Defense in insects
s have a wide variety of predators, including birds, reptiles, amphibians, mammals, carnivorous plants, and other arthropods. The great majority 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 adaptations is that their cost, in terms of time and energy, does not exceed the benefit that they provide to the organism. 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 little 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. 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.
Hiding
Walking sticks, many katydid species, and moths 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. 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.
Cost and benefit perspective
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. 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, 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 a form of defense which describes when a species resembles another recognized by natural enemies, giving it protection against predators. 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.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. An example can be found in the plain tiger, a non-edible butterfly, which is mimicked by multiple species, the most similar being the female danaid eggfly.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. Furthermore, insects may feign death, a response termed thanatosis. Beetles, particularly weevils, do this frequently. 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 mechanical defenses. Perhaps the most obvious is the cuticle. 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 which encase their abdomen with a mixture of materials like leaves, twigs, and stones.
Autotomy
, 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 where the cost is accentuated by the possibility that legs can be lost 20% of the time during molting. Harvestmen also use autotomy as a first line of defense against predators.Chemical defenses
Unlike pheromones, allomones 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 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 vomiting and blistering. Class I chemicals include bufadienolides, cantharidin, cyanides, cardenolides, and alkaloids, all of which have greater effects on vertebrates than on other arthropods. The most frequently encountered defensive compounds in insects are alkaloids. 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 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. 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.