All insects need shelter, food, and a place to breed. A very good place to get all three is inside a plant.

Over time, insects have become very specific as to what plant structures they attack, whether it be the leaf, flower, or root. To react to these specific attacks, plants have developed ways to begin forming protective structures at designated times, before it is too late and the effects of the attack are felt throughout the plant.

The attacking insects often excrete chemicals that send the plant into revolt. In some plants, development of galls is a response to the chemicals that insects secrete when they chew a plant. Other times, insect eggs being deposited, or a female ovipositor piercing the tissues, can initiate gall formation.

Most insects have what is called alteration of generation- a cycle of different forms of an organism that occurs during a single specie's life cycle. For example, the insect designated P1( parent), may deposit it's offspring in a specific plant structure. The next generation to emerge, F1( first generation), exits the structure. This generation will often a) physically appears different b) infect a different plant structure and c) cause a different type of gall structure than P1. The proceeding generation, F2 (second generation) emerges. F2 will often a) physically appear the same b) infect the same plant structure and c) cause a similar gall structure as P1.

For an example of alternation of generation see oak apple diagram.

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