The Insect Life Cycles
Introduction
A life cycle is the chain or sequence of biological events that occurs during the lifetime of an individual insect. The cycle is usually considered to begin with deposition of the egg and to end with egg laying by the adult female. A life cycle lies in the boundaries of a single generation. The life cycles have three distinct divisions. These divisions include: (1) Reproduction and embryonic development, (2) Post embryonic growth and development, and (3) Maturity. There is great variability among insects in the length of the life cycle, from a few days to many years, in the method of development, in the appearance and habits of the stages leading to adulthood and even between successive life cycles.
The insect seasonal cycle is related to the life cycle. The seasonal cycle is the sequence of life cycles of a species that occurs over a 1-year period.
I. Reproduction and embryonic development
Types of reproduction
1. Sexual reproduction: The most frequently observed form of reproduction in insects is sexual reproduction. In most species, both females and males are common, and mating occurs with the eggs fertilized in the female oviduct as they pass the duct of the sperm storage sac or spermatheca. Infrequently, fertilization may occur while eggs are still in the ovarioles or in the hemocoel.
2. Parthenogenesis: The asexual mode of reproduction or parthenogenesis is a derived and specialized characteristic in some insects. Males are not found in species with obligate parthenogenesis, and unfertilized eggs are females. Examples of this type of reproduction include the pear sawfly, Caliroa cerasi, several black flies, a number of weevils and many aphids.
3. Haplodiploidy: A combination of sexual and asexual modes, haplodiploidy, occurs in a few species of four insect orders: Coleoptera, Hymenoptera, Thysanoptera, and Hemiptera.The honey bee is one of the best known example of this mode, which is sometimes called facultative parthenogenesis. Here, unfertilized eggs (haploid) are males who become drones, and fertilized eggs (diploid) are females, most of whom become workers.
4. Paedogenesis: This is an unusual type of reproduction where the juvenile forms of insects are reproduced. This type of reproduction occurring in one beetle family (Micromalthidae), several genera of gall and fungus gnats, and common flower fly.
Embryonic development
An insect's egg is much too large and full of yolk to simply divide in half. Most insects do not undergo complete cell division during cleavage. During embryogenesis through 12-13 division cycles to yield about 5000 daughter nuclei. This process of nuclear division is known as superficial cleavage. As they form, the cleavage nuclei (often called "energids") migrate through the yolk toward the perimeter of the egg. They settle in the band of periplasm where they engineer the construction of membranes to form individual cells. The end result of "cleavage" is the blastoderm-a one-cell-thick layer of cells surrounding the yolk.
Blastoderm cells on one side of the egg begin to enlarge and multiply. This region, known as the germ band or ventral plate, is where the embryo's body will develop. The rest of the cells in the blastoderm become part of a membrane the serosa, that forms the yolk sac. Cells from the serosa grow around the germ band, enclosing the embryo in an amniotic membrane.
Development in most species proceeds when the germ band forms an infolding that eventually produces a three-layered embryo, the gastrula. The inner layer is the mesoderm which differentiates to form internal structures such as muscles, glands, heart, blood, fat body, and reproductive organs. The outer cell layer is the ectoderm which grows and differentiates to form the epidermis, the brain and nervous system, and most of the insect's respiratory (tracheal) system. Rudiments at either or both ends of the body give rise to a third embryonic tissue the endoderm that forms the midgut.
In most insect species, only one embryo forms in a single egg. But in some cases more than one embryo may be formed through a process of asexual division called polyembryony. In this case each embryo develops into active lava. Most insects producing eggs which are hatched outside the body of the female and are known as oviparous. Some insects bringing forth living or active young instead of laying eggs and are known as viviparous. Few insects producing living young by the hatching of the egg while still within the female and called ovoviparous.
II. Post-embryonic growth and development
1. Egg hatching: Insect eggs are of many shapes and sizes and are deposited by the females in various locations and ways. The hatching process or eclosion may begin immediately on completion of embryonic development or may be delayed for a time, thereby taking advantage of more favorable environmental conditions. Hatching is achieved by any number of means, but typically, hatching is a mechanical process, in which the larva crushes its way out of the chorion. The hatchling emerges as a first instar (larva or nymph).
2. Growth of immature: After eclosion, the immature young insects usually commence feeding and growing without delay. The immature is a virtual feeding machine at this point in the life cycle. This feeding behavior results in considerable growth in size of the insect. If it survives, it will periodically outgrow and replace its exoskeleton, a process known as molting. Molting is controlled by a molting hormone. The intervals between the molts are known as stages or stadia and the form assumed by an insect during a particular stadium is termed an instar. When an insect emerges from the egg, it is said to be in its first instar. Then, at the end of this stadium, the first molt occurs and the insect then assumes its second instar, and so on. Most insects molt at least three or four times, but in some cases, 30 or more molts may occur.
During molting the insect sheds its old exoskeleton by the process, called ecdysis and continues to fully expand the new one. Over the next few hours, sclerites will harden and darken form within the exocuticle. This process is called sclerotization or tanning gives the exoskeleton its final texture and appearance. An insect that is actively constructing new exoskeleton is said to be in a pharate condition. During the days or weeks of this process there may be very little evidence of change. Ecdysis occurs quickly in minutes to hours. A newly molted insect is soft and largely unpigmented (white or ivory). It is said to be in a teneral condition until the process of tanning is completed (usually a day or two).
3. Metamorphosis: One of the most characteristic features of insects is the fact that they almost always hatch in a morphologically different form from the adult. In order to achieve the adult stage, they have to pass through changes of form which are collectively termed metamorphosis. There are three different types of insect life cycle on the basis of metamorphosis.(a) Ametabolous insects undergo little or no structural change as they grow older. Immatures are called young. They are physically similar to adults in every way except size and sexual maturity. Other than size, there is no external manifestation of their age or reproductive state.
(b) Hemimetabolous insects exhibit gradual changes in body form during morphogenesis. Immatures are called nymphs or, if aquatic, naiads. Maturation of wings, external genitalia, and other adult structures occurs in small steps from molt to molt. Developmental changes that occur during gradual metamorphosis are usually visible externally as the insect grows, but adults retain the same organs and appendages as nymphs like eyes, legs, mouthparts, etc.
(c) Holometabolous insects have immature forms called larvae, that are very different from adults. Larvae are "feeding machines", adapted mostly for consuming food and growing in size. They become larger at each molt but do not acquire any adult-like characteristics. When fully grown, larvae molt to an immobile pupal stage and undergo a complete transformation. Larval organs and appendages are broken down (digested internally) and replaced with new adult structures that grow from imaginal discs, clusters of undifferentiated (embryonic) tissue that form during embryogenesis but remain dormant throughout the larval instars. The adult stage, which usually bears wings, is mainly adapted for dispersal and reproduction.
III. Maturity
1. Emergence of the adult: Adult emerge from the last immature exuviae in a way similar to immatures existing their old skins. As with many newly molted immatures, the new adult at first is soft and may lack much of its final pigmentation. At this time it is very vulnerable to natural enemies and other environmental adversities. The degree of sexual maturation at the time of adult emergence varies between sexes and among species. Most male insects emerge with a store of sperm, and some are fully capable of mating within short time. However, in some species the female may require a longer time to become sexually mature.
2. Mating behavior: Mating behavior consists of all the events involved in the transfer of sperm from the male to the female. The two most important events in this behavior are finding mate and copulation. Mate finding is an important behavior in all but a very few insects. In most instances locating a mate is necessary, and this act may involve complex processes, among which are certain cues to locate and recognize and appropriate mate. Such cues may be visual, olfactory, auditory, tactile, or combination of these.
Olfactory cues are probably the most commonly used type in mate finding. Many insect species secrete volatile chemical attractants that are highly specific and can be used by one sex to attract its opposite over considerable distances. Such volatiles are called sex pheromones. Male attracting sex pheromones are common in many insect orders, particularly Coleoptera, Lepidoptera and Diptera. In many moths female begin caking about 1 to 2 hours before dawn.
Copulation (joining of male and female genital structures) may follow immediately after mate finding or, in some species, after specific forms of courtship behavior. Depending of the species, a female may copulate several times during her lifetime or only once. Following the act of copulation insemination occurs (introducing of sperm into the female reproductive tract).
3. Oviposition: Once mating has occurred and eggs are matured and fertilized, egg deposition is the ultimate event in the life cycle of an insect. The act of layinf eggs by the adult female is called oviposition. Insects differ greatly in the ways they lay their eggs, or oviposit. Eggs may be laid singly (e.g. lacewings), in clumps (e.g. stink bugs), or in a contiguous mass (e.g. corn borer). Some insects, such as cockroaches produce an ootheca, which is a protective pod that contains several eggs ‘glued’ together by a secretion.
Insect’s eggs simply may be dropped o the ground, as with walkingsticks, or they may be placed on or in host tissues where newly eclosed immatures have ready access to food.
Insect seasonal cycle
Many insects have one generation each year, in which case they are said to be univoltine. In these insects the life cycle is synchronized to the seasonal cycle, often by the overwintering stage. Other insects have several or many generations in a single year in which case they are called multivoltine. The impact of this concept is obvious for a destructive insect having five generations in a season pose entirely different problems from one which completes its development in one season. In a few insects, such as the 17-year cicada, the life cycle may encompass many seasons.
Alternation of generations
In several groups of insects, succeeding generations are quite different in method of reproduction, morphology, and sometimes in feeding habits. This is called alternation of generations. Most of these have reproduction only by adults, although one generation may have sexual adults while the next may be parthenogenetic. A few have reproduction by adults and by immature forms. The life cycles are usually complicated, involving alternate hosts, winged and non-winged adults, different feeding patterns, etc. The aphids (Homoptera) and gall wasps (Hymenoptera) are the most typical of these groups.
