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03.05 Modes of Reproduction

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Bugs 101: Insect-Human Interactions

阿尔伯塔大学

4.8（20 個評分） | 5.5K 名學生已註冊

Of all the animals on earth, which are the strongest for their size? What about the fastest? Who were the first animals to evolve flight? Insects take all of these titles and more! As the most abundant animals on the planet, insects and other arthropods affect our lives in so many ways. From beneficial interactions like pollination and biological pest control, to the transmission of life threatening diseases; this course will teach you about the big ways that these little arthropods impact our lives. In Bugs 101: Insect-Human Interactions, you will be plunged into the diverse (and sometimes alien) world of arthropods to learn how they work, what they do, and how insects and humans interact every day. After completing this course, you will be able to: Describe the evolutionary relationships between insects and their arthropod relatives Inventory major groups of insects and their diversity Demonstrate evolutionary adaptations that make insects successful Discuss insect biology and human-insect interactions Evaluate positive and negative interactions between insects and humans Propose practical and symbolic roles insects play in human societies

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Biology, Entomology, Science, Ecology

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4.8（20 個評分）

5 stars
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VI

Aug 13, 2019

A study which is highly enjoyable and useful for professionals

CS

Aug 15, 2019

Overall a great course. Would, and have, recommend to others.

從本節課中

Module 03: The Business of Being an Insect Part 2

Building on the previous module, we continue to explore insect biology through an overview of the major physiological systems of insects. We look at the nervous and reproductive systems, and highlight some of the ways insects court, seduce, and reproduce.

03.05 Modes of Reproduction14:48

教學方

Dr. Maya Evenden
Professor of Entomology

腳本

Sperm transfer occurs in many ways in insects.

Some insects pass on free swimming sperm and

ejaculatory fluids while others package

sperm together with

protein-rich accessory gland secretions.

Remember, these sperm packets are

referred to as spermatophores.

An extreme example of sperm transfer occurs in

bed bugs in which males inseminate their mates,

not by entering the reproductive tract

through the genital opening,

but by pushing their sharp aedeagus gets through

the females exoskeleton to create a new opening.

The male then deposits his sperm

directly into the females hemolymph.

Unfortunately, this creates an open wound

leaving the female vulnerable to infection.

This form of sperm transfer is appropriately

referred to as traumatic insemination.

Beyond bedbugs and their violent mating encounters,

you may be surprised to learn that this type of

antagonistic interaction between sexes

is not uncommon in the animal world.

Sexual conflict whereby two sexes have conflicting

reproductive strategies is actually

quite common in insects.

Water striders just like bed bugs are

well-known examples of insects that

experience conflict between the sexes.

As with many insects,

mating in water strider involves

the male mounting the female from above.

Because at this position,

the female has no choice but to support the weight of

the male on the water surface for

as long as copulation occurs.

This is energetically costly to

the female and males tend to stay

attached to the females for as long as

possible to ensure offspring paternity.

Depending on the species,

matings in water striders can range from

four minutes to over seven hours.

Although the record for

the longest water strider mating encounter

belongs to a species known as

Aquarius najas or the river skater.

This extremely patient insect has been

reported to remain coupled for up to three months.

Because of this high energetic cost,

females typically accept fewer matings than males

initiate and tend to prefer to mate for shorter periods.

Females may even physically shake

off a male that attempts to initiate mating.

As a result, males of many species of

water strider have evolved

clasping mechanisms to help them hold on.

Evolutionary conflict doesn't stop there.

Females of the red backed water strider have

evolved what is essentially a genital shield,

almost like a chastity belt,

which blocks males access

to the female's reproductive system.

In addition to the clasping structures

seen on most water striders,

the males of this species use

a behavioral mechanism called intimidating courtship.

Once in copulatory position,

the male tops the surface of the water

creating ripples that could attract predators.

Because the male arrests on top of the female she is

more likely to be eaten by

an underwater predator than the males.

Given the choice between death by

a predator or an energetically costly mating encounter,

the females tend to quickly accept the mating

so the male stops signaling to potential predators.

Just as there is lots of

variation in insect mate finding,

courtship and mating behavior.

There are also many different modes

of insect reproduction.

Most insects reproduce sexually and eggs produced

by females are fertilized by sperm from males.

Most insects also have

direct fertilization that involves coupling of the sexes.

The vast majority of insects exhibit

oviparity which means that the females deposit

fertilized eggs externally onto something like a leaf

and embryos in these eggs complete

their development outside of the female's body.

This process of the egg passing from the genital opening

to the outside environment is called oviposition.

Oviposition is often the result of

a complex suite of

behaviors that ends with the female positioning

her offspring in a habitat where they are protected

and have access to resources

required for development and survival.

As such, many female insects have evolved

sophisticated sensory receptors to

detect optimal oviposition sites.

We will discuss more about oviposition site selection in

a future module when we learn about

host location in herbivorous insects.

Oviparity requires the female

to produce eggs that contain

a nutrient-rich yoke which provides

the developing embryo with essential proteins,

carbohydrates, lipids, and salts.

This mode of reproduction allows females

to produce large numbers of offspring.

The lack of parental care associated with

oviparity reduces the likelihood of offspring survival.

The outer layer of insect eggs that

functions to prevent water loss called

the chorion is sometimes

thickened to help protect against predation.

That being said,

a few insect species do care for their eggs.

In most cases, females carry out this costly practice,

although occasionally the role is reversed.

An example of this occurs in

the male giant water bug which

exhibits an unusual form of egg care.

After mating, female giant water bugs lay

fertilized eggs on the dorsal surface

of the males four wings.

The male protects the eggs until they

hatch and ensures that the eggs

obtain enough oxygen through

some impressive underwater acrobatics.

By sweeping and circulating water over the eggs on

his four wings and remaining

close to the air-water interface,

he can maximize the exit exposure to oxygen.

The females of some insect species

protect their offspring by

retaining the eggs inside

their body throughout embryonic development.

This reproductive strategy is

referred to as ovoviviparity.

The developing embryo remains within

the female's body where it receives

nourishment from the egg yolk until it hatches.

In some instances, hatching may even occur within

the female before the free-living juvenile

is deposited into the environment.

This mode of reproduction not only

provides extra protection for the young,

but is also an important strategy

for insects that rely on

resources that are only

available for short periods of time.

By reducing the external

development time of the offspring,

the juveniles can be highly competitive as

they are ready to eat soon after leaving their mother.

This allows them to take advantage of

temporary food sources recently located by their Mothers.

Dung and carrion feeding insects are often

ovoviviparous because of the temporary nature

of their food sources.

Although the vast majority of insects lay eggs,

some exhibit viviparity or live birth.

In viviparity, the embryos developed within the female.

It is different from ovoviviparity parody as

the female provides

the developing offspring with nutrients,

not from yolk but from accessory gland secretions,

hemolymph supplements, or even placental type tissues.

This type of development provides

a lot of protection for the offspring.

However, it restricts the number of young the female

can produce at once and over her lifetime.

In extreme cases, such as with tsetse flies,

the female will carry only a single offspring at a time.

The larva will complete most of its life cycle within

the female and is deposited just prior to pupation.

While sexual reproduction is the norm in insects,

most orders contain at least some species that exhibit

asexual reproduction by parthenogenesis.

Parthenogenesis occurs when unfertilized eggs

develop into embryos without male involvement.

Many members of the phasmatodea,

the stick insects, can reproduce using parthenogenesis.

Sometimes, a complex insect life cycle can involve stages

of both parthenogenesis and

oviparous sexual reproduction.

A good example of

this can be seen in some species of aphids,

in which females reproduce

asexually under optimal environmental conditions.

They produce daughters that are genetic clones of

themselves with two sets of

chromosomes both from the mother.

This allows the population size

to grow extremely quickly.

To make things more complicated,

newborn aphid daughters can be

ready to reproduce at birth,

making the mother aphid,

an expected grandmother as well.

These females usually reproduce over

several generations by viviparous parthenogenesis.

Asexual reproduction in these aphids will continue across

multiple generations until conditions

deteriorate as the colder seasons approach.

Females then produce male offspring

because genetic variation from

sexual reproduction becomes more

important than simply increasing population numbers.

This type of reproduction is referred to as holocyclic,

meaning that the periods of

asexual reproduction are

interspersed with sexual reproduction.

This means that male aphids maybe

almost completely absent most of the time,

and are produced only during certain times of the year.

Parthenogenesis also occurs in

hymenoptera insects such as ants, bees, and wasps.

They produce males through

parthenogenetic development from unfertilized eggs.

These males are haploid with

only a single set of chromosomes.

Meanwhile, fertilized eggs produced

diploid females with two sets of chromosomes.

This type of sex determination system

is known as haplodiploidy,

and will be discussed in a later module.

Other types of reproduction can be

found in a small number of insects.

For example, some species of holometabolous insects can

abbreviate their life-cycles by

skipping the pupal and adult stages altogether.

These insects reproduce by paedogenesis,

whereby offspring are produced

by reproductively mature juveniles.

Like parthenogenesis in aphids,

paedogenesis can be part of a complex life cycle.

Adult females may only be

produced during certain conditions,

while most of the time reproduction

occurs in female larvae.

Some species of gall midge is reproduced by paedogenesis.

The reproductive system within the larvae

matures faster than the rest of the individual.

So eggs are ready to be laid during the larval stage.

Paedogenesis can involve

either sexual or asexual reproduction.

Another odd mode of reproduction

in insects is polyembryony,

in which a single egg splits into multiple embryos.

This mode of reproduction is rare

and restricted predominantly to

endoparasitic insects like parasitic wasps.

It allows the development of multiple offspring within

a parasitized host with

relatively little energetic expense

for the older positing wasp.

Furthermore, the parasitic female

will only have to oviposit once,

thus reducing her exposure to

pollination and other threats.

In polyembroyny, the number of

embryos produced from a single egg varies

between species but can range from as few

as 10 to over 1,000.

These embryos do not receive nutrients from egg yolk,

but from the host they are parisitizing.

The larvae complete their development within

the host and as they emerge,

they spin pupal cases from which

the free-living adults later enclose.

We will learn more about these insects in

a future module when we delve into biological control.

The last quirky form of reproduction we

will discuss is hermaphroditism,

which occurs when both male

and female reproductive systems

are present within a single individual.

Although hermaphrodites can fertilize themselves,

they still tend to reproduce sexually.

An example of a hermaphroditic insect is

a historically important pest of citrus trees,

the cottony cushion scale,

which can reproduce through self-fertilization.

Now that you are more familiar

with insect reproductive systems,

in the next video,

we will examine how humans exploit this knowledge of

insect reproduction to control pest populations.

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