Entomology 3rd edition - C.Gillott - Chapter 21 doc

These changes areespecially noticeable in endopterygotes during the final larval and pupal stages.. At hatching, the fat content of a larva is typically low less than 1% in the caterpilla

alternat-a result, they undergo striking chalternat-anges (complete metalternat-amorphosis), sprealternat-ad over two molts,

in the formation of the adult (holometabolous development) The final juvenile instar hasbecome specialized to facilitate these changes and is known as the pupa (see also Chapter 2,Section 3.3)

In insect evolution increasing functional separation has occurred between the larvalphase, which is concerned with growth and accumulation of reserves, and the adult stage,whose functions are reproduction and dispersal Associated with this trend is a tendency for

an insect to spend a greater part of its life as a juvenile, which contrasts with the situation

in many other animals Thus, in apterygotes, the adult stage may be considerably longerthan the juvenile stage Furthermore, feeding (in the adult) serves to provide raw materialsboth for reproduction and for growth In exopterygotes and primitive endopterygotes adultsmay live for a reasonable period, but this is not usually as long as the larval phase Feeding

in the adult stage is primarily associated with reproductive requirements, though in someinsects it provides nutrients for an initial, short “somatic growth phase” in which the flightmuscles, gut, and cuticle become fully developed Many endopterygotes live for a relativelyshort time as adults and may feed little or not at all because sufficient reserves have beenacquired during larval life to satisfy the needs of reproduction

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CHAPTER 21

2 Growth

2.1 Physical Aspects

Growth in insects and other arthropods differs from that of mammals in various respects

In insects growth is almost entirely restricted to the larval instars, though in some speciesthere is a short period of somatic growth in newly enclosed adults when additional cuticlemay be deposited, and growth of flight muscles and the alimentary canal may occur As aconsequence, the length of the juvenile stage is considerably longer than that of the adult Anextreme example of this is seen in some mayfly species whose aquatic juvenile stage mayrequire 2 or 3 years for completion, yet give rise to an adult that lives for only a few hours ordays Growth in many animals is discontinuous or cyclic; that is, periods of active growth areseparated by periods when little or no growth occurs Nowhere is discontinuous growth betterseen than in arthropods, which must periodically molt their generally inextensible cuticle inorder to significantly increase their size (volume) It should be appreciated, however, that,though increases in volume may be discontinuous, increases in weight are not (Figure 21.1)

As an insect feeds during each stadium, reserves are deposited in the fat body, whose weightand volume increase In a hard-bodied insect this increase in volume may be compensatedfor by a decrease in the volume occupied by the tracheal system or by extension of theabdomen as a result of the unfolding of intersegmental membranes In many endopterygotelarvae, of course, the entire body is largely covered with extensible cuticle, and body sizeincreases almost continuously (but see below)

For many insects grown under standard conditions the amount of growth that occurs ispredictable from one instar to the next; that is, it obeys certain “growth laws.” For example,Dyar’s law, based on measurements of the change in width of the head capsule which occurs

at each molt, states that growth follows a geometric progression; that is, the proportionateincrease in size for a given structure is constant from one instar to the next Mathematically

expressed, the law states x /y = constant (value usually 1.2–1.4), where x = size in a given instar and y= size in previous instar (Figure 21.1) Thus, when the size of a structure isplotted logarithmically against instar number, a straight line is obtained, whose gradient isconstant for a given species (Figure 21.2) In those insects where it applies Dyar’s law can

be used to determine how many instars there are in the life history However, so many factors

FIGURE 21.1. Change in head width with time to illustrate Dyar’s law.

POSTEMBRYONIC DEVELOPMENT

FIGURE 21.2. Head width plotted logarithmically

against instar number in various species [After V B.

Wigglesworth, 1965, The Principles of Insect Physiology,

6th ed., Methuen and Co By permission of the author.]

affect growth rates and the frequency of ecdysis that the law is frequently inapplicable In

any event, the law requires that the interval between molts remains constant, but this is

rarely the case

As winged insects grow, they change shape; that is, the relative proportions of different

parts of the body change This disproportionate growth, which is not unique to insects, is

described as “allometric” (heterogonic, disharmonic) In other words, each part has its own

growth rate, expressed by the equation y = bx k (y = linear size of the part, x = linear

size of the standard (e.g., body length), b = initial growth index (y intercept), and k =

allometric coefficient) Normally, allometric growth is expressed as a log-log plot, when k

is the gradient of the slope (Figure 21.3)

Growth laws do not apply in situations where the number of instars is variable This

variability may be a natural occurrence, especially in primitive insects such as mayflies

that have many instars In addition, females that are typically larger than males may have

a greater number of instars than males Variability may also be induced by environmental

conditions For example, rearing insects at abnormally high temperature often increases the

number of instars, as does semistarvation In contrast, in some caterpillars crowding leads

to a decrease in the number of molts

Caterpillars and probably other larvae whose cylindrical body is covered with a thin

integument rely on hydrostatic (hemolymph) pressure to maintain the rigidity necessary

for locomotion However, this presents a problem with respect to their body form during

growth Mechanically, in a cylinder under internal pressure the hoop stress (around the

body) is twice the axial (lengthwise) stress Thus, a caterpillar theoretically should become

proportionately fatter as it grows, much like a balloon when inflated That it does not do

CHAPTER 21

FIGURE 21.3. Allometric growth in Carausius (Phasmida) [After V B Wigglesworth, 1965, The Principles of Insect Physiology, 6th ed., Methuen

and Co By permission of the author.]

so is due to the occurrence of axial pleats (transverse cuticular folds) that reduce the axialstress by unfolding as the insect enlarges (Carter and Locke, 1993)

2.2 Biochemical Changes during Growth

Like the physical changes noted above, biochemical changes that occur during bryonic development may also be described as allometric That is, the relative proportions

postem-of the various biochemical components change as growth takes place These changes areespecially noticeable in endopterygotes during the final larval and pupal stages At hatching,

the fat content of a larva is typically low (less than 1% in the caterpillar Malacosoma, for

example) and remains at about this level until the final larval stadium when fat is synthesizedand stored in large quantity, reaching about 30% of the dry body weight Though fat is thetypical reserve substance in most insects, members of some species store glycogen Again,this usually occurs in small amounts in newly hatched insects, but its proportion increasessteadily through larval development, and at pupation glycogen may be a significant com-ponent of the dry weight (one-third in the honey bee) Like fat, glycogen is stored in the fatbody

In contrast, the proportions of water, protein, and nucleic acids generally decline duringlarval development However, this is often not the situation in larvae that require large

amounts of protein for specific purposes, for example, spinning a cocoon In Bomybyx mori, for example, the hemolymph protein concentration increases sixfold in late larval

development, and about 50% of the total protein content of a mature larva is used incocoon formation The great increase in concentration of hemolymph protein often can beaccounted for almost entirely by synthesis, in the fat body, of a few specific proteins In the

fly Calliphora stygia, for example, the protein “calliphorin” makes up 75% (about 7 mg) of

POSTEMBRYONIC DEVELOPMENT

the hemolymph protein by the time a mature larva stops feeding The calliphorin is used in

the pupa as a major source of nitrogen (in the form of amino acids) for formation of adult

tissues and as a source of the energy required in biosynthesis Thus, at eclosion (emergence

of the adult), the hemolymph calliphorin content has fallen to 0.03 mg, and, 1 week after

emergence, the protein has entirely disappeared

During metamorphosis some of the above trends may be reversed The proportions of fat

and/or glycogen decline as these molecules are utilized in energy production In Calliphora

the fat content decreases from 7% to 3% of the dry weight through the pupal period In the

honey bee, which mainly uses glycogen as an energy source, the glycogen content drops

to less than 10% of its initial value as metamorphosis proceeds For most insects there is

little change in the net protein content during pupation, though major qualitative changes

occur as adult tissues develop In members of a few species a significant decline in total

protein content occurs during metamorphosis as protein is used as an energy source The

moth Celerio, for example, obtains only 20% of its energy requirements in metamorphosis

from fat, the remaining 80% coming largely from protein

Superimposed on the overall biochemical changes from hatching to adulthood are

changes that occur in each stadium, related to the cyclic nature of growth and molting

Fac-tors to be considered include the phasic pattern of feeding activity throughout the stadium,

synthesis of new and degradation of old cuticle, and net production of new tissues (though

some histolysis also occurs in each instar)

Measurement of oxygen consumption shows that it follows a U-shaped curve through

each stadium with maximum values being obtained at the time of molting The maxima are

correlated with the great increase in metabolic activity at this time, associated especially

with the synthesis of new cuticle and formation of new tissues In Locusta larvae there are

significant decreases in the carbohydrate and lipid contents of the fat body and hemolymph

at ecdysis, probably correlated with the use of these substrates to supply energy (Hill and

Goldsworthy, 1968) Conversely, as feeding restarts after a molt, these materials are again

accumulated

Changes in the amount of protein in the fat body and hemolymph of Locusta are

also cyclical, with maximum values occurring in the second half of each stadium (Hill

and Goldsworthy, 1968) The early increase in protein content is related to renewed feeding

activity after the molt Feeding activity reaches a peak in the middle of the stadium, providing

materials for growth of muscles (and presumably other tissues, though these were not studied

by Hill and Goldsworthy) and for the synthesis of cuticle Excess material is stored in the

fat body and hemolymph In the second half of the stadium feeding activity declines, and

this is followed by a decrease in the level of protein in the hemolymph and fat body Hill

and Goldsworthy (1968) suggested that the latter probably reflects the use of protein in the

synthesis of new cuticle However, recycled protein from the old cuticle may account for

most (about 80% in Locusta) of the protein content of the new cuticle.

3 Forms of Development

Through insect evolution there has been a trend toward increasing functional and

struc-tural divergence between juvenile and adult stages Juvenile insects have become more

concerned with feeding and growth, whereas adults form the reproductive and dispersal

phase This specialization of different stages in the life history became possible with the

introduction into the life history of a pupal instar, though the latter’s original function was

For example, in the firebrat, Thermobia domestica, between 45 and 60 molts have been

recorded

3.2 Hemimetabolous Development

Exopterygotes usually molt a fixed number of times, but, with the exception ofEphemeroptera, which pass through a winged subimago stage, never as adults In specieswhere the female is much larger than the male, she may undergo an additional larval molt.The number of molts is typically 4 or 5, though in some Odonata and Ephemeroptera whoselarval life may last 2 or 3 years a much greater and more variable number of molts occurs(e.g., 10–15 in species of Odonata, 15–30 in most Ephemeroptera)

In almost all exopterygotes the later juvenile instars broadly resemble the adult, exceptthat their wings and external genitalia are not fully developed Early instars show no trace ofwings, but, later, external wing buds arise as sclerotized, non-articulated evaginations of thetergopleural area of the wing-bearing segments Wings develop within the buds during thefinal larval stadium and are expanded after the last molt Other, less obvious, changes thatoccur during the growth of exopterygotes include the addition of neurons, Malpighiantubules, ommatidia, and tarsal segments, plus the differentiation of additional sensilla inthe integument This mode of development is described as hemimetabolous and includes apartial (incomplete) metamorphosis from larva to adult

3.3 Holometabolous Development

Holometabolous development, in which there is a marked change of form from larva

to adult (complete metamorphosis), occurs in endopterygotes and a few exopterygotes, forexample, whiteflies (Aleurodidae: Hemiptera), thrips (Thysanoptera), and male scale insects(Coccidae: Hemiptera) Perhaps the most obvious structural difference between the larvaland adult stages of endopterygotes is the absence of any external sign of wing development

in the larval stages The wing rudiments develop internally from imaginal discs that in mostlarvae lie at the base of the peripodial cavity, an invagination of the epidermis beneath thelarval cuticle, and are evaginated at the larval-pupal molt (see Section 4.2 and Figure 21.11)

As noted above, the evolution of a pupal stage in the life history has madeholometabolous development possible The pupa is probably a highly modified final ju-venile instar (Chapter 2, Section 3.3) which, through evolution, became less concernedwith feeding and building up reserves (this function being left to earlier instars) and more

POSTEMBRYONIC DEVELOPMENT

3.3.1 The Larval Stage

Among endopterygotes the extent to which the larval and adult habits and structurediffer [and therefore the extent of metamorphosis (Section 4.2)] is varied Broadly speaking,

in members of more primitive orders the extent of these differences is small, whereas theopposite is true, for example, in the Hymenoptera and Diptera Endopterygote larvae can

be arranged in a number of basic types (Figure 21.5) The most primitive larval form isthe oligopod Larvae of this type have three pairs of thoracic legs and a well-developedhead with chewing mouthparts and simple eyes Oligopod larvae can be further subdividedinto (1) scarabaeiform larvae (Figure 21.5A), which are round-bodied and have short legsand a weakly sclerotized thorax and abdomen, features associated with the habit of bur-rowing into the substrate, and (2) campodeiform larvae (Figure 21.5B), which are active,predaceous surface-dwellers with a dorsoventrally flattened body, long legs, strongly scle-rotized thorax and abdomen, and prognathous mouthparts Scarabaeiform larvae are typical

FIGURE 21.5. Larval types (A) Scarabaeiform (Popillia japonica, Coleoptera); (B) campodeiform damia convergens, Coleoptera); (C) eruciform (Danaus plexippus, Lepidoptera); (D) eucephalous (Bibio sp., Diptera); (E) hemicephalous (Tanyptera frontalis, Diptera); and (F) acephalous (Musca domestica, Diptera) [A–E, from A Peterson, 1951, Larvae of Insects By permission of Mrs Helen Peterson F, from V B Wigglesworth, 1959, Metamorphosis, polymorphism, differentiation, Scientific American, February 1959 By

(Hippo-permission of Mr Eric Mose, Jr.]

POSTEMBRYONIC DEVELOPMENT

of the Scarabaeidae and other beetle families; campodeiform larvae occur in Neuroptera,

Coleoptera-Adephaga, and Trichoptera

Polypod (eruciform) larvae (Figure 21.5C) have, in addition to thoracic legs, a varied

number of abdominal prolegs The larvae are generally phytophagous and relatively inactive,

remaining close to or on their food source The thorax and abdomen are weakly sclerotized in

comparison with the head, which has well-developed chewing mouthparts Eruciform larvae

are typical of Lepidoptera, Mecoptera, and some Hymenoptera [sawflies (Tenthredinidae)]

Apodous larvae, which lack all trunk appendages, occur in various forms in many

endopterygote orders but in common are adapted for mining in soil, mud, or animal or

plant tissues The variability of form concerns the extent to which a distinct head capsule

is developed In eucephalous larvae (Figure 21.5D), characteristic of some Coleoptera

(Buprestidae and Cerambycidae), Strepsiptera, Siphonaptera, aculeate Hymenoptera, and

more primitive Diptera (suborder Nematocera), the head is well sclerotized and bears normal

appendages The head and its appendages of hemicephalous larvae (Figure 21.5E) are

reduced and partially retracted into the thorax This condition is seen in crane fly larvae

(Tipulidae: Nematocera) and in the larvae of orthorraphous Diptera Larvae of

Diptera-Muscomorpha are acephalous (Figure 21.5F); no sign of the head and its appendages can

be seen apart from a pair of minute papillae (remnants of the antennae) and a pair of

sclerotized hooks believed to be much modified maxillae

Frequently a larva in the final instar ceases to feed and becomes inactive a few days

before the larval-pupal molt Such a stage is known as a prepupa In some species, the

entire instar is a non-feeding stage in which important changes related to pupation occur

For example, in the prepupal instar of sawflies, the salivary glands become modified for

secreting the silk used in cocoon formation

3.3.2 Heteromorphosis

In most endopterygotes the larval instars are more or less alike However, in some

species of Neuroptera, Coleoptera, Diptera, Hymenoptera, and in all Strepsiptera, a larva

undergoes characteristic changes in habit and morphology as it grows, a phenomenon

known as heteromorphosis (hypermetamorphosis) In such species several of the larval

types described above may develop successively (Figure 21.6) For example, blister beetles

(Meloidae) hatch as free-living campodeiform larvae (planidia, triungulins) that actively

search for food (grasshopper eggs and immature stages, or food reserves of bees or ants)

At this stage the larvae can survive for periods of several weeks without food Larvae that

locate food soon molt to the second stage, a caterpillarlike (eruciform) larva The insect

then passes through two or more additional larval instars, which may remain eruciform or

become scarabaeiform Some species overwinter in a modified larval form known as the

pseudopupa or coarctate larva, so-called because the larva remains within the cuticle of

the previous instar The pseudopupal stage is followed the next spring by a further larval

feeding stage, which then molts into a pupa

3.3.3 The Pupal Stage

The pupa is a non-feeding, generally quiescent instar that serves as a mold in which

adult features can be formed For many species it is also the stage in which an insect survives

adverse conditions by means of diapause (Chapter 22, Section 3.2.3) The terms “pupa”

and “pupal stage” are commonly used to describe the entire preimaginal instar This is,

CHAPTER 21

FIGURE 21.6. Heteromorphosis in Epicauta (Coleoptera) (A) Triungulin; (B) caraboid second instar; (C) final form of second instar; (D) coarctate larva; (E) pupa; and (F) adult [From J W Folsom, 1906, Entomology: With Special Reference to Its Biological and Economic Aspects, Blakiston.]

strictly speaking, incorrect because for a varied period prior to eclosion, the insect is a

“pharate adult,” that is, an adult enclosed within the pupal cuticle The insect thus becomes

an adult immediately after apolysis of the pupal cuticle and formation of the adult epicuticle(Chapter 11, Section 3.1) The distinction between the true pupal stage and the pharate adultcondition becomes important in consideration of so-called “pupal movements,” includinglocomotion and mandibular chewing movements (used in escaping from the protectivecocoon or cell in which metamorphosis took place) In most instances these movementsresult from the activity of muscles attached to the adult apodemes that fit snugly around theremains of the pupal apodemes (Figure 21.7)

FIGURE 21.7. Section through mandible of a decticous pupa to show adult apodemes around remains of pupal apodemes [After H E Hinton, 1946, A new classification

of insect pupae, Proc Zool Soc Lond 116:282–328 By

permission of the Zoological Society of London.]

POSTEMBRYONIC DEVELOPMENT

FIGURE 21.8. Pupal types (A) Decticous (Chrysopa sp., Neuroptera); (B) exarate adecticous (Brachyrhinus

sulcatus, Coleoptera); and (C) obtect adecticous (Heliothis armigera, Lepidoptera) [From A Peterson, 1951,

Larvae of Insects.By permission of Mrs Helen Peterson.]

Pupae are categorized according to whether or not the mandibles are functional and

whether or not the remaining appendages are sealed closely against the body (Figure

21.8) Decticous pupae, found in more primitive endopterygotes [Neuroptera, Mecoptera,

Trichoptera, and Lepidoptera (Zeugloptera and Dacnonypha)], have well-developed,

artic-ulated mandibles (moved by the pharate adult’s muscles) with which an insect can cut its

way out of the cocoon or cell Decticous pupae are always exarate; that is, the appendages

are not sealed against the body so that they may be used in locomotion Some neuropteran

pupae, for example, can crawl and some pupae of Trichoptera swim to the water surface

prior to eclosion Adecticous pupae, whose mandibles are non-functional and often

re-duced, may be either exarate or obtect In the latter condition the appendages are firmly

sealed against the body and are usually well sclerotized Adecticous exarate pupae are

char-acteristic of Siphonaptera, brachycerous Diptera, most Coleoptera and Hymenoptera, and

Strepsiptera In nematocerous Diptera, Lepidoptera (Heteroneura), and in a few Coleoptera

and Hymenoptera, pupae are of the adecticous obtect type

In muscomorph Diptera at the end of the final larval stadium the cuticle becomes

thickened and tanned The tanned cuticle is not shed but remains as a rigid coat (puparium)

around the insect A few hours after pupariation the larval epidermis apolyses so that a

pharate pupal instar is formed within the puparium, serving as in other endopterygotes as

the mold for adult tissues

An immobile pupa is vulnerable to attack by predators or parasites and to severe changes

in climatic conditions, particularly as the pupal stadium may last for a considerable time

To obtain protection against such adversities the pupa typically has a thick, tanned cuticle

Also, in many species it is enclosed within a cocoon or subterranean cell constructed by the

previous larval instar The cocoon may comprise various kinds of extraneous material, for

example, soil particles, small stones, leaves or other vegetation, or may be made solely of

silk In some endopterygotes the pupa is exposed (not surrounded by a protective cocoon)

but obtains additional protection by taking on the color of its surroundings Many parasitic

species remain within, and are thus protected by, the host’s body in the pupal stage

CHAPTER 21

4 Histological Changes During Metamorphosis

Though we have distinguished, in the preceding account, between hemimetabolousdevelopment (where partial metamorphosis occurs in the molt from larva to adult) andholometabolous development (in which metamorphosis is striking and requires two molts,larval-pupal and pupal-imaginal, for completion), the distinction is primarily useful indiscussions of insect evolution In a physiological sense the difference between partial andcomplete metamorphosis is a matter of degree rather than kind Indeed, as is described

in Section 6.1, the endocrine basis of growth, including molting and change of form, iscommon to all insects

4.1 Exopterygote Metamorphosis

In most exopterygotes the larval and adult forms of a species occupy the same habitat,eat the same kinds of food (though specific preferences may change with age), and aresubject to the same environmental conditions Accordingly, most organ systems of a juvenileexopterygote are smaller and/or less well-developed versions of those found in an adult andsimply grow progressively during larval life to accommodate changing needs Even larvalOdonata and Ephemeroptera that are aquatic and possess transient adaptive features such

as gills or caudal lamellae broadly resemble the adult stage The system that undergoes themost obvious change at the final molt is the flight mechanism In the last larval instar wingsdevelop within the wing buds as much folded sheets of integument, and, concurrently, thearticulating sclerites differentiate Direct flight muscle rudiments are present in larval instarsand are attached to the integument at points corresponding to the future locations of thesclerites Some of these (bifunctional muscles) may be important in leg movements duringlarval life (Chapter 14, Section 3.3.1) Like the direct flight muscles, the indirect flightmuscles grow progressively through larval life but remain unstriated and non-functionaluntil the adult stage

of undifferentiated cells, the imaginal discs and abdominal histoblasts The imaginal discsoccur as thickened regions of epidermis whose cells remain embryonic; that is, in the larvalinstars their differentiation is suppressed by the hormonal milieu existing in the insect at thistime At metamorphosis striking changes occur in the concentration of certain hormones,

as a result of which the cells can multiply and differentiate into specific adult organs andtissues (Figure 21.9) Experiments in which cells have been selectively destroyed by X-irradiation have shown that formation of imaginal discs occurs very early in embryogenesisand at specific sites Furthermore, each imaginal disc differentiates in a predetermined man-ner During larval development, the discs grow exponentially in relation to general bodygrowth and, typically, come to lie within an invagination, the peripodial cavity, beneath thecuticle (Figure 21.11) In contrast to the imaginal discs, the abdominal histoblasts, which

POSTEMBRYONIC DEVELOPMENT

FIGURE 21.9. Imaginal discs of Drosophila and their derivatives in the adult body [From H Wildermuth, 1970,

Determination and transdetermination in cells of the fruitfly, Sci Prog (Oxford) 58:329–358 By permission of

Blackwell Scientific Publications.]

are groups of loosely associated cells in the larval integument, do secrete larval cuticle

At metamorphosis, under hormonal influence, they divide and differentiate into the adult

abdominal epidermis, fat body, oenocytes, and some muscles

With the evolution of imaginal discs the way was open for the development of a larva

whose form is highly different from that of the adult, and capable of existing in a different

habitat from that of the adult, thus avoiding competition for food and space

To clarify the histological changes that occur in endopterygote metamorphosis, the

various organ systems will be considered separately

Epidermal cells carried over from the larval stage produce the cuticle of most adult

endopterygotes However, in Hymenoptera-Apocrita and Diptera-Muscomorpha the larval

epidermis is more or less completely histolyzed and replaced by cells derived from imaginal

discs and histoblasts In Muscomorpha, histolysis of the larval epidermis does not occur

until after pupariation

Appendage formation is also varied In lower endopterygotes formation of adult

mouth-parts, antennae, and legs begins early in the final larval stadium from larval epidermis

Cer-tain predetermined areas of the epidermis thicken, then proliferate and differentiate so that,

at pupation, the basic form of the adult appendages is evident During the pupal stadium the

final form of the adult appendages is expressed (Figure 21.10) In contrast, where the larval

appendages are very different from those of the adult, or are absent, the adult structures

develop from imaginal discs that undergo marked proliferation and differentiation in the last

larval instar and are evaginated from the peripodial cavity at the larval-pupal molt Wings

are formed in all endopterygotes from imaginal discs In most species their early

develop-ment is similar to the developdevelop-ment of paired segdevelop-mental appendages outlined above; that

is, the wing rudiments form in a peripodial cavity and become everted at the larval-pupal

molt (Figure 21.11) The forming wing bud in the peripodial cavity is initially a hollow,

CHAPTER 21

FIGURE 21.10. Sections through leg of Pieris (Lepidoptera) to show development of adult appendage (A) Leg

of last-instar larva 3 hours after ecdysis; (B) same as (A) but 1 day after ecdysis; (C) same as (A) but 3 days after ecdysis; (D) leg at beginning of prepupal stage showing presumptive areas of adult leg; and (E) leg of pupa [After

C.-W Kim, 1959, The differentiation center inducing the development from larval to adult leg in Pieris brassicae

(Lepidoptera), J Embryol Exp Morphol 7:572–582 By permission of Cambridge University Press.]

FIGURE 21.11. Sections through developing wing bud of first four larval instars of Pieris (Lepidoptera) [After

J H Comstock, 1918, The Wings of Insects, Comstock.]

POSTEMBRYONIC DEVELOPMENT

FIGURE 21.12. Transverse sections of developing wing of Drosophila (A, B) Successive stages in pharate

pupa; (C–G) stages in pupa; and (H) pharate adult [After C H Waddington, 1941, The genetic control of wing

development in Drosophila, J Genet 41:75–139 By permission of Cambridge University Press.]

fingerlike structure, but this becomes flattened so that the central cavity is more or less

obliterated, leaving only small lacunae (Figure 21.12A, B) A nerve and trachea that have

been associated with the imaginal disc now grow along each lacuna occasionally branching

in a predetermined pattern At the larval-pupal molt, hemolymph pressure forces the sides

of a wing bud apart so that there is sufficient space within it for the development of an adult

wing (Figure 21.12C, D) During the pupal stadium extensive proliferation of the epidermal

cells occurs within the wing bud, as a consequence of which the epidermis becomes folded

and closely apposed over most of the wing surface The epidermal layers remain separate

adjacent to the nerve and trachea, forming the definitive wing veins (Figure 21.12E–H)

The gut of endopterygotes typically changes its form markedly during metamorphosis

In Coleoptera the foregut and hindgut undergo relatively slight modification, this being

achieved by the activity of larval cells In higher endopterygotes these regions are partially

or entirely renewed from groups of primordial cells located at the junctions of the foregut

and midgut and midgut and hindgut, and adjacent to the mouth and anus The larval midgut

of all endopterygotes is fully replaced as a result of the activity of either regenerative cells

from the larval midgut, or undifferentiated cells at the junction of the midgut and hindgut,

or both In either arrangement, the histolyzed larval cells eventually are surrounded by adult

tissue To protect the insect from potential pathogens in the absence of a peritrophic matrix,

the differentiating pupal midgut epithelium releases a mixture of antibacterial peptides into

the gut lumen (Russell and Dunn, 1996)

Larval Malpighian tubules may be retained in some adult Diptera, but in other

en-dopterygotes they are partially or completely replaced at metamorphosis from special cells

located either along the length of each tubule or at the anterior end of the hindgut