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Insects physiology

Lecture 1

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Introduction

• The components that constitute the exoskeleton make an overwhelming

• contribution to the terrestrial success that arthropods can claim.

• Like the skin of vertebrates, the exoskeleton completely covers the insect and additionally provides an armor-like protective suit as a skeleton.

• Its most critical function is to serve as an interface between the insect and the environment, providing a barrier for the movement of water, ions, parasites, and environmental chemicals including insecticides.

Introduction

• This barrier is especially significant for small animals like insects that have a high surface-to-volume ratio and therefore present a relatively large amount of surface area to the environment.

• The nature of the exoskeleton has thus had profound implications for growth, respiration, locomotion, and, from a human perspective, the design of chemicals that must penetrate the integument to be used as control agents.

• The exoskeleton also plays an important structural role in determining the form of the insect body and making possible the dramatic changes in form that accompany metamorphosis.

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Introduction

• Insects exploit a variety of

diverse habitats and diets,made

possible by a developmental

plasticity in body form and

mouthpart structure.

Introduction• The rigidity that it provides

allows for the insertion of

muscles that can produce more

precise locomotor movements

than can the soft hydrostatic

exoskeletons of the annelid

worms.

• Although being surrounded by a

rigid suit of

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Strategies For Growth • armor might limit the movement

and environmental awareness of

insects, the integument that

makes up the exoskeleton is

elastic in some areas to make

flying and walking possible.

• Numerous sensory receptors that

are concentrated in strategic

areas provide windows to the

outside world that allow the

insect to respond appropriately to

the environment.

• The terms “integument” and

“exoskeleton” are often used

interchangeably, depending on

which functional component is

under consideration.

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Strategies For Growth

• The integument may comprise up to half the dry weight of some insects,representing a major investment of raw materials.

• However, because much of this is resorbed during molting and even periods of starvation, the integument could also be viewed as a food reserve.

• The hydrocarbons that are deposited in the exoskeleton are responsible for releasing particular behavioral sequences that are involved in mating.

Strategies For Growth

• Specific structures required for mate recognition, as well as chemicals such as pheromones and pigments that are deposited in the exoskeleton, are releasers for the stereotyped behaviors that are necessary for mating to occur.

• All of these features are provided by a single layer of epidermal cells and their secretions.

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Strategies For Growth • The exoskeleton provides a

number of advantages, it does pose a major problem for growth.

For insects with rigid exoskeletons to undergo significant amounts of growth, a new, larger exoskeleton must first be synthesized and the older one discarded.

During this period of molting, the insect is relatively helpless against predators because flight or defense is difficult.

Molting consumes time, energy, and metabolic resources.

Strategies For Growth

There is also a potential susceptibility for the loss of water because the insect can neither drink nor adjust its body to a changing environment.

To reduce this susceptible period during the molting cycle, more advanced insects have evolved toward a reduced number of molts.

Growth during the intermolt period is possible because the larvae of advanced holometabolous insects generally have relatively unsclerotized cuticles that can undergo a degree of stretching.

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Strategies For Growth • The growth and development of

insects is largely a function of the growth and development of their integuments.

• The cuticular molts that punctuate postembryonic growth are necessary if hard-bodied insects are to undergo any significant increases in size.

• Increases in body size do not always follow from molting

• Insects that are starved during the larval stage or molt to a diapause form may actually molt to smaller individuals if they molt at all.

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Strategies For Growth

• Apterygote insects continue to molt into the adult stage, but pterygoteinsects are incapable of molting as adults.

• The inability of adult pterygoteinsects to molt is probably

• the result of the degeneration of the epidermal cells that produce the wing once it is formed.

• After the molt to the adult stage, the epidermal cells that make up the wing degenerate and the loss of the water contained within them makes it possible for the wing membranes to move rapidly for fl ight

• .

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The three types of metamorphosis in

insects

Strategies For Growth • possible for the wing

membranes to move rapidly for fligh However, these dead cells can no longer initiate a molt nor synthesize a new cuticle.

• Without living epidermal cells, another wing cuticle could not be formed if the insect molted again.

• Thus, the death of the cells that make flight possible also makes molting as an adult impossible.

• Only the winged mayfly subimago is capable of a molt to another winged form, but the subimago is a poor flier because living epidermal cells must remain as part of the wing

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Strategies For Growth • In many holometabolous insects,

considerable growth can occur during a single larval instar in the absence of a molt because, with the exception of the

• head capsule, the cuticle is extensible enough to accommodate some increases in size.

• The last instar of Manduca sexta can grow from 1 g to over 9 g in weight without a molt because the pleated outer epicuticle of the exoskeleton is able to stretch to accommodate the growth that occurs within this instar.

• A molt may ultimately be necessary in order to acquire a larger head capsule and allow the sclerotizedmouthparts to increase in size so the rate of food intake isincreased to satisfy the demands of the larger body.

Strategies For Growth

• The molting cycle and

metamorphosis present

some interesting

developmental conditions.

• The particular

developmental stage of

an insect is referred to

as an “instar” or

“stadium,”

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INSTARS, STADIA, AND HIDDEN

PHASES

• One of the first steps in molting is

apolysis, in which the old cuticle

separates from the epidermis and

new cuticle begins to be produced.

• With the old cuticle no longer

directly attached to the epidermis, it

has effectively been discarded,

although it has not been shed, and

the newly formed cuticle now

represents the cuticle of the next

instar.

INSTARS, STADIA, AND HIDDEN PHASES

• For this reason, apolysis is

said to mark the passage to

the next instar, even though

ecdysis has not yet occurred

and the insect appears to still be in the skin of the

earlier instar.

• An instar is therefore

defined as the period

between two apolyses and

begins when the insect first

becomes detached from its

old skin.

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INSTARS, STADIA, AND HIDDEN PHASES

• The instar that is hidden under the old, unshed cuticle before ecdysis is referred to as the pharate instar.

• This distinction is more important for some instars than others.

• For example, some lepidopteransundergo diapause as pharate adults that are developmentally complete adults enclosed by the detached pupal cuticle.

• Although based on an external examination, it is easy to conclude that if the insect diapauses as a pupa, then the diagnosis would not be correct.

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INSTARS, STADIA, AND HIDDEN PHASES

• The stadium that an insect is in is defined by its ecdyses;

• a stadium represents the interval between one ecdysis and the next.

• Therefore, at apolysis, an insect passes to another instar but does not become the next stadium until after ecdysis .

INSTARS, STADIA, AND

HIDDEN PHASES• Apolysis, the separation of the

epidermal cells from the cuticle, marks the beginning of the molt and the next instar.

• The insect is an a pharatestage until ecdysis occurs, the casting off the old cuticle.

• Ecdysis marks the beginning of the next stadium.

• At the apolysis following the second instar, insect enters the third instar but is still in the second until after ecdysis