10
Behavior And Anatomy
It is easy to dismiss tiny animals like insects as essentially mindless, but spending some time observing them in the wild will reveal a great breadth of varied and complicated natural behaviors, easily on par with those shown by so-called higher animals, and typically tied into particular anatomical traits.
10.6 • Interspecies interactions
The behavior of social insects, such as the Asiatic Honeybee (Apis cerana), is remarkably complex and elaborate.
feeding behavior
Obtaining and consuming food is a problem that insects solve in many different ways—even among closely related species taking similar foods, there are multiple feeding strategies and behaviors.
Nectar is a food much-used by insects, and is consumed by moths, butterflies, bees, wasps, hoverflies, and others. Most nectar-feeding insects settle on the flower from which they feed, and insert their sucking mouthparts into the flower’s nectaries (often loading their own bodies with pollen from the flower’s stamens in the process, which they pass on to the female parts of the next flower, thus pollinating it). Some nectar-feeders use a foraging strategy called trap-lining, whereby they patrol the same route repeatedly so that the flower nectaries have time to refill—this requires brains capable of spatial mapping and memory.
Some moths are adapted to take nectar in flight, visiting flowers with pendent (hanging) heads and hovering in front of them as they feed, with their exceptionally long and sturdy proboscises. The feeding manner recalls hummingbirds, and some hawk moths are strikingly similar in appearance to hummingbirds, even down to a flattened fan of hairs on the abdomen tip like a tail, which helps them maintain a steady position as they hover. Bees have chewing as well as licking mouthparts, and some species make use of these to access flower nectaries by biting a hole through the bases of the petals, rather than reaching the nectar through the front of the flower.
Scorpion flies—scavengers that feed on dead insects—are adept at entering spider webs without becoming stuck and feeding on other insects caught in the mesh. They are long-legged and climb deliberately over the strands, holding their wings clear of the web, but will also fight with and sometimes even kill the web’s resident spider.
Wasps have biting mouthparts and are capable of dismembering prey as large as themselves.
The crop of a honeybee (sometimes called “honey-stomach”) can hold 75mg of nectar (a third of the bee’s total weight).
The morphological similarity between a hummingbird and a Hummingbird Hawk Moth (Macroglossum stellatarum) is very striking—the moth even has a bristly “tail” to stabilize it in flight.
hunting
Insects that actively chase down and kill prey are relatively rare, but include the dragonflies and robber flies, both of which are fast flying, with superb eyesight and strong legs to seize and control their prey. Some other predators use camouflage or mimicry to bring their prey to them—these include the orchid mantises, whose flowerlike bodies are even more attractive to flower-feeding insects than the actual flowers that they mimic.
Adult workers of the social wasps feed on sweet substances, but they must also collect protein-rich food for their larvae. Because they bring these foods to their nests, they carry out considerable processing to ensure they are only bringing home the choicest parts of the prey. It is not unusual to witness a wasp locked in a struggle with a still-living bee, fly, or moth, the wasp working away with its mandibles to cut off the prey’s wings and, often, its head, before flying away with the thorax and abdomen. Wasps will also use their mandibles to cut transportable chunks of meat from a large piece of carrion.
Horseflies of the genus Haematopota, the females of which feed on vertebrate blood, are very large for biting flies, but their wings are adapted for silent flight and they have soft rather than scratchy feet, allowing them to approach and settle on their prey undetected. The nonbiting males, however, fly with a noisy buzz and have much more conspicuous behavior in general.
breeding behavior
The behaviors involved in courtship, mating, and egg-laying may be very simple, or surprisingly complex. Insects have a very small window of time in which to breed, so devote considerable energy toward getting it right.
Choosing the right mate can be important, especially for female insects. Males therefore go to considerable lengths to try to improve their chances. One of the methods they use to appeal to a mate (and to warn off rivals) is song. Grasshoppers and crickets make chirping or reeling songs through stridulation—the rubbing together of body parts. Grasshoppers make the sound by rubbing a series of ridges on the hind leg against the forewing, while crickets rub their two forewings together. Cicadas produce their droning whirr of a song using their tymbals, paired membranous structures in the abdomen that vibrate through abdominal muscular action. Different species have their own unique songs, which is a great help in identifying them (especially as they can be remarkably difficult to find).
Patterned wings, especially when only present in the male of the species, are often involved in courtship displays. Male picture-winged flies (family Ulidiidae) raise and flick their boldly patterned wings, and also wave their legs, while approaching a potential mate. This may be enough to convince the female to mate, but she sometimes retrospectively rejects the male by ejecting his sperm from her body (and then eating it). Male demoiselle damselflies (family Calopterygidae) gather at the waterside and perform short showy flights, flicking their dark-banded wings to attract the attention of the plain-winged females.
Female bush crickets use their prominent bladelike ovipositors to stab a hole into vegetation, into which they lay their eggs.
For most other damselflies there is no courtship ritual—males (which are usually colorful) simply approach the drabber females and try to take hold of them behind the head with their cerci. This position, known as tandem, is the precursor to mating. In some species of damselflies, a proportion of females are andromorphs, meaning they have male-like coloring and patterning and are less likely to be approached by males. Attention from lots of males at the same time can result in injury to the female, and places her at risk of predation, so in years of high population density the andromorphic females often survive in larger numbers than typical females, and still secure enough matings to lay eggs. In other years, the andromorphs may have lower breeding success than typical females. Over time, things even out and both typical and andromorphic females persist in the population.
Male demoiselle damselflies flash their dark-marked wings in courtship displays to attract females.
The loud, tuneless “song” of cicadas is produced through abdominal vibrations.
rebel workers
In a honeybee nest, only the queen produces eggs—the workers, though anatomically female, are sterile and exist only to support the nest and the queen’s offspring. However, if a nest loses its queen, a certain proportion of workers defy their biological destiny and lay eggs of their own. Because they have not mated, these eggs hatch as male larvae (the same is true when queens lay unfertilized eggs—see Chapter 9). These males leave the nest when mature and could potentially contribute their rebel mothers’ genes to a new nest, if they manage to find a virgin queen.
parental care
Most female insects do no parenting beyond laying their eggs in a suitable place, and males do even less. However, a few insects do demonstrate lasting and committed care of their young.
Earwig females are unusually devoted parents. A male and female pair live together in a nest chamber, but after mating, the female chases away her male partner. She lays her eggs in the chamber and remains with them over the week they take to hatch, defending them from potential predators and carefully cleaning them to protect them from fungus. When they hatch, she feeds the larvae on regurgitated food, and they may also eat her if she dies before they leave the nest.
Female Diploptera punctata cockroaches also care for and feed their young. Rather than regurgitating food, they secrete a protein-rich, milklike food for them from specialized abdominal glands known as brood sacs. This food is consumed by the unborn embryos (this species of cockroach is one of the few that is viviparous—gives birth to live young).
Shield bug females also take care of their young—indeed, one European species (Elasmucha grisea) is known as the Parent Bug for this reason. A female Parent Bug lays her egg cluster on a birch or alder leaf and shelters it with her body. She places on the eggs the bacteria that the larvae will need to digest plant matter—the bacteria reach the developing larvae through the eggshell. The mother bug stays with the small larvae when they are newly hatched, “herding” them back to the cluster if they try to stray. She only leaves them when they reach their third instar and separate from their family group.
Female earwigs—unusually among invertebrates—care for their larvae.
Although solitary bees and wasps do not typically tend their eggs or larvae, they do provide them with a home and a supply of food that will sustain them until pupation age—the nest, which may be a burrow or a mud pot, has its entrance sealed to keep out predators. It will be provisioned with food—most bee species supply pollen and nectar, while the solitary wasps leave a paralyzed prey item. Some species do remain in attendance after the egg or eggs hatch, providing additional food. This behavior is taken much further among communally living (eusocial) bees and wasps, with adults providing continuous care (see here).
This male Giant Water Bug is laden with egg masses placed there by females.
fatherly love
The female Giant Water Bug (Lethocerus deyrolli) lays her egg mass on her mate’s back. He will carry the eggs with him until they hatch, going about his daily foraging activities but also making sure that the eggs are kept wet, and driving away potential predators. A male may mate repeatedly and carry eggs from several females—females prefer to lay their eggs on males who are already carrying some eggs, and thus demonstrating their parenting skills.
A mother Parent Bug stands guard near her cluster of larvae.
seasonal behavior
Seasonal change—whether from summer to fall and winter in temperate areas, or the tropical shift from rainy to dry—necessitates different behavioral strategies, as well as anatomical adaptations.
Most insects, being exothermic, only function well and have full activity within a certain, relatively narrow, temperature range. This means that they rest in a safe place when the temperature is outside that range (often overnight). For those that live in temperate climes, temperature ranges of 104°F (40°C) or more may be expected between summer and winter and an extended period of inactivity may be needed in the coldest (or hottest) periods. Many insects pass the winter months in one of their inactive forms (egg or pupa). Those that overwinter as larvae or adults will enter a period of inactivity (hibernation, or diapause) whereby all metabolic processes are drastically slowed. They may also find or create a shelter (hibernaculum) in which to hibernate, to protect them from the worst of the cold. Some species can tolerate having their body tissues frozen and can overwinter in damp soil, but those that cannot will choose dry hibernacula that are sheltered from the wind.
The beautiful Oleander Hawk Moth (Daphnis nerrii) is a rare migrant visitor to Europe from Africa and western Asia.
A Peacock Butterfly (Aglais io) disturbed during hibernation may not be warm enough to be able to fly, but a sudden flash of its large, bright eyespots will hopefully discourage a would-be predator.
In hibernation, all developmental processes cease, and metabolic function slows almost to nothing. Hibernating insects consume virtually no oxygen, and bodily fat stores are eked out over the cold season. The physiological “decision” to enter a state of hibernation is determined by genes and triggered by various environmental cues, including hours of daylight, air temperature, and changes in food supply (fewer fresh leaves for herbivores, reduced numbers of prey for carnivores). When it is readying itself to become active again, the insect can help raise its internal temperature by vibrating its wings, and by basking in sunlight. Large-winged insects change their wing position in response to air temperature, spreading their wings to absorb more heat and closing them to conserve it. Dragonflies adopt a tail-up position called obelisking to avoid overheating on hot days—the posture reduces the surface area exposed to the sun.
Butterflies that hibernate in their adult form have drab, camouflaged undersides, though the wings’ upper surfaces may be colorful.
A dragonfly can keep cool by “obelisking”—this posture reduces the amount of surface area exposed when the sun is directly overhead.
Some insects hibernate communally, benefiting from safety in numbers. Ladybugs often form large aggregations inside buildings. They leave chemical compounds as they walk, which guide other ladybugs to the hibernaculum. Moth larvae of the genus Choristoneura spin silk shelters in foliage to protect themselves. Migration is another tactic to avoid bad weather, though covering the necessary distance is possible only for strong-flying insects in their adult form.
too hot
Inactivity in hot, dry conditions is known as estivation. This may occur in the dry season in tropical areas, and in high summer in warm temperate areas. The main hazard facing the insects that estivate is water loss through evaporation. When estivating, insects can reduce water loss in this way by drastically slowing down their respiration rate. However, they can revive to a normal, active state much more quickly than insects that are disturbed from hibernation. The Winter Ant (Prenolepis imparis), found across North America, is an example of an insect that is adapted to activity in low temperatures—it estivates in hot summer weather, when most other ants are at their most active.
INSECT MIGRATION
When a habitat is hospitable only at certain times of year, the insects that live in it will usually hibernate or estivate through the lean months. Another solution is to migrate.
The mass migration of Monarch Butterflies through the southern US and Mexico is one of nature’s great wonders.
Migration is most familiar to us as a bird behavior. Many species breed in temperate regions during the spring and summer and move to or beyond the equator for the winter months. This behavior is largely driven by insects—most migratory birds are insectivores, and insects are inactive in cold weather, whether they winter as eggs, larvae, pupae, or adults.
Because of their short adult life spans, relatively few insects are capable of undertaking migrations of this distance—breeding is their priority. However, some strong fliers do make long journeys. The most famous of them is the Monarch Butterfly (Danaus plexippus), populations of which migrate in the fall from southern Canada and northern and central United States to Florida and Mexico, or down the western side of the Rockies to Southern California, where they form huge gatherings at overwintering sites. In spring, a return journey takes place, but these butterflies are not the same individuals that set out the previous fall—up to four generations are born and die over the course of the journey.
Several insects that mainly occur in Africa, including the Vagrant Emperor Dragonfly (Anax ephippiger) and Painted Lady Butterfly (Vanessa cardui), migrate north into Europe and western Asia in summer when their populations are unusually high in their normal range. The year 2019 saw millions of Painted Ladies reach the UK, but most years numbers are much lower. The butterflies will breed in their new home, and there is evidence from radar records that the new generation heads back south in fall, traveling at high altitude. Their breeding cycle is rapid and their migratory pattern flexible, allowing them to make the most of local conditions.
Vagrancy
Insects that occur outside their usual range have often been introduced by people, deliberately or accidentally (for example, in imported food or timber). But some insects stray huge distances more or less under their own steam, albeit perhaps with considerable wind assistance. A handful of Monarchs cross the Atlantic to western Europe during the fall most years, and the migratory Green Darner Dragonfly (Anax junius) has also made the crossing on occasion.
Most lost migrants will die and leave no mark, but once in a while, vagrancy can lead to colonization. Monarch butterflies are rare in mainland western Europe but are more likely to make it to the Canary Islands. The species became established across the archipelago in the 19th century after several mass arrivals—though its long-term survival was only possible because its larval food-plant (milkweed) exists on the islands.
The migratory Vagrant Emperor Dragonfly is a rare visitor to the UK but has made the journey from Africa even in the depths of winter.
Painted Ladies migrate from North Africa into northwestern Europe every summer.
eusocial insects
The social organization of bee, ant, wasp, and termite colonies is truly remarkable. Studying how such colonies function reveals how layered and complex insect behavior can be.
Eusociality is rare in the animal world. It is a system whereby several (perhaps thousands) of individual adult insects cooperate to manage a nesting space, sharing care of young and other tasks. Usually only one or a few of the adults actually reproduce, and the “workers” may belong to distinct “castes,” each with its own particular anatomy and social role.
The European or Western Honeybee (Apis mellifera) is the most well-known of the world’s 10 or so species of eusocial honeybees, and the only one that has been fully domesticated.
Ants follow the same scent-marked pathways as they head out from their nests to forage.
The most famous and well-studied eusocial animal is the honeybee. Within a nest, all eggs are laid by the queen, who is larger than the worker females. There are no different worker castes, but a worker bee’s role changes as it ages (see Chapter 9). Once it is foraging outside the nest, it brings back pollen and nectar as food for the larvae. Inside the nest, workers share information about foraging sites through their “waggle dances”—a figure-eight movement whose orientation and speed corresponds to a location in the outside world. The crop of the honeybee is exceptionally expandable, and when full of nectar can make up a third of the insect’s total weight.
Termite mounds are imposing structures that offer homes to various other organisms besides termites.
Olfactory communication is very important in eusocial insects. A pair of brain structures called the corpora pedunculata, or mushroom bodies, are key to learning and memory through scent signals, and are well developed in eusocial insects. Worker ants establish foraging trails, and others follow the chemical cues left behind along these trails. This enables them to make their way directly back to their nest over 330 feet (100m) or more. They will assist fellow workers that are struggling with a heavy burden—in this way, ants can kill prey much larger than themselves and carry it back to the nest.
Termites belong to a different biological grouping from other eusocial insects but have much in common with ants in particular. They build enormous nest mounds from their own droppings and other materials, including clay-rich soil. The mounds are oriented to ensure optimal temperatures inside. Termites feed mainly on dead and decomposing plant material and may encourage and maintain colonies of edible fungi within the nest.
breeding and spreading
An insect colony may persist for many years, though those of social wasps and bumblebees only survive for one season. Among ants, new reproductive females with wings leave the nest and mate with a male or males from another nest. After this, they find a site to establish a new colony and shed their wings. The fertilized eggs they lay become new female worker ants. In social wasps, new queens mate in fall and then hibernate. The following spring, they find a nest site and build the foundations of a new nest from chewed wood fibers. The queen lays a few eggs and feeds the larvae when they hatch. Once they mature into adult workers, they take over the work of adding to the nest and caring for the queen’s subsequent offspring.
MUTUALISM AND COMMENSALITY
There are numerous examples of mutually beneficial relationships between two species in nature. However, in many relationships only one of the two benefits, neither helping nor hurting the other.
Ants tend their aphid “herds” with great care, as the aphids provide them with a generous supply of honeydew.
Mutualistic or symbiotic relationships among insects are exemplified by the link between ants and aphids. Aphids are true bugs, which feed on plant sap by puncturing the plant with their piercing mouthparts. They excrete excess fluid, and this “honeydew” is loaded with sugar, which provides food for many other species—for example, several species of tree-dwelling butterflies feed mainly on aphid honeydew in their adult stages. Ants also value aphid honeydew and often corral and guard “herds” of aphids, protecting them from predators and harvesting their honeydew.
When one species relies on another for its survival, but does so without affecting the “host,” the relationship is called commensalism. This is distinct from mutualism, where both species benefit, and also from parasitism, where one species is harmful to the other. Pure commensality is rare in nature. However, large ant nests often host a number of other species that live apparently unnoticed in the nest, feeding on detritus and not bothering the host insects in any obvious way. The large “gallery” tunnel complexes used by certain wood-boring beetles also provide home, shelter, and food for various other insects and invertebrates, which exploit the wood that decays around the edges of the tunnels.
There are many mutualistic relationships where one partner appears to get a lot more from the arrangement than the other. One example is seen in burying beetles, and the tiny mites that they often carry on their bodies. These beetles lay their eggs on corpses of vertebrate animals, as do the mites. The mites use the beetles as transportation, and in return they help reduce competition for the beetles at carcasses, by killing off fly eggs and larvae. However, the mites do also eat the burying beetles’ eggs and larvae.
Termite activities enrich the soil, allowing plant communities to grow that are different from surrounding areas.
Housing estates
The huge, robust mound nests made by certain species of termites provide homes for many animals. The nests are made from compressed soil particles, hardened by the baking sun. Termite mounds in the African savanna can reach tremendous size—more than 100 feet (30m) across. These huge mounds often support more trees than the surrounding areas, as their soil is more fertile, providing patches of rich habitat that can sustain many other animals.
Some termites make their nests underground, with a network of tunnels linking various nesting chambers to each other and to the surface. Their nests and tunnels may be shared by mites and springtails.
Burying beetles provide a taxi service for mites—both lay their eggs on carrion.
interspecies interactions
Insects prey on many other animals—including other insects. They also parasitize them, live in their homes, and even, in a few instances, have mutually beneficial relationships.
Beyond the simple links between predator and prey, it is among the social insects that we see some of the more complex interspecies relationships. Ants are well known for herding and guarding colonies of aphids in order to drink the honeydew that the aphids secrete. Other insects also benefit from the protection and care of ants, but in the case of the Large Blue Butterfly (Phengaris arion) the ants are duped into giving far more. When a Large Blue caterpillar reaches its fourth instar, it drops down from its food plant and is quickly found by ants of the species Myrmica sabuleti. Fooled by pheromones and sounds produced by the caterpillar, the ants take it into their nest and treat it as a larval queen ant, feeding it in preference to the colony’s own larvae (even sometimes feeding it on their own larvae) and protecting it from danger, until it pupates and leaves the nest as an adult butterfly.
Phoresy is the transport of one species by another, and is usually benign in nature. Water mites hitch rides between water bodies by riding on damselflies, and scavenging mites travel on the backs of burying beetles. Parasitoid wasps of the genus Trichogramma use phoresy in a more sinister manner. They can sense anti-aphrodisiac pheromones released by female butterflies of their host species. The butterflies release these pheromones after they have mated, to discourage further male attention. But to the wasps, the pheromones indicate that the butterfly is likely to lay eggs soon. On detecting the pheromone, the wasp will climb on to the butterfly’s body, so as to be ready to lay its own eggs on the butterfly’s eggs when they are laid. Mantisflies carry out a similar trick with spiders (see Chapter 8), though in this case it is the larvae that hitch a ride, rather than the adult females.
Through pollination, insects enable plants to reproduce sexually and to access a wide gene pool.
Ants assidiously guard their “herd” of aphids and move them to good feeding areas.
queens and slaves
Several species of bees are brood parasites, laying their eggs in other bees’ nests. Female cuckoo bumblebees (Psithyrus spp.) actually enter and live in the nests of their host bumblebee species, laying many eggs and sometimes even attacking and killing the resident queen. The resident workers take care of the cuckoo bee’s eggs as if they were their own. The slave-maker ants take over other nests’ workforces in an even more dramatic manner. Workers of these ant species seek out nests of other ants, and abduct pupae from them, sometimes several thousand, to bring to their own nests and raise as co-opted workers.
The slavemaker ants have specialist workers which are tasked with finding other ant nests to attack. Once one of these “scouts” has found a suitable nest, it goes back to its own nest, leaving a scent trail, which others will follow back to the targeted nest to carry out the raid, usually meeting very little resistance. The ants that emerge from the stolen pupae imprint on their new colony completely, and may even end up carrying out raids on their original nests.
The Large Blue Butterfly lays its eggs on thyme plants (Thymus spp.), but the caterpillars switch to a more carnivorous diet in their final instar.
HOST–PARASITE RELATIONSHIPS
Even the most healthy-looking human being may still be host to a variety of tiny parasitic animals, both internal and external. Parasitism is a fact of life for virtually all living things.
A parasite is defined as an organism that lives on or in another organism, feeding on its body tissues in some way, and usually causing some harm (though this is not necessarily serious or life-threatening). The parasite depends on its host for survival, at least during one of its life stages, if not its entire life. Parasites that live on the outside of the body are known as ectoparasites, while those that live inside are endoparasites.
Some insects live as parasites, while others are hosts to them, and some species may be both host and parasite at the same time. A few insects are parasitic on humans, among them the fleas. Most species of flea have a preferred host, but will take blood from other vertebrates as well—the so-called Human Flea (Pulex irritans) has dozens of recorded hosts other than humans, and the Cat Flea (Ctenocephalides felis) will happily bite people as well as cats. The Chigoe Flea (Tunga penetrans), found in tropical Central and South America and also (as a non-native species) southern Africa, affects humans and other species and actually burrows into the skin, creating painful lesions. The Head Louse (Pediculus humanus) and the Pubic Louse (Pthirus pubis), however, are obligate parasites of humans. Unlike fleas, which only habitually live on their host’s bodies in their adult form, the lice are full-time residents, laying their eggs on head hair and pubic hair respectively and feeding on blood.
Larvae of the fly Dermatobia hominis are parasites of humans and other mammals.
Most blowflies (family Calliphoridae) lay their eggs on carrion or dung, but some lay eggs on wounds on the bodies of living vertebrate animals.
Insects themselves are infected by a range of endoparasites, especially protozoa (single-celled organisms), which the insects usually take into their bodies while feeding. The protozoa may cause considerable damage by feeding on internal body tissues, and a heavy infestation can kill the host. Some parasitic protozoa are used as natural control agents to reduce populations of harmful insects. Other kinds of protozoa, including the species that causes malaria, need to be passed on to another organism to complete their life cycle.
The Human Botfly
This species of fly, Dermatobia hominis, found in Mexico and South and Central America, is a particularly alarming parasite of humans and other mammals. The female botfly captures a female mosquito and affixes an egg to the mosquito’s mouthparts, where it hatches. In this way, when the mosquito bites human skin, the botfly larva is transferred to the wound and burrows deep inside. The larva feeds on its host’s tissues and grows to its full size over the following eight weeks, before (assuming the host has not managed to remove it) falling out to pupate in soil. Removing a living larva is very difficult, as it resists being pulled out, and if its body breaks in the attempt, the wound will probably become infected. If the entry wound is covered with petroleum jelly the larva will suffocate after several hours, and can then be extracted with a pair of tweezers.
The Head Louse is a tenacious parasite of humans and is found almost everywhere on Earth.
Fleas can spread serious diseases, including typhus and bubonic plague.
PARASITOIDS
Insects are not always very invested in care of their young. The parasitoids are an exception to this, but their idea of good parenting is many people’s idea of the scariest horror story.
Large White butterflies are the host for the parasitoid wasp Cotesia glomerata.
Parasitoidism is a particularly gruesome form of parasitism, in which the host is eventually killed by its parasite, but not before the parasite has used the host’s living body as food and shelter for an entire stage of its life. The majority of known insect parasitoids are in the order Hymenoptera—in particular the ichneumon, braconid, and chalcid wasps.
Female parasitoids may use many different hosts, but many specialize in one or a few host species and attack it at a particular life stage. For example, the female braconid wasp Cotesia glomerata of Europe and Asia injects her eggs into the bodies of caterpillars from the genus Pieris, particularly the Large White (P. brassicae). The parasitized caterpillar appears to behave normally but inside it the eggs hatch and the wasp larvae feed on its body tissues. After a couple of weeks, the larvae emerge from the caterpillar’s body, killing it, and form their pupae around it—although sometimes they themselves are attacked before they can pupate by another specialist parasitoid—the ichneumon Lysibia nana. In some cases, parasitoid attacks can have dramatic impacts on their hosts’ population, and the populations of parasitoid and host tend to rise and fall year on year in a regular cycle.
Some parasitoid wasps, such as the potter or mason wasps (family Vespidae), construct a nest and place one or more of their hosts inside it, stinging them first to paralyze them and prevent their escape. The wasp lays an egg in or on each host’s body, and the larva consumes it gradually.
All insect life stages are vulnerable to attack from parasitoids. Here, two adult parasitoid wasps emerge from parasitized stink bug eggs.
Parasitoid larvae chew their way out of their host’s body before they pupate in a cluster around the remains.
Internal battles
Hosts attempt to thwart parasitoids in various ways. Some try to escape their attackers, and many have an immune response to the injected eggs. The parasitoids are very persistent and in some cases lay so many eggs in the host that the immune system is overwhelmed. The act of egg-laying sometimes also introduces a virus to the host’s body, which compromises its immune system.
In a few cases, the parasitoids can actually modify the host’s behavior. For example, host caterpillars parasitized by endoparasitoid wasps of the genus Glyptapanteles will try to defend the wasp pupae that form around its dying body.