Final answer:
Controls are not needed in the Arthropods experiment described in the study guide because they allow comparison between groups that are different in only one independent variable.
Step-by-step explanation:
In the Arthropods experiment described in the study guide, controls are not needed because experimental controls allow comparison between groups that are different in only one independent variable. This means that the experiment is designed to specifically test the effect of the independent variable on the dependent variable. By having a control group and an experimental group that differ only in the presence or absence of the independent variable, researchers can attribute any differences in the dependent variable to the independent variable.