Defense in insects
One of the best known modern examples of the role that evolution has played in insect defenses is the link between melanism and the peppered moth (Biston betularia). Peppered moth evolution over the past two centuries in England has taken place, with darker morphs becoming more prevalent over lighter morphs so as to reduce the risk of predation. However, its underlying mechanism is still debated.[2]
Hiding
Walking sticks (order Phasmatodea), many katydid species (family Tettigoniidae), and moths (order Lepidoptera) are just a few of the insects that have evolved specialized cryptic morphology. This adaptation allows them to hide within their environment because of a resemblance to the general background or an inedible object.[2] When an insect looks like an inedible or inconsequential object in the environment that is of no interest to a predator, such as leaves and twigs, it is said to display mimesis, a form of crypsis.
Insects may also take on different types of camouflage, another type of crypsis. These include resembling a uniformly colored background as well as being light below and dark above, or countershaded. Additionally, camouflage is effective when it results in patterns or unique morphologies that disrupt outlines so as to better merge the individual into the background.[2]
Cost and benefit perspective
Butterflies (order Lepidoptera) are a good example of the balancing act between the costs and benefits associated with defense. In order to take off, butterflies must have a thorax temperature of 36–40 °C (97–104 °F). This energy is derived both internally through muscles and externally through picking up solar radiation through the body or wings. When looked at in this light, cryptic coloration to escape from predators, markings to attract conspecifics or warn predators (aposematism), and the absence of color to absorb adequate solar radiation, all play key roles in survival. Only when these three affairs are in balance does the butterfly maximize its fitness.[3]
Mimicry
Mimicry is a form of defense which describes when a species resembles another recognized by natural enemies, giving it protection against predators.[2] The resemblance among mimics does not denote common ancestry. Mimicry works if and only if predators are able to learn from eating distasteful species. It is a three part system that involves a model species, a mimic of that species, and a predatory observer that acts as a selective agent. If learning is to be successful, then all models, mimics, and predators must co-exist, a notion feasible within the context of geographic sympatry.[4]
Mimicry is divided into two parts, Batesian mimicry and Müllerian mimicry.
Batesian mimicry
In Batesian mimicry, an aposematic inedible model has an edible mimic. Automimics are individuals that, due to environmental conditions, lack the distasteful or harmful chemicals of conspecifics, but are still indirectly protected through their visibly identical relatives.[2] An example can be found in the plain tiger (Danaus chrysippus), a non-edible butterfly, which is mimicked by multiple species, the most similar being the female danaid eggfly (Hypolimnas misippus).
Müllerian mimicry
In Müllerian mimicry, a group of species benefit from each other's existence because they all are warningly colored in the same manner and are distasteful. The best examples of this phenomenon can be found within the butterfly genus Heliconius.
Behavioral responses
Behavioral responses to escape predation include burrowing into substrate and being active only through part of the day.[1] Furthermore, insects may feign death, a response termed thanatosis. Beetles, particularly weevils, do this frequently.[2] Bright colors may also be flashed underneath cryptic ones. A startle display occurs when prey takes advantage of these markings after being discovered by a predator.[2] The striking color pattern, which often includes eyespots, is intended to evoke prompt enemy retreat.[1] Better formed eyespots seem to result in better deterrence.[2]
Mechanical defenses
Insects have had millions of years to evolve mechanical defenses. Perhaps the most obvious is the
Some insects uniquely create retreats that appear uninteresting or inedible to predators.
Autotomy
Autotomy, or the shedding of appendages,[2] is also used to distract predators, giving the prey a chance to escape. This highly costly mechanism is regularly practiced within stick insects (order Phasmatodea) where the cost is accentuated by the possibility that legs can be lost 20% of the time during molting.[7] Harvestmen (order Opiliones) also use autotomy as a first line of defense against predators.[8]
Chemical defenses
Unlike pheromones, allomones harm the receiver at the benefit of the producer.[2] This grouping encompasses the chemical arsenal that numerous insects employ. Insects with chemical weaponry usually make their presence known through aposematism. Aposematism is utilized by non-palatable species as a warning to predators that they represent a toxic danger.[3] Additionally, these insects tend to be relatively large, long-lived, active, and frequently aggregate.[2] Indeed, longer-lived insects are more likely to be chemically defended than short lived ones, as longevity increases apparency.[9]
Throughout the arthropod and insect realm, however, chemical defenses are quite unevenly distributed. There is great variation in the presence and absence of chemical arms among orders and families to even within families.[9] Moreover, there is diversity among insects as to whether the defensive compounds are obtained intrinsically or extrinsically.[10][page needed] Many compounds are derived from the main food source of insect larvae, and occasionally adults, feed, whereas other insects are able to synthesize their own toxins.[2]
In
Classification
Gullan and Cranston have divided chemical defenses into two classes. Class I chemicals irritate, injure, poison, or drug individual predators. They can be further separated into immediate or delayed substances, depending on the amount of time it takes to feel their effects. Immediate substances are encountered topographically when a predator handles the insect while delayed chemicals, which are generally contained within the insect's tissues, induce
Pasteels, Grégoire, and Rowell-Rahier [9] grouped chemical defenses into three types: compounds that are truly poisonous, those that restrict movement, and those that repel predators. True poisons, essentially Class I compounds, interfere with specific physiological processes or act at certain sites.[9] Repellents are similar to those classified under Class II as they irritate the chemical sensitivity of predators. Impairment of movement and sense organs is achieved through sticky, slimy, or entangling secretions that act mechanically rather than chemically. This last grouping of chemicals has both Class I and Class II properties. Again, these three categories are not mutually exclusive, as some chemicals can have multiple effects.[9]
Examples
Assassin bugs
When startled, the assassin bug Platymeris rhadamanthus (family Reduviidae),[8] is capable of spitting venom up to 30 cm at potential threats. The saliva of this insect contains at least six proteins including large amounts of protease, hyaluronidase, and phospholipase which are known to cause intense local pain, vasodilation, and edema.[10]
Cockroaches
Many cockroach species (order Blattodea) have mucus-like adhesive secretions on their posterior. Although not as effective against vertebrates, these secretions foul the mouths of invertebrate predators, increasing the chances of the cockroach escaping.[10]
Termites
The majority of termite soldiers secrete a rubberlike and sticky chemical concoction that serves to entangle enemies, called a
Among termite species in the Apicotermitinae that are soldierless or where soldiers are rare, mouth secretions are commonly replaced by abdominal dehiscence. These termites contract their abdominal muscles, resulting in the fracturing of the abdominal wall and the expulsion of gut contents. Because abdominal dehiscence is quite effective at killing ants, the noxious chemical substance released is likely contained within the termite itself.[12]
Ants
The subfamily Dolichoderinae, which also does not possess a stinger, has a different type of defense. The anal gland secretions of this group rapidly polymerize in air and serve to immobilize predators.[10]
Leaf beetles
Leaf beetles produce a spectrum of chemicals for their protection from predators. In the case of the subtribe Chrysomelina (Chrysomelinae), all live stages are protected by the occurrence of isoxazolin-5-one derived glucosides that partially contain esters of 3-nitropropanoic acid (3-NPA, beta-nitropropionic acid).[13] The latter compound is an irreversible inhibitor of succinate dehydrogenase.[14] Hence, 3-NPA inhibits the tricarboxylic acid cycle. This inhibition leads to neurodegeneration with symptoms similar to those caused by Huntington's disease.[15] Since leaf beetles produce high concentrations of 3-NPA esters, a powerful chemical defense against a wide range of different predators is obvious. The larvae of Chrysomelina leaf beetles developed a second defensive strategy that is based on the excretion of droplets via pairs of defensive glands at the back of the insects. These droplets are immediately presented after mechanical disturbance and contain volatile compounds that derive from sequestered plant metabolites. Due to the specialization of leaf beetles to a certain host plant, the composition of the larval secretion is species-dependent.[16][17] For instance, the red poplar leaf beetle (Chrysomela populi) consumes the leaves of poplar plants, which contain salicin. This compound is taken up by the insect and then further transformed biochemically into salicylaldehyde, an odor very similar to benzaldehyde. The presence of salicin and salicylaldehyde can repel potential predators of leaf beetles.[17]
The hemolymph toxins originate from autogenous de novo biosynthesis by the Chrysomelina beetle.
The larvae of leaf beetles from the subfamilies of e.g., Criocerinae and Galerucinae often employ fecal shields, masses of feces that they carry on their bodies to repel predators. More than just a physical barrier, the fecal shield contains excreted plant volatiles that can serve as potent predator deterrents.[20]
Wasps
Ant attacks represent a large predatory pressure for many species of wasps, including Polistes versicolor. These wasps possess a gland located in the VI abdominal sternite (van de Vecht's gland) that is primarily responsible for making an ant-repellent substance. Tufts of hair near the edge of the VI abdominal sternite store and apply the ant repellent, secreting the ant repellent through a rubbing behavior.[21]
Collective defenses in social insects
Many chemically defended insect species take advantage of clustering over solitary confinement.
Termites (order
Some species of bee, mainly that of the genus
Alarm
Immunity
Insects, like nearly every other organism, are subject to
Role of phenotypic plasticity
Phenotypic plasticity is the capacity of a single genotype to exhibit a range of phenotypes in response to variation in the environment.[30] For example, in Nemoria arizonaria caterpillars, the cryptic pattern changes according to season and is triggered by dietary cues. In the spring, the first brood of caterpillars resembles oak catkins, or flowers. By the summer when the catkins have fallen, the caterpillars discreetly mimic oak twigs.[31] No intermediate forms are present in this species, although other members of the genus Nemoria, such as N. darwiniata, do exhibit transitional forms.[30]
In social insects such as ants and termites, members of different castes develop different phenotypes. For example, workers are normally smaller with less pronounced mandibles than soldiers. This type of plasticity is more so determined by cues, which tend to be non-harmful stimuli, than by the environment.[30]
Phenotypic plasticity is important because it allows an individual to adapt to a changing environment and can ultimately alter their evolutionary path. It not only plays an indirect role in defense as individuals prepare themselves physically to take on the task of avoiding predation through camouflage or developing collective mechanical traits to protect a social hive, but also a direct one. For example, cues elicited from a predator, which may be visual, acoustic, chemical, or vibrational, may cause rapid responses that alter the prey’s phenotype in real time.[32]
See also
- Insect ecology
- Antipredator adaptation
- Behavioral ecology
References
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- ^ J. E. Huheey (1984). "Warning coloration and mimicry". In William J. Bell & Ring T. Cardé (ed.). Chemical Ecology of Insects. London: Chapman and Hall. pp. 257–297.
- ^ Nation, James L. Insect Physiology and Biochemistry. Boca Raton, FL: CRC Press, 2002.
- ^ John R. Meyer (March 8, 2005). "Trichoptera". ENT 425 - General Entomology. North Carolina State University. Archived from the original on July 28, 2010. Retrieved March 23, 2011.
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