Defense in insects
Encyclopedia
Insect
s have a wide variety of predators, including bird
s, reptile
s, amphibian
s, mammal
s, carnivorous plant
s, and other arthropod
s. The great majority (80–99.99%) of individuals born do not survive to reproductive age, with perhaps 50% of this mortality rate attributed to predation. In order to deal with this ongoing escapist battle, insects have evolved a wide range of defense mechanisms. The only restraint on these adaptation
s is that their cost, in terms of time and energy, does not exceed the benefit that they provide to the organism in question. The further that a feature tips the balance towards beneficial, the more likely that selection will act upon the trait, passing it down to further generations. The opposite also holds true; defenses that are too costly will have a slim to zero chance of being passed down. Examples of defenses that have withstood the test of time include hiding, escape by flight or running, and firmly holding ground to fight as well as producing chemicals and social structures that help prevent predation.
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 up to debate.
), many katydid species (family Tettigoniidae
), and moths (order Lepidoptera
) are just a few of the plethora of insects that have evolved specialized cryptic
morphology. This adaptation allows them to effectively hide within their environment because of a resemblance to the general background or an inedible object. 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 mimensis, a form of crypsis.
Insects may also take on different types of camouflage
, another type of cypsis. These include resembling a uniformly colored background as well as being light below and dark above, or coundershaded
. 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.
) 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 (96.8–104 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.
.
Mimicry is divided into two parts, Batesian mimicry
and Müllerian mimicry
.
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. 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
).
, a group of species benefit from each others' 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 Heliconius
butterfly genus. Like in Batesian mimicry, the mimics are not necessarily related, although they obviously are in Heliconius.
s, are especially fond of this sort of acting. 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. The striking color pattern, which often includes eyespots, is intended to evoke prompt enemy retreat. Better formed eyespots seem to result in better deterrence.
. Although its main role lies in support and muscle attachment, when extensively hardened by the cross-linking of proteins and chitin, or sclerotized
, the cuticle acts as a first line of defense. Additional physical defenses include modified mandibles, horns, and spines on the tibia and femur. When these spines take on a main predatory role, they are termed raptorial
.
Some insects uniquely create retreats that appear uninteresting or inedible to predators. This is the case in caddisfly larvae (order Trichoptera
) which encase their abdomen with a mixture of natural materials like leaves, twigs, and stones.
, or the shedding of appendages, 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. Harvestmen (order Opiliones
) also use autotomy as a first line of defense against predators.
s harm the receiver at the benefit of the producer. 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. Additionally, these insects tend to be relatively large, long-lived, active, and frequently aggregate. Indeed, longer-lived insects are more likely to be chemically defended than short lived ones, as longevity increases apparency.
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. Moreover, there is diversity among insects as to whether the defensive compounds are obtained intrinsically or extrinsically. 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.
In reflex bleeding
, insects dispel their blood, hemolymph, or a mixture of exocrine secretions and blood as a defensive maneuver. As previously mentioned, the discharged blood may contain toxins produced within the insect source or externally from plants that the insect consumed. Reflexive bleeding occurs in specific parts of the body; for example, the beetle families Coccinellidae
(ladybugs) and Meloidae bleed from the knee joints.
and blistering. Class I chemicals include bufadienolides, cantharidin, cyanides, cardenolides, and alkaloids, all of which have greater effects on vertebrates than on other arthropods. In fact, the most frequently encountered defensive compounds in insects are alkaloids.
In contrast to Class I chemicals, Class II chemicals are essentially harmless. They stimulate scent and taste receptors so as to discourage feeding. They tend to have low molecular weight and are volatile and reactive, including acids, aldehydes, aromatic ketones, quinones, and terpenes. Furthermore, they may be aposematic, indicating through odors the presence of chemical defenses. The two different classes are not mutually exclusive, and insects may use combinations of the two.
Pasteels, Grégoire, and Rowell-Rahier also grouped chemical defenses, albeit in a different manner. Weaponry is partitioned into chemical 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. 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. As with Class I and Class II compounds, these three categories are not mutually exclusive, as some chemicals can have multiple effects.
, the assassin bugs and thread-legged bugs. Interestingly, the assassin bug species Platymeris rhadamanthus is capable of spitting venom up to 30 cm. The saliva of this insect, loaded with a mixture of at least six proteins including excess amounts of protease
, hyaluronidase
, and phospholipase
causes intense local pain, vasodilation
, and edema
.
, and it is usually coupled with specialized mandibles. In nasute species of termites (contained within the subfamily Nasutitermitinae), the mandibles have receded. This makes way for an elongated, syringic nasus
capable of squirting liquid glue. When this substance is released from the frontal gland reservoir and dries, it becomes sticky and is capable of immobilizing attackers. It is highly effective against other arthropods, including spiders, ants, and centipedes.
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.
is the defense of choice for many ants (family Formicidae
). It is injected from an ovipositor
that has been evolutionarily modified into a stinging apparatus. These ants release a complex venom mixture that can include histamine
. Within the Formicinae
subfamily, the stinger has been lost and instead the poison gland forcibly ejects the fluid of choice, formic acid
. Some carpenter ants (genus Camponotus) also have mandibular glands that extend throughout their bodies. When these are mechanically irritated, the ant forcibly commits suicide and by exploding, spilling out a sticky, entangling substance.
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.
, cycloalexy is adopted. Either the heads or ends of the abdomen, depending on where noxious compounds are secreted, make up the circumference of a circle. The remaining larvae lie inside this defensive ring where the defenders repel predators through threatening attitudes, regurgitation
, and biting.
Termites (order Isoptera), like eusocial
ants, wasps, and bees, rely on a caste system to protect their nests. The evolution of fortress defense is closely linked to the specialization of soldier mandibles. Soldiers can have biting-crushing, biting-cutting, cutting, symmetrical snapping, and asymmetrical snapping mandibles. These mandibles may be paired with frontal gland secretion, although snapping soldiers rarely utilize chemical defenses. Termites take advantage of their modified mandibles in phragmosis
, which is the blocking of the nest with any part of the body; in this case of termites, nest entrances are blocked by the heads of soldiers.
Alarm pheromone
s warn members of a species of approaching danger. Because of their altruistic nature, they follow the rules of kin selection
. They can illicit both aggregational and dispersive responses in social insects depending on the alarm caller’s location relative to the nest. Closer to the nest, it causes social insects to aggregate and may subsequently produce an attack against the threat. In nonsocial insects, these compounds typically stimulate dispersal regardless of location. Chemical alarm systems are best developed in aphid
s and treehoppers (family Membracidae) among the nonsocial groups. Alarm pheromones take on a variety of compositions, ranging from terpenoids in aphids and termites to acetate
s, an alcohol, and a ketone in honeybees
to formic acid and terpenoids in ants.
s caused by viruses, bacteria, fungi, protozoa, and nematode
s. These encounters can either kill the insect or greatly weaken it. Insects protect themselves against these detrimental microorganisms in two ways. Firstly, the body-enveloping chitin
cuticle, in conjunction with the tracheal system and the gut lining, serve as major physical barriers to entry. Secondly, hemolymph
itself plays a key role in repairing external wounds as well as destroying foreign organisms within the body cavity. Interestingly, insects, along with having passive immunity, also show evidence of acquired immunity
.
Social insects additionally have a repertoire of behavioural and chemical 'border-defences' and in the case of the ant, groom venom or metapleural gland
secretions over their cuticle .
is the capacity of a single genotype
to exhibit a range of phenotypes in response to variation in the environment. 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 catkin
s, or flowers. By the summer when the catkins have fallen, the caterpillars discretely mimic oak twigs. No intermediate forms are present in this species, although other members of the Nemoria
genus, such as N. darwiniata, do exhibit transitional forms.
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.
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.
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
s have a wide variety of predators, including bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s, reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
s, amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...
s, mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s, carnivorous plant
Carnivorous plant
Carnivorous plants are plants that derive some or most of their nutrients from trapping and consuming animals or protozoans, typically insects and other arthropods. Carnivorous plants appear adapted to grow in places where the soil is thin or poor in nutrients, especially nitrogen, such as acidic...
s, and other arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s. The great majority (80–99.99%) of individuals born do not survive to reproductive age, with perhaps 50% of this mortality rate attributed to predation. In order to deal with this ongoing escapist battle, insects have evolved a wide range of defense mechanisms. The only restraint on these adaptation
Adaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
s is that their cost, in terms of time and energy, does not exceed the benefit that they provide to the organism in question. The further that a feature tips the balance towards beneficial, the more likely that selection will act upon the trait, passing it down to further generations. The opposite also holds true; defenses that are too costly will have a slim to zero chance of being passed down. Examples of defenses that have withstood the test of time include hiding, escape by flight or running, and firmly holding ground to fight as well as producing chemicals and social structures that help prevent predation.
One of the best known modern examples of the role that evolution has played in insect defenses is the link between melanism
Melanism
Melanism is an undue development of dark-colored pigment in the skin or its appendages, and the opposite of albinism. It is also the medical term for black jaundice.The word is deduced from the , meaning black pigment....
and the peppered moth
Peppered moth
The peppered moth is a temperate species of night-flying moth. Peppered moth evolution is often used by educators as an example of natural selection.- Distribution :...
(Biston betularia). Peppered moth evolution
Peppered moth evolution
The evolution of the peppered moth over the last two hundred years has been studied in detail. Originally, the vast majority of peppered moths had light colouration, which effectively camouflaged them against the light-coloured trees and lichens which they rested upon...
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 up to debate.
Hiding
Walking sticks (order PhasmatodeaPhasmatodea
The Phasmatodea are an order of insects, whose members are variously known as stick insects , walking sticks or stick-bugs , phasmids, ghost insects and leaf insects...
), many katydid species (family Tettigoniidae
Tettigoniidae
The family Tettigoniidae, known in American English as katydids and in British English as bush-crickets, contains more than 6,400 species. It is part of the suborder Ensifera and the only family in the superfamily Tettigonioidea. They are also known as long-horned grasshoppers, although they are...
), and moths (order Lepidoptera
Lepidoptera
Lepidoptera is a large order of insects that includes moths and butterflies . It is one of the most widespread and widely recognizable insect orders in the world, encompassing moths and the three superfamilies of butterflies, skipper butterflies, and moth-butterflies...
) are just a few of the plethora of insects that have evolved specialized cryptic
Cryptic
Cryptic can refer to:* Crypsis, of animals that are difficult to observe* Cryptic crossword, a crossword with cryptic clues* Cryptic era, earliest period of the Earth* Cryptic, an album by Edge of Sanity* Cryptic Writings, an album by Megadeth...
morphology. This adaptation allows them to effectively hide within their environment because of a resemblance to the general background or an inedible object. 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 mimensis, a form of crypsis.
Insects may also take on different types of camouflage
Camouflage
Camouflage is a method of concealment that allows an otherwise visible animal, military vehicle, or other object to remain unnoticed, by blending with its environment. Examples include a leopard's spotted coat, the battledress of a modern soldier and a leaf-mimic butterfly...
, another type of cypsis. These include resembling a uniformly colored background as well as being light below and dark above, or coundershaded
Countershading
Countershading, or Thayer's Law, is a form of camouflage. Countershading, in which an animal’s pigmentation is darker dorsally, is often thought to have an adaptive effect of reducing conspicuous shadows cast on the ventral region of an animal’s body...
. 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.
Cost and benefit perspective
Butterflies (order LepidopteraLepidoptera
Lepidoptera is a large order of insects that includes moths and butterflies . It is one of the most widespread and widely recognizable insect orders in the world, encompassing moths and the three superfamilies of butterflies, skipper butterflies, and moth-butterflies...
) 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 (96.8–104 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
Cryptic
Cryptic can refer to:* Crypsis, of animals that are difficult to observe* Cryptic crossword, a crossword with cryptic clues* Cryptic era, earliest period of the Earth* Cryptic, an album by Edge of Sanity* Cryptic Writings, an album by Megadeth...
coloration to escape from predators, markings to attract conspecifics or warn predators (aposematism
Aposematism
Aposematism , perhaps most commonly known in the context of warning colouration, describes a family of antipredator adaptations where a warning signal is associated with the unprofitability of a prey item to potential predators...
), 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.
Mimicry
Mimicry is a unique form of defense and describes when a species resembles another recognized by natural enemies, giving it protection against predators. The superficial resemblance among mimics does not necessarily 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 sympatrySympatric speciation
Sympatric speciation is the process through which new species evolve from a single ancestral species while inhabiting the same geographic region. In evolutionary biology and biogeography, sympatric and sympatry are terms referring to organisms whose ranges overlap or are even identical, so that...
.
Mimicry is divided into two parts, Batesian mimicry
Batesian mimicry
Batesian mimicry is a form of mimicry typified by a situation where a harmless species has evolved to imitate the warning signals of a harmful species directed at a common predator...
and Müllerian mimicry
Müllerian mimicry
Müllerian mimicry is a natural phenomenon when two or more harmful species, that may or may not be closely related and share one or more common predators, have come to mimic each other's warning signals...
.
Batesian mimicry
In Batesian mimicry, an aposematicAposematism
Aposematism , perhaps most commonly known in the context of warning colouration, describes a family of antipredator adaptations where a warning signal is associated with the unprofitability of a prey item to potential predators...
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. An example can be found in the Plain Tiger (Danaus chrysippus
Danaus chrysippus
Danaus chrysippus, known as the Plain Tiger or African Monarch, is a common butterfly which is widespread in Asia and Africa. It belongs to the Danainae subfamily of the brush-footed butterfly family, Nymphalidae...
), a non-edible butterfly, which is mimicked by multiple species, the most similar being the female Danaid Eggfly (Hypolimnas misippus
Hypolimnas misippus
The Danaid Eggfly, Mimic, or Diadem is a widespread species of nymphalid butterfly. It is well known for polymorphism and mimicry. Males are blackish with distinctive white spots that are fringed in blue...
).
Müllerian mimicry
In Müllerian mimicryMüllerian mimicry
Müllerian mimicry is a natural phenomenon when two or more harmful species, that may or may not be closely related and share one or more common predators, have come to mimic each other's warning signals...
, a group of species benefit from each others' 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 Heliconius
Heliconius
Heliconius comprise a colorful and widespread brush-footed butterfly genus distributed throughout the tropical and subtropical regions of the New World. These butterflies utilize Passion flower plants as their larval food source and rely on bright wing color patterns to signal their distastefulness...
butterfly genus. Like in Batesian mimicry, the mimics are not necessarily related, although they obviously are in Heliconius.
Behavioral responses
Behavioral responses to escape predation include burrowing into substrate and being active only through a restricted period of the day. Furthermore, insects may feign death, a response termed thanatosis. Beetles, particularly weevilWeevil
A weevil is any beetle from the Curculionoidea superfamily. They are usually small, less than , and herbivorous. There are over 60,000 species in several families, mostly in the family Curculionidae...
s, are especially fond of this sort of acting. 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. The striking color pattern, which often includes eyespots, is intended to evoke prompt enemy retreat. Better formed eyespots seem to result in better deterrence.
Mechanical defenses
Insects have had millions of years to evolve a variety of mechanical defenses. Perhaps the most obvious is the cuticleArthropod cuticle
The cuticle forms the outer skeleton of arthropods, including insects.-Anatomy:The cuticle is made up of two layers; the epicuticle which is a thin and waxy water resistant outer layer and contains no chitin, and another layer under it called the procuticle...
. Although its main role lies in support and muscle attachment, when extensively hardened by the cross-linking of proteins and chitin, or sclerotized
Sclerite
A sclerite is a hardened body part. The term is used in various branches of biology for various structures including hardened portions of sponges, but it is most commonly used for the hardened portions of arthropod exoskeletons....
, the cuticle acts as a first line of defense. Additional physical defenses include modified mandibles, horns, and spines on the tibia and femur. When these spines take on a main predatory role, they are termed raptorial
Raptorial
The term raptorial implies much the same as "predatory", but most often refers to modifications of an arthropod's foreleg that make it function for the grasping of prey while it is consumed, where the gripping surfaces are formed from the opposing faces of two successive leg segments...
.
Some insects uniquely create retreats that appear uninteresting or inedible to predators. This is the case in caddisfly larvae (order Trichoptera
Trichoptera
The caddisflies are an order, Trichoptera, of insects with approximately 12,000 described species. Also called sedge-flies or rail-flies, they are small moth-like insects having two pairs of hairy membranous wings...
) which encase their abdomen with a mixture of natural materials like leaves, twigs, and stones.
Autotomy
AutotomyAutotomy
Autotomy or self amputation is the act whereby an animal severs one or more of its own appendages, usually as a self-defense mechanism designed to elude a predator's grasp...
, or the shedding of appendages, 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
Phasmatodea
The Phasmatodea are an order of insects, whose members are variously known as stick insects , walking sticks or stick-bugs , phasmids, ghost insects and leaf insects...
) where the cost is accentuated by the possibility that legs can be lost 20% of the time during molting. Harvestmen (order Opiliones
Opiliones
Opiliones are an order of arachnids commonly known as harvestmen. , over 6,400 species of harvestmen have been discovered worldwide, although the real number of extant species may exceed 10,000. The order Opiliones can be divided into four suborders: Cyphophthalmi, Eupnoi, Dyspnoi and Laniatores...
) also use autotomy as a first line of defense against predators.
Chemical defenses
Unlike pheromones, allomoneAllomone
An allomone is any chemical substance produced and released by an individual of one species that affects the behaviour of a member of another species to the benefit of the originator but not the receiver...
s harm the receiver at the benefit of the producer. This grouping encompasses the chemical arsenal that numerous insects employ. Insects with chemical weaponry usually make their presence known through aposematism. Aposematism
Aposematism
Aposematism , perhaps most commonly known in the context of warning colouration, describes a family of antipredator adaptations where a warning signal is associated with the unprofitability of a prey item to potential predators...
is utilized by non-palatable species as a warning to predators that they represent a toxic danger. Additionally, these insects tend to be relatively large, long-lived, active, and frequently aggregate. Indeed, longer-lived insects are more likely to be chemically defended than short lived ones, as longevity increases apparency.
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. Moreover, there is diversity among insects as to whether the defensive compounds are obtained intrinsically or extrinsically. 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.
In reflex bleeding
Autohaemorrhaging
Autohaemorrhaging or reflex bleeding is the action of animals deliberately ejecting haemolymph or blood from themselves. If the animal has toxic compounds in its blood then it may be an effective chemical defence mechanism....
, insects dispel their blood, hemolymph, or a mixture of exocrine secretions and blood as a defensive maneuver. As previously mentioned, the discharged blood may contain toxins produced within the insect source or externally from plants that the insect consumed. Reflexive bleeding occurs in specific parts of the body; for example, the beetle families Coccinellidae
Coccinellidae
Coccinellidae is a family of beetles, known variously as ladybirds , or ladybugs . Scientists increasingly prefer the names ladybird beetles or lady beetles as these insects are not true bugs...
(ladybugs) and Meloidae bleed from the knee joints.
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 vomitingVomiting
Vomiting is the forceful expulsion of the contents of one's stomach through the mouth and sometimes the nose...
and blistering. Class I chemicals include bufadienolides, cantharidin, cyanides, cardenolides, and alkaloids, all of which have greater effects on vertebrates than on other arthropods. In fact, the most frequently encountered defensive compounds in insects are alkaloids.
In contrast to Class I chemicals, Class II chemicals are essentially harmless. They stimulate scent and taste receptors so as to discourage feeding. They tend to have low molecular weight and are volatile and reactive, including acids, aldehydes, aromatic ketones, quinones, and terpenes. Furthermore, they may be aposematic, indicating through odors the presence of chemical defenses. The two different classes are not mutually exclusive, and insects may use combinations of the two.
Pasteels, Grégoire, and Rowell-Rahier also grouped chemical defenses, albeit in a different manner. Weaponry is partitioned into chemical 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. 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. As with Class I and Class II compounds, these three categories are not mutually exclusive, as some chemicals can have multiple effects.
Assassin bugs
Some insects inject their venom directly into kayla, as is the case with the over 5,000 species in the family ReduviidaeReduviidae
Reduviidae is a large, cosmopolitan family of predatory insects in the suborder Heteroptera...
, the assassin bugs and thread-legged bugs. Interestingly, the assassin bug species Platymeris rhadamanthus is capable of spitting venom up to 30 cm. The saliva of this insect, loaded with a mixture of at least six proteins including excess amounts of protease
Protease
A protease is any enzyme that conducts proteolysis, that is, begins protein catabolism by hydrolysis of the peptide bonds that link amino acids together in the polypeptide chain forming the protein....
, hyaluronidase
Hyaluronidase
The hyaluronidases are a family of enzymes that degrade hyaluronic acid.In humans, there are six associated genes, including HYAL1, HYAL2, HYAL3, and PH-20/SPAM1.-Use as a drug:...
, and phospholipase
Phospholipase
A phospholipase is an enzyme that hydrolyzes phospholipids into fatty acids and other lipophilic substances. There are four major classes, termed A, B, C and D, distinguished by the type of reaction which they catalyze:*Phospholipase A...
causes intense local pain, vasodilation
Vasodilation
Vasodilation refers to the widening of blood vessels resulting from relaxation of smooth muscle cells within the vessel walls, particularly in the large arteries, smaller arterioles and large veins. The process is essentially the opposite of vasoconstriction, or the narrowing of blood vessels. When...
, and edema
Edema
Edema or oedema ; both words from the Greek , oídēma "swelling"), formerly known as dropsy or hydropsy, is an abnormal accumulation of fluid beneath the skin or in one or more cavities of the body that produces swelling...
.
Cockroaches
Many cockroach species (order Blattodea) have mucous-like adhesive secretions on their posterior. Although not as affective against vertebrates, these secretions foul the mouths of invertebrate predators, increasing the chances of the cockroach escaping.Termites
The majority of termite soldiers secrete a rubberlike and sticky chemical concoction that serves to entangle enemies, called a Fontanellar gunFontanellar gun
The fontanellar gun is a type of specialized weaponry used by the soldiers of the North American termite subfamily Nasutitermitinae to ward off predators...
, and it is usually coupled with specialized mandibles. In nasute species of termites (contained within the subfamily Nasutitermitinae), the mandibles have receded. This makes way for an elongated, syringic nasus
Nasus
Nasus is Latin for nose or snout, and appears in many related terms:Fish species:*Gogonasus, a Devonian fish and link to early tetrapods*Barbus nasus, a Cyprinid fish*Bassozetus nasus, a cusk-eel*Chondrostoma nasus, nase...
capable of squirting liquid glue. When this substance is released from the frontal gland reservoir and dries, it becomes sticky and is capable of immobilizing attackers. It is highly effective against other arthropods, including spiders, ants, and centipedes.
Among termite species in the Apicotermitinae that are soldierless or where soldiers are rare, mouth secretions are commonly replaced by abdominal dehiscence
Dehiscence
Dehiscence can refer to:*Dehiscence is the spontaneous opening at maturity of a plant structure, such as a fruit, anther, or sporangium, to release its contents.*Wound dehiscence is a previously closed wound reopening....
. 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.
Ants
VenomVenom
Venom is the general term referring to any variety of toxins used by certain types of animals that inject it into their victims by the means of a bite or a sting...
is the defense of choice for many ants (family Formicidae
Ant
Ants are social insects of the family Formicidae and, along with the related wasps and bees, belong to the order Hymenoptera. Ants evolved from wasp-like ancestors in the mid-Cretaceous period between 110 and 130 million years ago and diversified after the rise of flowering plants. More than...
). It is injected from an ovipositor
Ovipositor
The ovipositor is an organ used by some animals for oviposition, i.e., the laying of eggs. It consists of a maximum of three pairs of appendages formed to transmit the egg, to prepare a place for it, and to place it properly...
that has been evolutionarily modified into a stinging apparatus. These ants release a complex venom mixture that can include histamine
Histamine
Histamine is an organic nitrogen compound involved in local immune responses as well as regulating physiological function in the gut and acting as a neurotransmitter. Histamine triggers the inflammatory response. As part of an immune response to foreign pathogens, histamine is produced by...
. Within the Formicinae
Formicinae
Formicinae is a subfamily within the Formicidae containing ants of moderate evolutionary development.Formicines retain some primitive features such as the presence of cocoons around pupae, the presence of ocelli in workers, and little tendency toward reduction of palp or antennal segmentation in...
subfamily, the stinger has been lost and instead the poison gland forcibly ejects the fluid of choice, formic acid
Formic acid
Formic acid is the simplest carboxylic acid. Its chemical formula is HCOOH or HCO2H. It is an important intermediate in chemical synthesis and occurs naturally, most notably in the venom of bee and ant stings. In fact, its name comes from the Latin word for ant, formica, referring to its early...
. Some carpenter ants (genus Camponotus) also have mandibular glands that extend throughout their bodies. When these are mechanically irritated, the ant forcibly commits suicide and by exploding, spilling out a sticky, entangling substance.
The subfamily Dolichoderinae
Dolichoderinae
Dolichoderinae is a subfamily of ants, which includes species such as the Argentine ant , the erratic ant, the odorous house ant, and the cone ant. This subfamily is distinguished by having a single petiole and a slit-like orifice, rather than the round acidopore encircled by hairs that typifies...
, 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.
Collective defenses in social insects
Many chemically defended insect species take advantage of clustering over solitary confinement. Among some insect larvae in the orders Coleoptera and HymenopteraHymenoptera
Hymenoptera is one of the largest orders of insects, comprising the sawflies, wasps, bees and ants. There are over 130,000 recognized species, with many more remaining to be described. The name refers to the heavy wings of the insects, and is derived from the Ancient Greek ὑμήν : membrane and...
, cycloalexy is adopted. Either the heads or ends of the abdomen, depending on where noxious compounds are secreted, make up the circumference of a circle. The remaining larvae lie inside this defensive ring where the defenders repel predators through threatening attitudes, regurgitation
Vomiting
Vomiting is the forceful expulsion of the contents of one's stomach through the mouth and sometimes the nose...
, and biting.
Termites (order Isoptera), like eusocial
Eusociality
Eusociality is a term used for the highest level of social organization in a hierarchical classification....
ants, wasps, and bees, rely on a caste system to protect their nests. The evolution of fortress defense is closely linked to the specialization of soldier mandibles. Soldiers can have biting-crushing, biting-cutting, cutting, symmetrical snapping, and asymmetrical snapping mandibles. These mandibles may be paired with frontal gland secretion, although snapping soldiers rarely utilize chemical defenses. Termites take advantage of their modified mandibles in phragmosis
Phragmosis
Phragmosis is any method by which an animal defends itself in its burrow by using its own body as a barrier. An example can be seen in the mygalomorph spider Idiosoma nigrum , which protects itself in its burrow by positioning itself so as to block the burrow with its abdomen, which is unusually hard...
, which is the blocking of the nest with any part of the body; in this case of termites, nest entrances are blocked by the heads of soldiers.
Alarm pheromone
Pheromone
A pheromone is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual...
s warn members of a species of approaching danger. Because of their altruistic nature, they follow the rules of kin selection
Kin selection
Kin selection refers to apparent strategies in evolution that favor the reproductive success of an organism's relatives, even at a cost to the organism's own survival and reproduction. Charles Darwin was the first to discuss the concept of group/kin selection...
. They can illicit both aggregational and dispersive responses in social insects depending on the alarm caller’s location relative to the nest. Closer to the nest, it causes social insects to aggregate and may subsequently produce an attack against the threat. In nonsocial insects, these compounds typically stimulate dispersal regardless of location. Chemical alarm systems are best developed in aphid
Aphid
Aphids, also known as plant lice and in Britain and the Commonwealth as greenflies, blackflies or whiteflies, are small sap sucking insects, and members of the superfamily Aphidoidea. Aphids are among the most destructive insect pests on cultivated plants in temperate regions...
s and treehoppers (family Membracidae) among the nonsocial groups. Alarm pheromones take on a variety of compositions, ranging from terpenoids in aphids and termites to acetate
Acetate
An acetate is a derivative of acetic acid. This term includes salts and esters, as well as the anion found in solution. Most of the approximately 5 billion kilograms of acetic acid produced annually in industry are used in the production of acetates, which usually take the form of polymers. In...
s, an alcohol, and a ketone in honeybees
Honey bee
Honey bees are a subset of bees in the genus Apis, primarily distinguished by the production and storage of honey and the construction of perennial, colonial nests out of wax. Honey bees are the only extant members of the tribe Apini, all in the genus Apis...
to formic acid and terpenoids in ants.
Immunity
Insects, like nearly every other organism on Earth, are subject to infectious diseaseInfectious disease
Infectious diseases, also known as communicable diseases, contagious diseases or transmissible diseases comprise clinically evident illness resulting from the infection, presence and growth of pathogenic biological agents in an individual host organism...
s caused by viruses, bacteria, fungi, protozoa, and nematode
Nematode
The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...
s. These encounters can either kill the insect or greatly weaken it. Insects protect themselves against these detrimental microorganisms in two ways. Firstly, the body-enveloping chitin
Chitin
Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
cuticle, in conjunction with the tracheal system and the gut lining, serve as major physical barriers to entry. Secondly, hemolymph
Hemolymph
Hemolymph, or haemolymph, is a fluid in the circulatory system of some arthropods and is analogous to the fluids and cells making up both blood and interstitial fluid in vertebrates such as birds and mammals...
itself plays a key role in repairing external wounds as well as destroying foreign organisms within the body cavity. Interestingly, insects, along with having passive immunity, also show evidence of acquired immunity
Adaptive immune system
The adaptive immune system is composed of highly specialized, systemic cells and processes that eliminate or prevent pathogenic growth. Thought to have arisen in the first jawed vertebrates, the adaptive or "specific" immune system is activated by the “non-specific” and evolutionarily older innate...
.
Social insects additionally have a repertoire of behavioural and chemical 'border-defences' and in the case of the ant, groom venom or metapleural gland
Metapleural gland
Metapleural glands are secretory glands that are unique to ants and basal in the evolutionary history of ants. They are responsible for the production of an antibiotic fluid that then collects in in a reservoir on the posterior of the ant's alitrunk...
secretions over their cuticle .
Role of phenotypic plasticity
Phenotypic plasticityPhenotypic plasticity
Phenotypic plasticity is the ability of an organism to change its phenotype in response to changes in the environment. Such plasticity in some cases expresses as several highly morphologically distinct results; in other cases, a continuous norm of reaction describes the functional interrelationship...
is the capacity of a single genotype
Genotype
The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...
to exhibit a range of phenotypes in response to variation in the environment. 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 catkin
Catkin
A catkin or ament is a slim, cylindrical flower cluster, with inconspicuous or no petals, usually wind-pollinated but sometimes insect pollinated . They contain many, usually unisexual flowers, arranged closely along a central stem which is often drooping...
s, or flowers. By the summer when the catkins have fallen, the caterpillars discretely mimic oak twigs. No intermediate forms are present in this species, although other members of the Nemoria
Nemoria
Nemoria is a genus of moth in the family Geometridae.-Species:Species include:* Nemoria arizonaria* Nemoria bistriaria – Hübner, 1818* Nemoria mimosaria – * Nemoria outina* Nemoria saturiba – Ferguson, 1969...
genus, such as N. darwiniata, do exhibit transitional forms.
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.
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.
See also
- Insect ecologyInsect ecologyInsect ecology is the scientific study of how insects, individually or as a community, interact with the surrounding environment or ecosystem....
- Antipredator adaptation
- Defense mechanisms
- Behavioral ecologyBehavioral ecologyBehavioral ecology, or ethoecology, is the study of the ecological and evolutionary basis for animal behavior, and the roles of behavior in enabling an animal to adapt to its environment...