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REPRODUCTION

How do the sexes find each other?

Odour

Female moths have scent glands on the abdomen that secrete pheromones.

These chemicals attract males of the same species. They are generally simple molecules, (such as alcohols with 2 double bonds). Male moths can detect as little as 10 -16 g of these pheromones with their enlarged plumose antennae.

46% males can detect a female 4km away and 26% males can detect a female 11 km away.

Some male moths also secrete a pheromone to induce the female to mate.

Vision

Butterflies use colour and movement. Males will pursue coloured paper imitations of females. They will also try to chase away coloured imitations of other males.

Some male flies form compact swarms. Females are attracted by the sight of these swarms, enter and select a mate.

Male fireflies (beetles) are attracted by, and fly to, the light pulses emitted bythe usually wingless females. The pulse pattern is species specific.

Sound

Male mosquitoes are attracted to the note produced by the wing vibration of the female.

Female crickets, some grasshoppers and cicadas are attracted by the sound produced by their males. Crickets use their burrows to resonate the sound; cicadas use the enlarged empty space in their abdomens. Some leafhoppers communicate by substrate vibration.

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INTERNAL GENITALIA

The female reproductive system includes a pair of ovaries, lateral oviducts, a common oviduct and a vagina. Generally each ovary is composed of several ovarioles to produce multiple eggs (ova). Most female insects also have one or more spermathecae where sperm can be stored for some time and can be nourished by secretions from the spermathecal glands.

Accessory glands add various coatings to eggs before they are laid. These usually aid in the adhesion of the eggs to a substrate.

The male reproductive system consists of paired testes (where sperm are produced), vas deferens (tubes from the testes), seminal vesicles, (where sperm are stored), accessory glands, (which provide seminal fluid and the spermatophore) and a common ejaculatory duct .

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MATING

MATING STRATEGIES

FERTILIZATION

Aquatic insect ancestors could discharge sperm into water but terrestrial insects must transfer sperm to the female without allowing it to dry out. This is often done via spermatophores, packets of sperm. In some insects the spermatophores also have edible portions for the female.

Most insect eggs have a coating to protect the embryo from desiccation. However, they also possess micropyles, small holes to allow the entry of sperm. (There are also other holes - aeropyles - connecting to air bubbles within the egg shell membranes).

Fertilization occurs within the oviduct. As the fertilized egg moves down the oviduct, it is coated with secretions from various accessory organs.

SPERM COMPETITION

Males employ several methods to protect their sperm from other males.

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EGG / YOUNG DEVELOPMENT

TYPES OF REPRODUCTION / EGG LAYING

1. Oviparity = egg laying. Many female insects have ovipositors - strong projections of their external genitalia to allow insertion of eggs into a substrate.

2. Viviparity = birth of live young. The eggs are retained and hatch within the female. Some blow fly larvae are born nearly ready to pupate.

3. Paedogenesis = reproduction in immature forms. E.g. In some small flies egg cells develop parthenogenetically into larvae which then emerge through the mother's skin. In some aphids and cockroaches, developing embryos within the oviducts of the female have themselves, developing embryos within them.

4. One embryo can give rise to many individuals by separation of the cells early in development. This is common in the development of many parasitic wasps.

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DEVELOPMENT

CONTROL OF REPRODUCTION

Reproduction in insects is by production of hormones in the corpora allata in the brain. Gonotropin is produced which in most insects is equivalent to juvenile hormone.

SEX DETERMINATION

In most insects, females have 2X chromosomes and males, XO.

However, in hermaphroditic insects, there are both male and female tissues in the same individual.

Some insects can reproduce parthenogenetically. Eggs are unfertilised and produce only female offspring.

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