Answer:
Brain size increase due to find out improved quality diets whereas cecum size decrease due to reduction of its need.
Step-by-step explanation:
Brain size increased due to the need to increase the mental capacity of an organism in order to explore the environment for the survival such as search for improved quality diet and the cecum size decreased because omnivorous diet improved due to the reduction of its need as a component of the digestive system. Insects have different respiration system as compared to vertebrates because vertebrates uses lungs for respiration whereas the insects uses tracheal tubes for the absorption of oxygen and removal of carbondioxide gas.