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Information on the basics of Entomology

Introduction Contents►

An Introduction To Entomology ¹

Kingdom: Animalia, Phylum: Arthropoda

Subphylum: Hexapoda: Class: Insecta: Entomology

Metamorphosis

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General Characteristics of Metamorphosis

Types of Metamorphosis

The Adult Stage

The Pupal Stage

References Citations Sample Examinations

Table 1 (Contents)

General Characteristics of Metamorphosis

Insects attain maximum size by undergoing a succession of molts or ecdyses. The number of molts that an insect passes through is quite constant for the species, and the form assumed by the animal between any two ecdyses is called an instar. The animal's existence is thus made up of a succession of instars (growth) during which the insect is immature, followed by the attainment of the final adult instar (metamorphosis). In the simplest and most generalized insects the instars resemble one another and only differ from the adult in the absence of wings and the incomplete development of the reproductive system. Where the adult is primitively wingless, as in silverfish and springtails the change from young to adult is so slight as to be ignored, and metamorphosis, involving only a development of the reproductive system, is usually considered to be absent. The insect orders in this category may be grouped under the Ametabola.

 However, in winged insects the adult is in sharp contrast to the young stage not only because of its wings, but also on account of the sexual appendages that in the adult have become prominent. Such forms are said to undergo a metamorphosis. Here the degree of metamorphosis varies considerably, irrespective of wings, depending on whether the young stages resemble their adults or not. A growth stage of a cockroach, eg., has the general appearance of the adult. But the young stage of a housefly is a grub and has no resemblance to the final stage with its wings, complex body form and mouthparts.

Metabolous insects pass through a distinctive metamorphosis, and classifications have been based on this.

The Heterometabola are insects whose young stages, called nymphs, closely resemble the adult in body form and type of mouthparts. These young stages have compound eyes and, where wings develop, their growth is external and clearly visible throughout most or all of the nymphal instars. Included are the following orders: Isoptera, Zoraptera, Orthoptera, Embioptera, Dermaptera, Homoptera, Heteroptera, Mallophaga, Anoplura, Thysanoptera, Ephemeroptera, Odonata and Plecoptera. The last three orders, having nymphs adapted for aquatic life, have sometimes been grouped accordingly as Hemimetabola.

The Holometabola, including the orders Coleoptera, Neuroptra, Mecoptera, Trichoptera, Lepidoptera, Diptera, Hymenoptera, Strepsiptera and Siphonaptera, have young stages known as larvae that differ noticeably from the adult in body form and mouthparts. These young stages do not have compound eyes, but in their place are lateral ocelli. Also, their wings develop from within pockets of the hypodermis and are not visible from the outside during growth. So great is the difference between the larva and the adult that an instar known as the pupa has been developed to connect the gulf between them.

This resting stage is really one of much physiological and developmental activity, and it is here that many larval tissues, e.g. the muscles and the alimentary canal, are broken down by phagocytic or other processes and the new adult tissue is constructed from many growth centers, usually called imaginal discs. The change from larva to pupa is often accompanied by a period of inactivity at the end of the last larval instar.

The origin of the phenomenon of holometaboly is vague. That it is associated with divergent specializations of larvae and adults in varying degrees is from observation. Therefore, it is not unexpected to find among the orders composing this group, as, for instance, in many Coleoptera, larvae that are nymph-like in that they are well cuticularized and have well-developed legs, and mouthparts resembling those of the adults.

The forms of larvae vary markedly and indicate the degree of metamorphosis passed through. A campodeiform larva resembles some of the ametabolous Thysanura and hve well-developed legs, antennae, cerci and mouthparts, e.g. many Coleoptera. An eruciform larva is fleshy and thin skinned, its legs may be in the form of supporting struts rather than organs of active locomotion, prolegs are often found on the abdomen, and there are no cerci, e.g. caterpillars of Lepidoptera and sawflies. A grub is an apodous larva, which resembles the erusiform type, e.g. some Coleoptera, Diptera and Hymenoptera.

Pupal modifications are also found; thus the exarate character of the Hymenoptera, Mecoptera and Neuroptera, is that in which the cases of the adult appendages lay are not fused along their length to the body. In obtect pupae wing and leg cases are merged with the body wall, e.g. most Lepidoptera and Diptera. In the specialized Diptera the last larval skin is retained as a barrel-shaped puparium over the pupa within. Such protected pupae are coarctate.

In the Heterometabola the development of adult form is a gradual process and the appendages, including mouthparts, antennae and legs, grow directly into those of the adult. Wings develop gradually as external dorsolateral extensions of the meso- and metathoracic body wall. All the Heterometabola have this kind of wing development and for that reason the alternative name Exopterygota is sometimes given to the group.

Larvae of the Holometabola have, for the most part, mouthparts with a form and mode of working different from that of their adults, their legs are reduced in size and complication or even absent, and wing growth is hidden from view; thus, the alternative name Endopterygota.. It is in the pupal stage of Holometabola that adult appendages show up on the surface.

The development of adult appendages in the larva is only one of the many aspects of metamorphosis. The wings that suddenly appear in the pupa of the butterfly grow gradually through each of the five larval instars, but instead of growing externally as in the Heterometabola (Exopterygota) they appear as outgrowths from the bottom of invaginations of the body wall. This obliging fold of the body wall forming a sac, opening at the surface by a minute pore, hides the growing wing bud within it, making it the main difference between endopterygote and exopterygote development.

At pupation the sac carrying the wing disc or bud at its base straightens out by contraction of its walls and the wing bud is in this way brought to view. Similar limb buds occur for the adult legs and mouthparts that always grow in association with the corresponding larval organs. These buds are also called imaginal discs and their existence is typical of all endopterygote insects.

Thus there are two postembryonic processes that occur in the lives of all insects. Both require the process of molting, the shedding of the old and the production of the new cuticle. The first of these, growth, is the attribute of the young insect; the second, metamorphosis, is the attainment at the final molt of the condition of sexual maturity. These processes are under hormonal control from neurosecretory cells of the brain, the corporata allata and the prothoracic glands. The interchange of these is complex, depending on a precisely timed activation. The result of this interplay is the suppression of adult characters from appearance during youth and the final attainment of sexual maturity when the juvenile features are lost. The prothoracic gland produces the hormones necessary for molting but only when activated to do so by the neurosecretory cells of the brain.

The molting which result is the route by which either further juvenile stages or the final adult stage can be attained. In the presence of the secretion of the corpora allata the juvenile condition results. When these glands stop functioning, usually at the end of the larval period, the molting process, still activated by the prothoracic gland, gives rise to the adult.

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Types of Metamorphosis

There are four types of metamorphosis (1) Ametabolous, (2) Paurometabolous, (3) Hemimetabolous and (4) Holometabolous.

Ametabolous Metamorphosis. -- In this type the only appreciable difference from the immature to the adult is the maturation of the sex organs (e.g., silverfish)

Paurometabolous Metamorphosis. -- Here the various mymphs closely resemble the adult except for body proportions. The steps are egg to nymphs to adult (e.g., grasshopper, milkweed bug).

Hemimetabolous Metamorphosis. -- All the Hemimetabolaare aquatic. The steps are egg to naiad to adult. The naiad is very different from the adult in appearance (e.g., dragonfly, mayfly, stonefly).

The wings are developed externally in both the hemi- and paurometabolous insects, and these two are often considered together under the Paurometabola. The developing wing is called a wingpad.

Holometabolous Metamorphosis. -- Here the steps are egg to larva to pupa and adult. Wings are developed internally beneath the cuticula. The pupal is often called the "resting stage," but this in inaccurate because thee is a complete histolysis of larval tissues and a reconstruction to the adult.

There are five larval types found in the Holometabola:

Campodeiform larvae are extremely active and usually predatory. They have a flattened body with a prognathous head, and their thoracic legs are well developed for running. Their jaws are designed for cutting and tearing. Their cerci and antennae are often well developed (e.g., many beetles, neuropterans and trichopterans: Fig. ent73).

Eruciform larvae are the opposite of campodeiform by being sluggish, like caterpillars. They have a hypognathous round body. The head is well developed but with very short antennae, and with thoracic legs and abdominal prolegs (e.g., Lepidoptera, Mecoptera and some Hymenoptera: Fig. ent70).

Scarabaeiform larvae inhabit the soil or plant tissues, and some species are enclosed in tunnels. They are quite helpless grubs especially when exposed on the surface of the soil. They are hypognathous with a usually curved body (e.g., June beetle: Fig. ent72).

Vermiform larvae are very advanced with a reduced head. The body is elongated and wormlike, legless (e.g., maggots: Fig. ent71).

Elateriform larvae are prognathous with a hard and elongated cylindrical body (e.g., wireworms: Fig. ent74)

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The Adult Stage

The adult is the perfect or sexual stage of an insect. In this stage the sexual organs mature and locomotory appendages are sufficient for propagation. The storage of energy varies considerably among species. Some adults can only take water or liquid; some adults use fat bodies that were stored in a previous immature stage, and some, like chironomid midges, cannot even imbibe liquids. Much of metamorphosis is directed toward producing an adult that can propagate the species.

Types of Reproduction. -- Adult insects show various types of reproduction. In oviporous reproduction an egg is formed and the female lays the egg covered by a chorion. In polyembryony there is a lot of division in the egg to give many individuals from one egg. In vivipary the insect is born alive although its origin was still from an egg. In parthenogenesis the young are produced form infertile eggs, as in aphids. Some species do not have males, as in the white-fringed beetle. Some may have a combination of fertile and infertile periods. Paedogenesis involves reproduction in immature stages. The maggot young fasten themselves on the parent and consume them. Several generations may pass in this manner.

Egg Shape Variation. -- The various kinds of sculpturing found on insect eggs are formed by epithelial cells.

Many insects have the means of fixing their eggs so that they will have a proper environment for development. Examples are nits, the eggs of lice, that are glued to the hair of their host and the ovipositor can cut holes into wood where the eggs are laid. Lacewings lay their eggs on a stalk, which is a piece of silk with the egg attached to the end. However, some insects depend wholly on large numbers of eggs for survival of the species.

Molting Process. -- The entire cuticle is cast off and a new one is formed. This includes lenses of the eyes, mandibles, linings of the fore and hind guts, tracheal trunks and linings of the genital chamber. Hypodermal cells must lay down the new cuticula. At the same time the cells must secrete a fluid, which will erode away much of the endocuticle. This can be resorbed by the hypodermal cells and used again. When most of the cuticula has eroded away the insect can molt. New cuticula is very soft and pliable. It remains so for several hours before being oxidized during which time it can be expanded to accomodate the larger size of the insect. When the insect casts its skin, the integument underneath is not pigmented and it remains white for an hour or more.

The instar is the form of an insect after the molt. Following egg hatching there can be a first, second and more molts. The number of instars varies but it is normally four. Some insects can regress in molting (e.g., carpet beetle).

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The Pupal Stage

The pupa is an immature stage in development of the Holometabola. It is a stage of quiescence primarily even though there can be some movement as is typical of mosquitoes. It is also the stage of breakdown of tissue and the buildup of others. Various other names often assigned to this stage are chrysalis, puparium and cocoon.

There are three types of pupae: (1) exarate, where the developing wings, mouthparts and legs are visible externally (e.g., Hymenoptera and Coleoptera: Figs. ent75 & ent76).

(2) obtect, where the mouthparts, legs and wings are seen as incompletely formed structures glued down as an integral part of the pupal case (e.g., Lepidoptera: Fig. ent77).

(3) coarctate, where an exarate pupa is formed within the last larval skin. The exterior is generally smooth and seedlike (e.g., Diptera: Fig. ent78).

Details of Insect Taxonomic Groups

Examples of beneficial species occur in almost every insect order, and considerable information on morphology and habits has been assembled. Therefore, the principal groups of insect parasitoids and predators provide details that refer to the entire class Insecta. These details are available at <taxnames.htm>.

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References

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