[daniescholtz]

1. What are the purpose of ants? is it mainly pollination?
2. What are the purpose of flies?
3. What are the purpose of mosquitoes?

[Don_1]

There isn't exactly a 'purpose' to any life form. But I think I know what you are getting at.

Ants play differing roles throughout the world. There are many different species of ant, over 20,000, in fact. Some certainly do play a role in pollination, while others help to keep other insect populations in check.

As for flies, again there are many different species, some, like the ants, play a role in pollination, while others are predators. The maggots (larvae) of some flies feed on fruits and nuts, while others play a significant role in the breakdown of dead animal tissue. Some maggots are even used in medical treatments to clean infected wounds of dead and decaying tissue.

Now for those pesky mosquitoes. What a pain they are. But it is only the female which will give you those irritating bites. They need blood for egg production. Mosquitoes, and some flies, can be a serious health risk. Some mosquitoes can carry malaria, yellow fever and dengue fever, some flies can carry sleeping sickness. But even the dreaded mosquito has its advantages; their larvae help to keep algae and bacteria in check in water.

Then you must consider that all three, in both their larvae and adult stage, provide food for birds, reptiles, amphibians, fish and other animals. Even man is known to eat some, such as Honeypot ants.

[daniescholtz]

What a great explanation, thank you.

[Sprool]

I like Richard Dawkins views that all these creatures are just shells for the DNA that lives within them. It is the genes that continue to evolve and the best ones at surviving will be available to reproduce further generations. Ants and mosquitos are all highly successful packages for the genetic data within them, they each seek a different niche in the ecosystem. Their purpose is to survive.

[Jessica H]

Ants also allow air to circulate in the soil by digging tunnels.   Plant roots need oxygen, just like us.   So ants help your garden grow.   I'm not sure how big the effect really is, but I try this argument to keep my husband from killing all the ants in our yard.   Fireants, though, he can kill.  No purpose for them!

[CliffordK]

I'm not sure about mosquitoes...
Perhaps bird-food?
The larvae would also be fish food.

As far as other insects.  Flies and etc.  They also help get rid of the stuff we don't want.
They will help chew up manure.
Help strip and get rid of roadkill.

Ants would be the same. 
Imagine the amount of organics that die each year, decompose, and regrow.  Ants will also carry it home...  dead worms & etc.

[Sprool]

and of course pollination.