Insect mouth parts fall into different categories depending on the insect type. Each has a certain tool for dealing with certain prey. Houseflies utilize spongy pads that secrete saliva over their food, dissolving the food source which can later be sucked up through their straw-like mouth. Other insects like mosquitoes have a needle-like mouthpart useful for breaking through skin and sucking blood (only female mosquitoes suck blood, male mosquitoes suck plant juices). Still other insects, like ants, have powerful jaws that grip and cut food sources. Some can slice through human skin (Fire Ants).
Structure of Insect Mouth Parts (In general, as each insect differs):
Mandibles - These are hard jaws meant for gripping and biting, most often found on insects like ants.
Maxillae - Secondary jaws, usually past the primary jaws for further destruction of the prey.
Labrum - The upper lip of the mouth, commonly found in insects such as caterpillars and butterflies.
Labium - The lower lip of the mouth, again, commonly found in caterpillars and butterflies.
Ants utilize mandibles, maxillae, labium and labrum. The mandibles are closed by powerful jaw muscles that break the food down into smaller pieces. Behind the mandibles, the maxillae is there to 'taste' the food (showing the propensity of ants to enjoy items like sugar, and judge whether the food source is something they find appealing or not). Finally, the labium and the labrum chew the food and the food then goes into the mouth of the insect for final eating and digestion.
Butterflies, on the other hand, utilize what is called a proboscis. This whip like organ serves as it's tongue and can suck nectar out of flowers. It is also a very sensitive organ and communicates a lot of information back to the butterfly.