A locus is not an insect! It is a group of points that is made by a point moving according to a given rule.
As an example, a dog is chained to a post. The chain is 1.5 m long, so the dog can only move 1.5 m from the post at maximum. Sketch a diagram of the locus of the dog, if he aims to get as far away as possible from the post and consequently rotates round it.
The movement is circular, with a radius of 1.5 m. Therefore, the locus looks like this diagram.
The following pages will give you more information on loci (the plural of locus).