There are four main reasons to study the factors which in nature regulate the equilibrium between insects and bacteria. Each of these reasons is of such a nature that it could be the subject for a review of its own. However, we will not elaborate but only state the arguments briefly as follows: 1. More than 106 insect species have been described and the number of individuals has been estimated to be as large as 1018 (Wiggelsworth 1968). These high numbers show that during evolution insects have been extremely successful in the competition with other forms of life. Thus, from these numbers alone one would predict that insects are successful in dealing with infections, since otherwise they would not be so numerous.