Notice the small holes in this picture. They were made by "powder post beetles". Their name is derived from what is left of the wood after they digest it. Notice the "powder" streaming from the three lower holes. This is a pretty good indication that the beetles are actively eating and pushing the powder from the galleries that they are making in the wood.
Powder post beetles ate into this sillplate and joist. Notice how they have weakened the sillplate and the joist is crushing it down. Powder post beetles can do considerable damage when left un-treated.
Pictured are Powder Post Beetle larvae. All beetles go through "complete metamorphosis" ie. egg, larvae, pupae,and adult life stages. The larvae do the majority of the damage to the wood. There are several different families of powder post beetles and each family has many species. The time it takes them to go through the various life stages can range from 1 year to several years. Even the same specie varies in the amount of time from egg to adult, depending on temperature and the amount of moisture in the wood in which they are feeding.
In this picture are several powder post beetle larvae. Note the different sizes as they go through various stages of their development.