WE all know what a danger the housefly is to our health, but few people realize that man himself has invited this menace by his dislocation of the balance of Nature, and that it is only under the conditions created by man that the housefly (as its name implies) can thrive. In his fifth talk Professor Tatter,sall will describe how flies, ants, beetles, cockroaches, weeds. worms, and slugs desert their natural habitat to come and batten on man.