Either, carbon, hydrogen, nitrogen, oxygen, phosphorus, or sulfur. I would go with carbon because of the photosynthesis and cellular respiration<span />
Carbon is the most versatile element on the planet. It is found in all the living things and is recycled when the living beings die.
This element in the form of carbon dioxide is utilized by the plants to synthesize glucose (in which this element is also present) that is used in the form of food for the plants. The reaction for photosynthesis follows the equation:
It is also released by animals in the form of carbon dioxide as a cellular respiration product.
Arthropods have an exoskeleton & jointed limbs. Insects and some other land arthropods breathe through a system of tiny body tubes called tracheae. Some examples of arthropods are a spider or a scorpion.