The life cycle of a species serves as a unit of biological time. From the development of the eggs in the female and the sperm in the male and their union there is a series of changes, often involving several unlike forms which lead to adulthood and the production of progeny for the next generation. This sequential growth from the fertilized egg to a sexually mature adult constitutes the life cycle of the individual. 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.
Insect eggs are of many shapes and sizes and are deposited by the females in various locations and ways; they may be smooth ellipses or be ribbed or sculptured in various ways; others may be provided with processes of different kinds to ensure survival. The length of time required for hatching varies with each insect species and is affected by many external factors such as temperature and moisture.
Slide showing a butterfly depositing eggs on a leaf.
Slide showing eggs of a tent caterpillar laid in a frothy mass around a twig.
Slide showing the eggs of a bug.
The process of hatching or eclosion from the egg varies greatly in different groups. After eclosion, the young insect usually commences feeding and growing without delay. Although most insects lay eggs, a few deposit living young instead of eggs.
Slide showing the embryos within the egg; note the red eye spots on the insects almost ready to hatch.
As the insect integument is not cellular it cannot grow by addition of cells. It also has very little stretch and so cannot be extended to accommodate growth. Because of this, the insect must periodically shed its old skin and replace it with a larger one. This process is called a molt and is controlled by a molting hormone.
Slide showing an adult cicada following molting from the pupa; note the cast skin of the pupa or exuvia.
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. The average is five or six molts. The molts represent significant biological landmarks in the events which occur during the life of an insect. The final instar is the mature form which is known as the adult stage. Entry into this stage is controlled by the absence of a hormone, the so-called juvenile hormone, the presence of which keeps the insect from reaching maturity.
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 two main types of metamorphosis in insects:
Other terms describing this type of metamorphosis are direct, incomplete, or gradual. The immature stages or nymphs resemble the adult except that the wings develop gradually as external pads, the genitalia are incomplete, and there are often color differences as well as the obvious size differences. The principal changes during growth are in size, body proportions, the development of the ocelli, the size of the wings and occasionally in the form of the antennae, mouthparts, abdomen and other structures.
Slide showing first stage nymphs of a bug recently emerged from the egg. Note that the nymphs have well formed legs and a shape similar to an adult bug.
This term is applied to those insects which pass through a complex series of changes, the first after the egg stage being the larva. The larva passes through several instars which are more or less grub-like and are usually similar in form but differ in size. Larvae do not have wings or compound eyes; they may or may not have thoracic legs and they sometimes possess leg-like appendages called prolegs on the abdomen.
Slide showing a young larva of the pine butterfly.
Slide showing a mature larva of the pine butterfly.
Following the molt of the last larval instar, the insect transforms into a stage called the pupa. This stage is the resting inactive instar in most insects undergoing metamorphosis. The insect is incapable of feeding during this stage, and is quiescent. It is the transitional phase during which the larval body and its organs are remodeled to fit the requirements of the adult. Although normally motionless, many pupae are capable of limited locomotion, particularly toward the end of the stadium.
Slide showing a pupa of the pine butterfly.
During the pupal instar, insects are particularly vulnerable as they are limited in movement and lacking defenses. These liabilities are overcome in many ways such as concealment, protective coloration or hard covering and enclosure within a cocoon. The cocoon is usually prepared by the last-instar larva and may be constructed of wood chips, sand and gravel, body hairs or vegetable matter, but chiefly from silk produced by the larva. Many insect larvae do not fabricate free-existing cocoons but construct cells in the ground or in their habitat, usually lined with silk, in which to pupate. Emergence from the cocoon or pupal chamber may be by the adult chewing or cutting its way out by means of special organs, by rupturing of the cocoon with muscular contractions, or by softening the cocoon with a secretion whereby the adult may push its way out.
Slide showing the last instar larva of the Douglas-fir tussock moth spinning its cocoon.
Slide showing the completed cocoons.
Just prior to emergence of the adult, the pupa usually darkens markedly in color. After emergence, there is usually a resting period while the still soft tissues harden. The time of emergence varies greatly as does the time it takes before the adult is able to carry on its activities. Adult insects are seldom sexually mature immediately upon emergence. Most must mate before they can deposit viable eggs but some have the ability to produce young without fertilization (parthenogenesis). When this happens, the young produced are all of the same sex.
Slide showing mother aphids and parthenogenetically produced young on a leaf.
Adult insects have a normal life span ranging from a few days to several years, depending on the species.
The life cycle is the development of the individual from egg to egg. Many forest 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. The overwintering stage determines which stage of the insect begins the seasonal cycle and this is usually determined by the requirements of the species. For example, insects whose young larvae require new foliage usually overwinter as eggs or very young larvae. Entry into the overwintering stage is usually stimulated by declining day-length in the fall and the process of overwintering is called diapause. Other forest 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.
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