against predatory invertebrates, with fine‐structured catching apparatuses, and not against vertebrate predators, we exposed them to both, an invertebrate (Triops cancriformis or Notontecta maculata) and a vertebrate predator (Leucaspius delineatus). Our results show that the length of these spinules as well as spinules‐covered areas vary, likely depending on the predator the prey is exposed to. We further present first indications of a Daphnia species‐specific elongation of the spinules and an increase of the spinules‐bearing areas. Although we cannot exclude that spinescence is altered because it is developmentally connected to changes in body shape in general, our results suggest that the inducible alterations to the spinule length and spinules‐covered areas disclose another level of predator‐induced changes in two common Daphnia species. The predator‐induced changes on this level together with the large‐scale and ultrastructural defensive traits may act as the overall morphological defense, adjusted to specific predator regimes in nature.     

Predators shape distribution and promote diversification of morphological defenses in <Emphasis Type="Italic">Leucorrhinia</Emphasis>, Odonata

Predators strongly influence species assemblages and shape morphological defenses of prey. Interestingly, adaptations that constitute effective defenses against one type of predator may render the prey susceptible to other types of predators. Hence, prey may evolve different strategies to escape predation, which may facilitate adaptive radiation of prey organisms. Larvae of different species in the dragonfly genus Leucorrhinia have various morphological defenses. We studied the distribution of these larvae in relation to the presence of predatory fish. In addition, we examined the variation in morphological defenses within species with respect to the occurrence of fish. We found that well-defended species, those with more and longer spines, were more closely associated with habitats inhabited by predatory fish and that species with weakly developed morphological defenses were more abundant in habitats without fish. The species predominantly connected to lakes with or without fish, respectively, were not restricted to a single clade in the phylogeny of the genus. Our data is suggestive of phenotypic plasticity in morphological defense in three of the studied species since these species showed longer spines in lakes with fish. We suggest that adaptive phenotypic plasticity may have broadened the range of habitats accessible to Leucorrhinia. It may have facilitated colonization of new habitats with different types of predators, and ultimately, speciation through adaptive radiation.     

Temporal environmental variation and phenotypic plasticity: a mechanism underlying priority effects

Understanding the role of history in the formation of communities has been a major challenge in community ecology. Here, we explore the role of phenotypic plasticity and its associated trait‐mediated indirect interactions as a mechanism behind priority effects. Using organisms with inducible defenses as a model system, we examine how aquatic communities initially containing different predator environments are affected at the individual and community level by the colonization of a second predator. Snails and tadpoles were established in four different caged‐predator environments (no predator, fish, crayfish or water bugs). These four communities were then crossed with three predator colonization treatments (no colonization, early colonization, or late colonization) using lethal water bugs as the predator. The snails responded to the caged predator environments with predator‐specific behavioral and morphological defenses. In the colonization treatments, snails possessing the wrong phenotype attempted to induce phenotypic changes to defend themselves against the new risk. However, snails initially induced by a different predator environment often suffered high predation rates. Hence, temporal variation in predation risk not only challenged the snail prey to try to track this environmental variation through time by adjusting their defensive phenotypes, but also caused trait‐mediated interactions between snails and the colonizing predator. For tadpoles within these communities, there was little evidence that the morphological responses of snails indirectly effected tadpole predation rates by colonizing water bugs. Unexpectedly, predation rates on tadpoles by colonizing water bugs were generally higher in the three caged‐predator treatments, suggesting that water bugs elevated their foraging activity in response to potentially competing predators. In summary, we demonstrate an important priority effect in which the initial occurrence of one species of predator can facilitate predation by a second predator that colonizes at a later date (i.e. a TMII) suggesting that phenotypic plasticity can be an important driver behind priority effects (i.e. historical exposure to predators).     

Entrapped carrion increases indirect plant resistance and intra‐guild predation on a sticky tarweed

Many plants employ indirect defenses against herbivores; often plants provide a shelter or nutritional resource to predators, increasing predator abundance, and lessening herbivory to the plant. Often, predators on the same plant represent different life stages and different species. In these situations intraguild predation (IGP) may occur and may decrease the efficacy of that defense. Recently, several sticky plants have been found to increase indirect defense by provisioning predatory insects with entrapped insects (hereafter: carrion). We conducted observational studies and feeding trials with herbivores and predators on two sticky, insect‐entrapping asters, Hemizonia congesta and Madia elegans, to construct food webs for these species and determine the prevalence of IGP in these carrion‐provisioning systems. In both systems, intraguild predation was the most common interaction observed. To determine whether IGP was driven by resource abundance, whether it reduced efficacy of this indirect defense and whether stickiness or predator attraction was induced by damage, we performed field manipulations on H. congesta. Carrion supplementation led to an increase in predator abundance and IGP. IGP was asymmetric within the predator guild: assassin bugs and spiders preyed on small stilt bugs but not vice versa. Despite increased IGP, carrion provisions decreased the abundance of the two most common herbivores (a weevil and a mealybug). Overall seed set was driven by plant size, but number of seeds produced per fruit significantly increased with increasing carrion, likely because of the reduction in the density of a seed‐feeding weevil. Observationally and experimentally, we found that carrion‐mediated indirect defense of tarweeds led to much intraguild predation, though predators effectively reduced herbivore abundance despite the increase in IGP.     

Sluggish Movement and Repugnant Odor Are Positively Interacting Insect Defensive Traits in Encounters with Frogs

Sluggish movement is common in chemically defended insects. We have recently shown that sluggish movement can be beneficial to prey when it fails to release the attack response of an ambush (=motion-oriented) predator. Here, we test the hypothesis that sluggish movement and chemical defense (i.e., repugnant odor) together are more defensive than either alone. We manipulated the movement and odor of lubber grasshoppers to produce four prey types: (1) sluggish-moving and high odor, (2) sluggish-moving and low odor, (3) fast-moving and high odor, and (4) fast-moving and low odor. We then offered these prey to frogs. In two independent experiments, frogs attacked prey type 1 (i.e., sluggish-moving and high-odor prey) significantly later than they attacked the other prey types. Hence, the defenses of sluggish movement and repugnant odor can act together to produce a prey that is better defended than prey with either defense alone. This may help explain why these two traits commonly cooccur in insects.     

Cardenolide‐defended milkweed bugs do not evoke learning in Nephila senegalensis spiders

Antipredator defense of herbivorous insects often relies on the potential toxicity of defensive chemicals sequestered from their host plants. The colorful Lygaeinae (Heteroptera: Lygaeidae) store a concentrated mixture of toxic cardenolides (cardiac glycosides) in specialized storage compartments of the bugs' integument, from which they are released upon attack. Larvae and adults of the large milkweed bug Oncopeltus fasciatus (Heteroptera: Lygaeinae) are specialized to feed on cardenolide‐containing milkweeds in the plant genus Asclepias and display a conspicuous red and black colorations. To investigate whether O. fasciatus gained improved protection by feeding on a toxic host plant (Asclepias syriaca), compared to a nontoxic alternative (sunflower seeds), we fed nymphs and adults of O. fasciatus to the golden orb‐weaver Nephila senegalensis. While visually oriented vertebrates, such as avian predators, have been intensively investigated for their reaction to defensive compounds and aposematic coloration, less attention has been paid to invertebrate predators. Their different perceptual abilities can provide important opportunities for testing hypotheses on warning coloration and chemical defenses. The predation trials showed that the bugs fed on Asclepias were significantly less likely to be killed than the bugs reared on a cardenolide‐free diet. This suggests that sequestered cardenolides in O. fasciatus nymphs and adults represent a significant fitness advantage on an individual level against this invertebrate predator. Yet, when testing for avoidance learning in the spiders, negative experience did not change the way how similar prey was attacked at the next encounter. In this case, visual or chemical aposematism thus does not seem to matter for predator learning.     

Eocene tortoise beetles from the Green River Formation in Colorado,U.S.A. (Coleoptera: Chrysomelidae: Cassidinae)

Abstract The fossil history of leaf beetles (Chrysomelidae) is relatively poorly documented despite an abundance of available material. Of particular interest is the origin and radiation of the diverse tortoise beetles, a derived group within Cassidinae s.l. (=Cassidinae + Hispinae) defined by the exophagous life history and specialized morphology of the immature stages. Cassidinae is also a group with relatively few fossil records that can be assigned with any degree of certainty. Here we report two of the oldest definitive tortoise beetle fossils, Eosacantha delocranioides gen.n. et sp.n. and Denaeaspis chelonopsis gen.n. et sp.n. , from the Eocene Green River Formation (ca. 47 million years old) in northwestern Colorado, U.S.A. Owing to the fine level of preservation, many important features can be observed and are coded into the recent cladistic analysis for the subfamily. Phylogenetic analysis highlights that both genera have affinities with modern lineages, one restricted to the Old World and the other restricted to the Neotropics. Although Cassidinae as a whole extend into the Cretaceous, the available information suggests that the tortoise beetles perhaps originated and diversified during the Early Tertiary. As such, the morphological and biological transitions from the leaf‐mining hispiforms to the distinctive tortoise‐like cassidiforms, with their elaborate defensive larval shields and other unique behaviours, probably took place during the latest Paleocene or earliest Eocene. These Green River fossils are the oldest yet to document the specialized morphology associated with the transition in cassidine feeding and immature biology.     

Experimental assessment of trophic ecology in a generalist spider predator: Implications for biocontrol in Uruguayan crops

Conservative biological control promotes the use of native natural enemies to limit the size and growth of pest populations. Although spiders constitute one of the most important groups of native predators in several crops, their trophic ecology remains largely unknown, especially for several generalist taxa. In laboratory, we assessed the predatory behaviour of a wandering spider (the wolf spider Lycosa thorelli (Keyserling, 1877) against several arthropods varying in size and trophic positions, all found in South American soybean and rice crops. As prey we used the bug Piezodorus guildinii (Westwood, 1837) as well as larvae and adults of the moth Spodoptera frugiperda (Smith, 1797), both being considered important pests in Uruguayan crops. We also used several non-pest arthropods as prey, sarcophagid flies, carabid beetles and wolf spiders. All prey were attacked in more or less high, although not statistically differing, proportions. However, carabids were not consumed, and bugs were consumed in significantly lower proportions than flies. A negative correlation was found between prey size and acceptance rate. Immobilization times were longer against larvae when compared to moths and flies, while predatory sequences were longer for bugs when compared to flies, moths and spiders. In addition, we found a positive effect of prey size on predatory sequence length and complexity. Our results confirm the ability of spiders to attack and feed upon prey with different morphologies, included well-defended arthropods, and their potential use as natural enemies of several pests in South American crops.     

Not attackable or not crackable—How pre‐ and post‐attack defenses with different competition costs affect prey coexistence and population dynamics

It is well‐known that prey species often face trade‐offs between defense against predation and competitiveness, enabling predator‐mediated coexistence. However, we lack an understanding of how the large variety of different defense traits with different competition costs affects coexistence and population dynamics. Our study focusses on two general defense mechanisms, that is, pre‐attack (e.g., camouflage) and post‐attack defenses (e.g., weaponry) that act at different phases of the predator—prey interaction. We consider a food web model with one predator, two prey types and one resource. One prey type is undefended, while the other one is pre‐ or post‐attack defended paying costs either by a higher half‐saturation constant for resource uptake or a lower maximum growth rate. We show that post‐attack defenses promote prey coexistence and stabilize the population dynamics more strongly than pre‐attack defenses by interfering with the predator's functional response: Because the predator spends time handling “noncrackable” prey, the undefended prey is indirectly facilitated. A high half‐saturation constant as defense costs promotes coexistence more and stabilizes the dynamics less than a low maximum growth rate. The former imposes high costs at low resource concentrations but allows for temporally high growth rates at predator‐induced resource peaks preventing the extinction of the defended prey. We evaluate the effects of the different defense mechanisms and costs on coexistence under different enrichment levels in order to vary the importance of bottom‐up and top‐down control of the prey community.     

Predatory Versatility in Females of the Scorpion <Emphasis Type="Italic">Bothriurus bonariensis</Emphasis> (Scorpiones: Bothriuridae): Overcoming Prey with Different Defensive Mechanisms

Scorpions are dominant predators in some environments. Nevertheless, most studies of predatory behavior in scorpions have focused on diet composition whereas some other relevant aspects, such as predatory strategy, have been poorly explored. Herein we evaluate the prey acceptance and predatory strategy of the scorpion Bothriurus bonariensis against sympatric prey with different defenses. As prey, we selected earwigs (Forficula cf. auricularia) which use pincer-like defensive appendages, hard-bodied isopods (Armadillium vulgare) known for their conglobation defensive strategy, soft bodied isopods (Porcellio cf. scaber), which secrete noxious substances as defense mechanisms, cockroaches with limited defensive mechanisms (Blatta cf. orientalis.) and spiders (Lycosa cf. poliostoma) which possess venomous fangs. Prey were offered to 21 adults of B. bonariensis in random order until all prey had been offered to all scorpions. Prey consumption and the number of attempts needed for capture were recorded. We also evaluated the effect of sting use on immobilization time as well as the prey capture strategies on the most consumed prey. We found that despite using a similar number of attempts for capturing all prey, spiders and armadillid isopods were less consumed than other prey. Immobilization times were longer for earwigs than for armadillid isopods and cockroaches. Scorpions used alternative predatory strategies against these aforementioned prey, although the stinger was used against all of them. These results show that scorpions are able to use different predatory strategies which might allow them to include prey with diverse defensive strategies in their diet.     

Chemical defense of toad tadpoles under risk by four predator species

Many organisms use inducible defenses as protection against predators. In animals, inducible defenses may manifest as changes in behavior, morphology, physiology, or life history, and prey species can adjust their defensive responses based on the dangerousness of predators. Analogously, prey may also change the composition and quantity of defensive chemicals when they coexist with different predators, but such predator‐induced plasticity in chemical defenses remains elusive in vertebrates. In this study, we investigated whether tadpoles of the common toad (Bufo bufo) adjust their chemical defenses to predation risk in general and specifically to the presence of different predator species; furthermore, we assessed the adaptive value of the induced defense. We reared tadpoles in the presence or absence of one of four caged predator species in a mesocosm experiment, analyzed the composition and quantity of their bufadienolide toxins, and exposed them to free‐ranging predators. We found that toad tadpoles did not respond to predation risk by upregulating their bufadienolide synthesis. Fishes and newts consumed only a small percentage of toad tadpoles, suggesting that bufadienolides provided protection against vertebrate predators, irrespective of the rearing environment. Backswimmers consumed toad tadpoles regardless of treatment. Dragonfly larvae were the most voracious predators and consumed more predator‐naïve toad tadpoles than tadpoles raised in the presence of dragonfly cues. These results suggest that tadpoles in our experiment had high enough toxin levels for an effective defense against vertebrate predators even in the absence of predator cues. The lack of predator‐induced phenotypic plasticity in bufadienolide synthesis may be due to local adaptation for constantly high chemical defense against fishes in the study population and/or due to the high density of conspecifics.     

Advantages of social foraging in crab spiders: Groups capture more and larger prey despite the absence of a web

Among group‐living spiders, subsocial representatives in the family of crab spiders (Thomisidae) are a special case, as they build protective communal leaf nests instead of extensive communal capture webs. It could thus be inferred that antipredator benefits (e.g., enhanced protection in larger nests) rather than foraging‐related advantages (e.g., capture of more and larger prey) promote sociality in this family. Nonetheless, subsocial crab spiders do share prey, and if this behaviour does not reflect mere food scramble but has a cooperative character, crab spiders may offer insights into the evolution of social foraging applicable to many other cooperative predators that hunt without traps. Here, we performed a comparative laboratory feeding experiment on three of the four subsocial crab spider species—Australomisidia ergandros, Australomisidia socialis and Xysticus bimaculatus—to determine if crab spiders derive advantages from foraging in groups. In particular, we tested artificially composed groups of five sibling spiderlings vs. single siblings in terms of prey capture success and prey size preference. Across species, groups had higher prey capture success (measured in terms of capture rates and capture latency) and were more likely to attack large, sharable prey—dynamics leading to reduced food competition among group members in favour of living and foraging in groups. Within groups, we further compared prey extraction efficiency among the three applied social foraging tactics: producing, scrounging and feeding alone. In A. ergandros, individuals were exceptionally efficient when using the non‐cooperative scrounger tactic, which entails feeding on the prey provided by others. Thus, our multispecies comparison confirms foraging advantages in maintaining a cooperative lifestyle for crab spiders, but also demonstrates the relevance of research into exploitation of cooperative foraging in this family.     

Tree species diversity alters plant defense investment in an experimental forest plantation in southern Mexico

How plant species diversity affects traits conferring herbivore resistance (e.g., chemical defenses), as well as the mechanisms underlying such effects, has received little attention. One potential mechanism for the effect of diversity on plant defenses is that increased plant growth at high diversity could lead to reduced investment in defenses via growth–defense trade‐offs. We measured tree growth (diameter at breast height) and collected leaves to quantify total phenolics in 2.5‐year‐old plants of six tropical tree species (N = 597 plants) in a young experimental plantation in southern Mexico. Selected plants were classified as monocultures or as polycultures represented by mixtures of four of the six species examined. Tree species diversity had a significant negative effect on total phenolics, where polycultures exhibited a 13 percent lower mean concentration than monocultures. However, there was marked variation in the effects of diversity on defenses among tree species, with some species exhibiting strong reductions in phenolic levels in mixtures, whereas others were unresponsive. In addition, tree species diversity had no effect on growth, nor was the negative effect of diversity on chemical defenses mediated by a growth–defense trade‐off. These results demonstrate that tree diversity can alter investment in chemical defenses in long‐lived tree species but that such effect may not always be under strong control by plant endogenous resource allocation trade‐offs. Regardless of the underlying mechanism, these findings have important implications for predicting effects on consumers and ecosystem function.     

Aversive reactions of two invertebrate predators to European red–black insects

Prey species gain protection by imitating signals of unpalatable models in defensive mimicry. Mimics have been traditionally classified as Batesian (palatable mimic resembling an unpalatable model) or Müllerian (unpalatable mimic resembling a similarly unpalatable model). However, recent studies suggest that rather than discrete categories, the phenomenon of mimicry can be better understood as a continuum. The level of unpalatability of defended prey is a key factor in determining the type of mimetic relationship. Herein, we used insects (ladybugs and true bugs) from a putative European “red–black” mimetic complex as experimental models of defended species and crickets as a control prey. We offered the prey to two species of sympatric invertebrate predators (praying mantis and spider) and video recorded the interactions. We tested three alternative hypotheses, namely (i) the three red–black species tested are similarly defended against both predators; (ii) some red–black species are better defended than others against both predator species, and (iii) the effectiveness of the red–black species defenses is predator dependent. Both predators attacked all prey types with a similar frequency. But while all three red–black species similarly elicited aversive behaviors in spiders, the mantises' aversive reactions varied depending on the prey species. Our results provide support to the third hypothesis, suggesting that the same prey species can fall into different parts of the spectrum of palatability–unpalatability depending on the type of predator.     

Ontogenetic change in effectiveness of chemical defence against different predators in Oxycarenus true bugs

Many prey species change their antipredator defence during ontogeny, which may be connected to different potential predators over the life cycle of the prey. To test this hypothesis, we compared reactions of two predator taxa – spiders and birds – to larvae and adults of two invasive true bug species, Oxycarenus hyalinipennis and Oxycarenus lavaterae (Heteroptera: Oxycarenidae) with life-stage-specific chemical defence mechanisms. The reactions to larvae and adults of both true bug species strikingly differed between the two predator taxa. The spiders were deterred by the defences of adult bugs, but the larval defences were ineffective against them. By contrast, birds attacked the larvae considerably less often than the adult bugs. The results indicate a predator-specific ontogenetic change in defence effectiveness of both Oxycarenus species. The change in defence is likely linked to the life-stage-specific composition of secretions in both species: whereas secretions of larvae are dominated by unsaturated aldehydes, secretions of adults are rich in terpenoids, which probably serve dual function of defensive chemicals and pheromones. Our results highlight the variation in defence between different life stages and the importance of testing responses of different types of predators.