Natural history ( insects )
Tagged: bird, cat, dog, fish, insectLife cycle
Egg
Most insects begin their independent lives as fertilized eggs. The chorion, or eggshell, is commonly pierced by respiratory openings that lead to an air-filled meshwork inside the shell. For some insects (e.g., cockroaches) a batch of eggs is cemented together to form an egg packet or ootheca. Insects may pass unfavourable seasons in the egg stage. Eggs of the lucerne flea Sminthurus (Collembola) and of some grasshoppers (Orthoptera) pass summer droughts in a dry shrivelled state and resume development when moistened. Most eggs, however, retain their water although they may pass the winter in a state of arrested development, or diapause, usually at some early stage in embryonic development.
Dried eggs of Aedes mosquitoes enter a state of dormancy after development is complete; they quickly hatch when placed in water. The hatching of young larvae is achieved in several ways. Some, such as caterpillars, bite their way out of the egg. Many, such as the flea, have hatching spines with which they cut a slit in the shell; others force off a preformed egg cap. In order to exert this force, the young larva swallows air; after hatching, it continues to distend itself in this way until the cuticle hardens. Once formed, the insect cuticle cannot grow. Growth can occur only by a series of molts (ecdyses) during which new and larger cuticles form and old cuticles are shed. Molting makes possible large changes in body form.
Types of metamorphosis
In the most primitive wingless insects (apterygotes) such as the silverfish Lepisma, there is almost no change in form throughout growth to the adult. These are known as ametabolous insects. Among insects such as grasshoppers (Orthoptera), true bugs (Heteroptera), and homopterans (e.g., aphids, scale insects), the general form is constant until the final molt, when the larva undergoes substantial changes in body form to become a winged adult with fully developed genitalia. These insects, termed hemimetabolous, are said to undergo incomplete metamorphosis. The higher orders of insects i.e., Lepidoptera (butterflies and moths), Coleoptera (beetles), Hymenoptera (ants, wasps, and bees), and Diptera (true flies) are termed holometabolous because larvae are totally unlike adults. These larvae undergo a series of molts with little change in form before they enter into complete metamorphosis, which includes molting first into pupae and then into fully winged adults.
Types of larvae
Larvae, which vary considerably in shape, are classified in five forms: eruciform (caterpillar like), scarabaeiform (grublike), campodeiform (elongated, flattened, and active), elateriform (wireworm like), and vermiform (maggot like). The three types of pupae are obtect, with appendages more or less glued to the body; exarate, with the appendages free and not glued to the body; and coarctate, essentially exarate but remaining covered by the cast skins (exuviae) of the next to the last larval instar (name given to the form of an insect between molts).
Role of hormones
Both molting and metamorphosis are controlled by hormones. Molting is initiated by a hormone from neurosecretory cells in the brain. The hormone acts upon a prothoracic gland, an endocrine gland in the prothorax; this gland, in turn, secretes the molting hormone, a steroid known as ecdysone, which, by its action on the epidermis, stimulates growth and cuticle formation. Metamorphosis likewise is controlled by a hormone. Throughout the young larval stages a small gland behind the brain, called the corpus allatum, secretes the juvenile hormone (also known as neotenin).
So long as this hormone is present in the blood the molting epidermal cells lay down a larval cuticle. In the last larval stage, juvenile hormone is no longer produced, and the insect undergoes metamorphosis into an adult. Among holometabolous insects the pupa develops in the presence of a very small amount of juvenile hormone. Although a state of arrested development may occur during any stage, diapause occurs most commonly in pupae. In temperate latitudes many insects overwinter in the pupal stage (e.g., cocoons). The immediate cause of diapause, failure to secrete the growth and molting hormones, usually is induced by a decrease in daylength as summer wanes. In addition to the changes in form during development, many insects exhibit polymorphism as adults. For example, the worker and reproductive castes in ants and bees may be different; termites have a soldier caste as well as reproductives and persistent larvae; adult aphids (Homoptera) may be winged or wingless; and some butterflies show striking seasonal dimorphism. The general interpretation of all such differences is that, although the capacity to develop different forms is present in the genes of every member of a given species, particular lines of development are evoked by environmental stimuli. Hormones, including perhaps the juvenile hormone, may be agents for the control of such changes.
Reproduction
The life of the adult insect is geared primarily to reproduction. Since reproduction is sexual in almost all insects, mating must be followed by impregnation of the female and fertilization of eggs. Usually the male seeks out the female. In butterflies in which vision is important, the colour of the female in flight can attract a male of the same species. In mayflies (Ephemeroptera) and certain midges (Diptera) the males dance in swarms to provide a visual attraction for females. In certain beetles (e.g., fireflies and glowworms) parts of the fat body in the female have become modified to form a luminous organ that attracts the male. Male crickets and grasshoppers attract females by their chirping songs, and the male mosquito is lured by the sound emitted by the female in flight. The most important element in mating, however, is odour. Most female insects secrete odorous substances called pheromones that serve as specific attractants and excitants for males. The male likewise may produce scents that excite the female.
Certain scales (androconia) on the wings of many male butterflies function in this way. Assembling scents, active in small quantities, are well known in female gypsy moths and silkworms as male attractants. The queen substance in the honeybee serves the same purpose. Mating and egg production require appropriate temperatures and adequate nutrition. The need for protein is particularly important, and in insects such as Lepidoptera (butterflies and moths), which take only sugar and water in the adult stage, necessary protein is derived from larval reserves. Temperature and nutrition often influence hormone secretion. Juvenile hormone or hormones from the neurosecretory cells commonly are needed for egg production. In the absence of these hormones reproduction is arrested, and the insect enters a reproductive diapause. This phenomenon occurs in the potato beetle Leptinotarsa during the winter. A few insects (e.g., the stick insect Carausius) rarely produce males; the eggs develop without fertilization in a process known as parthenogenesis. During summer months in temperate latitudes, aphids occur only as parthenogenetic females in which embryos develop within the mother (viviparity). In certain gall midges (Diptera) oocytes start developing parthenogenetically in the ovaries of the larvae; the young larvae escape by destroying the body of their mother in a process called paedogenesis.
Sensory perception and reception
Touch
Insects have an elaborate system of sense organs. Tactile hairs, concentrated on the antennae, palps, legs, and tarsi, cover the entire body surface. The hairs serve to inform the insect about its surroundings and its body position (a phenomenon known as proprioception). For example, contact between the hairs on the feet and the ground inhibits movement and may lead to a state of sleep in some insects. Modified mechanical sense organs in the cuticle called campaniform organs detect bending strains in the integument. Such organs exist in the wings and enable the insect to control its movements. Campaniform organs, well developed in small clublike halteres (the modified hind wings of dipterans), serve as strain gauges and enable the fly to control its equilibrium.
Sound
Exceedingly sensitive organs called sensilla are concentrated in organs of hearing—e.g., bushy antennae of the male mosquito or tympanal organs in the front legs of crickets or in abdominal pits of grasshoppers and many moths. In moths these sensitive organs can perceive the high-pitched sounds emitted by bats as they hunt by echolocation. Insects complement organs of sound reception with sound-producing organs, which usually are (as in crickets) wing membranes that vibrate in response to movement of a stiff rod across a row of stout teeth. Sometimes (as in cicadas) a timbal (membrane) in the wall of the thorax is set in vibration by a rapidly contracting muscle attached to it.
Chemicals
Chemical perceptions by the thin-walled sensilla may be comparable to the human sense of taste or smell. Many insect chemoreceptors are specialized according to specific behaviour patterns. For example, although approximately equivalent to humans in the perception of flower odours and sugar sweetness, honeybees are exceedingly sensitive to the queen substance, which is scentless to humans. And male silkworm moths are excited by infinitesimal traces of the female sex pheromone, even in the presence of odours that are intensely strong to humans.
Sight
Although the insect eye provides very poor form perception, insects by using a process of scanning (i.e., moving the eye rapidly across a field of view) probably can form adequate visual impressions of their surroundings. Insects have good colour vision; colour perception commonly extends (as in ants and bees) into the ultraviolet, although it often fails to extend into the deep red. Many flowers have patterns of ultraviolet reflection invisible to the human eye but visible to the insect eye.
Behaviour
Instincts
The insect orients itself by making orientation responses to the stimuli it receives. Formerly, insect behaviour was described as a series of forced movements in response to stimuli. That hypothesis has been supplanted by one that holds that the insect has a central nervous system with built-in patterns of behaviour or instincts that can be called forth by environmental stimuli; these instincts are modified by the insect’s internal state, which has been affected by preceding stimuli. Searching for food or an egg-laying site, catching prey, and mating are a few examples of complex behaviour. Experimental studies of details of behaviour have provided significant information about the properties of the sense organs. Patterns of behaviour range from comparatively simple reflex responses (e.g., the avoidance of adverse stimuli, the grasping of a rough surface on contact with the claws) to the elaborate behavioral sequences involved in hunting, capturing, and eating prey. The highest developments of behaviour, found in social insects such as the ants, bees, and termites, are based on the instinct principle.
An interesting example of a behavioral pattern is that found in the leaf-cutter bee Megachile. The female bee first locates a site for her nest in rotten wood and shapes the nest into a long tunnel; then she seeks out preferred shrub leaves from which to build a cell and cuts first a disc for a cell cap, then a series of oval pieces for the walls. After preparing the nest, she stores a mixture of pollen and honey, lays an egg, and finally closes the cell with more cut leaves. The leaf-cutter bee repeats this sequence until the nest is filled. Each act can be performed only in this set sequence. The insect does not stop to repair any damage to the nest but proceeds undeterred to the next step in her behavioral pattern. The honeybee society is more flexible than that of the leaf-cutter bee. Behavioral sequences of individuals are predictable, but the choice of acts or duties within the hive can be influenced by the needs of the colony. A capacity for learning does exist, and must exist, in any insect that has to find its nest; but learning capacity plays a relatively small part in the overall pattern of honeybee behaviour.
Insect societies
Both in complexity of behaviour and learning capacity, solitary bees and wasps are the equals of social wasps or honeybees. Social insects, however, have developed a division of labour in which the members must do the work required at the proper time. If the society is to succeed, its needs must be communicated to the individual, and the individual must act. These needs may be met by a temporary change in behaviour during which appropriate instinctive acts are performed or by changes in development that lead to the appearance of appropriate castes. Commonly, both behavioral and developmental changes are initiated by pheromones, which act as chemical messengers that convey information from one member of a colony to another.
Insect societies are gigantic families, the offspring of a single female. In the honeybee the single queen in the hive secretes the pheromone known as the queen substance (oxodecenoic acid); it is taken up by the workers and passed throughout the colony by food sharing. So long as the queen substance circulates, all members are informed that the queen is present. If the workers are deprived of queen substance, they proceed at once to build queen cells and feed the young larvae with a special salivary secretion known as royal jelly to produce more queens. Pheromones liberated by termite soldiers or reproductive adults control the development of soldiers and reproductive forms. Alarm substances and other pheromones control much of the behaviour in ants. A remarkable form of communication is the dance language in the honeybee, in which the direction and approximate distance of a foraging site can be conveyed by one worker to another. For additional information on the social behaviour of bees, ants, and termites, see below in sections on the Hymenoptera and the Isoptera.
Ecology
Terrestrial insects
Insects feed on every sort of organic matter, and their methods of feeding and digestion have become modified accordingly. The major climatic hazards faced by terrestrial insects are temperature extremes and desiccation. Different species function best at various optimal temperatures. If conditions are too hot, an insect seeks out a cool, moist, and shady spot. If exposed to the sun, an insect positions itself so as to present the smallest amount of body surface to the heat. If conditions are too cool, insects remain in the sun to warm themselves. Many butterflies must spread their wings to collect heat before they can fly. A moth raises its temperature by vibrating its wings or “shivering†before taking flight.
The heat generated in this way is conserved by hairs or scales that maintain an insulating layer of air around the body. The optimum muscle temperature for flight is from 38° to 40° C (100° to 104° F). In extremely cold weather the danger for insects is freezing, and insects that survive winters in cold latitudes are called cold hardy. A few insects (e.g., some caterpillars and aquatic midge larvae) tolerate ice formation in body fluids, although it is probable that the cell contents do not freeze. In most insects, however, cold hardiness means resistance to freezing. This resistance results partly from accumulation of large quantities of glycerol as an antifreeze and partly from physical changes in the blood that permit supercooling, without freezing, to temperatures far below the freezing point. Resistance to desiccation includes development of hard waterproofing waxes and exaggeration of water-conserving mechanisms.
Aquatic insects
Major adaptational changes apart from remarkable modifications of the legs for swimming concern respiration of aquatic insects. Some occur in insects that rise to the water surface to take atmospheric air into their tracheal systems. Mosquito larvae use only the last pair of abdominal spiracles, which open at the tip of a respiratory siphon. Water beetles (e.g., Dytiscus) have converted the space between the protective sheaths on the hind wings (elytra) and the abdomen into an air-storage chamber. Air-breathing insects can prolong the period of submergence by trapping air among their surface hairs. This air film acts as a physical gill and makes possible oxygen uptake from water. Other adaptations to an aquatic environment have occurred in larvae that obtain all their oxygen from the water. In midge larvae, abundant tracheae (breathing tubes) supply the entire thin cuticle. Caddisfly larvae (Trichoptera) and mayfly larvae (Ephemeroptera) have tracheal gills. In large dragonfly larvae, the gills are inside the rectum, and the water is pumped in and out through the anus.
Protection from enemies
Insects may derive some protection from a horny or leathery integument; but they also have various chemical defenses. Some caterpillars carry among their body-surface hairs special irritating hairs, which break up into barbed fragments containing a poisonous substance that causes intense itching and serves as a protection against most birds. Dermal glands of many insects discharge repellent or poisonous secretions over the cuticle; other insects are protected by poisons that are present continuously in the blood and tissues. Such poisons often are derived from the plants on which the insects feed.
In many hymenopterans (ants, bees, wasps) accessory glands of the female, which usually pour out a secretion over the egg, have become modified to produce toxic proteins. These poisons, injected into the nervous system of the prey of solitary wasps, paralyze it; in this state the prey serves as food for the wasp larva. Similar stings are used by hymenopterans, including ants, wasps, and bees, for self-defense. Concealment is an important protective device for insects. Vast numbers hide beneath stones or the bark of trees. Others rely on protective coloration. Although insect colours depend partly on pigmentation in the outer body covering (cuticle), the most important pigments occur in epidermal cells below the cuticle. Butterfly and moth pigments are deposited inside flattened hairs, or scales, which cover the wings. Some of the most brilliant insect colours are not the result of pigmentation; they are physical interference colours produced by fine laminae in the surface of the scales. Protective coloration may take the form of camouflage (cryptic coloration) in which the insect is confused with its background. The coloration of many insects copies a specific background with extraordinary detail. Stick insects (Carausius) can accommodate their colour to that of a changing background by moving pigment granules in their epidermal cells. Some caterpillars have patterns that develop in response to a background; however, these are irreversible. Insects such as caterpillars, which rely on cryptic coloration, combine it with a rigid deathlike position. Alternatively, insects that are well provided with chemical defenses generally show conspicuous warning, or aposematic, coloration. Experiments have proved that predators such as birds quickly learn to associate such coloration labels with nauseous or dangerous prey. Finally, insects without nauseous qualities may gain protection by mimicry, that is, by developing the conspicuous coloration found in distasteful species (see also coloration; mimicry).
Population regulation
The factors that limit the numbers of insect species are complex. Experimental studies of a population of grain beetles in a jar containing wheat show that the complexities increase if a second species is added. With insects in natural habitats, competing not only with members of their own species but with numerous other species as well, the obstacles to survival become increasingly great. Competition among species is reduced to some extent by adaptation of species to niches, or habitats, for which other insects do not compete. Formerly, controversy arose over whether numbers were always density dependent (i.e., limited by the density of the species itself) or whether catastrophic actions, notably the vagaries of weather, were often of prime importance. It has since become recognized that the ultimate factor in the control of numbers is competition within the species for food and other needs; but in many circumstances, before competition for food becomes significant, numbers are reduced by external factors.
Competition within a species often is reduced by wholesale migration to new localities. Migration may occur by active flight, as in aphids and locusts, largely directed by the wind. Another important factor in the regulation of populations is balanced polymorphism of species, in which the prevalence of individuals with given characteristics changes according to the action of natural selection as the state of the environment changes.
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