Final answer:
The excretory system in spiders and insects utilizes Malpighian tubules to filter waste from the hemolymph and eliminate it with minimal water loss, an efficient system crucial for maintaining osmotic balance in arthropods.
Step-by-step explanation:
The excretory system of spiders and insects is a structure known as Malpighian tubules. Therefore, the correct answer is (a) Malpighian tubules. Malpighian tubules are specialized excretory structures found in arthropods. For example, in cockroaches, these tubules are involved in filtering waste from the hemolymph, which is a mixture of blood and interstitial fluid. Uric acid is the main waste product excreted by these tubules, and it is typically expelled as a thick paste or powder to conserve water. This is a highly efficient system that helps insects to maintain osmotic balance and eliminates nitrogenous wastes without a great loss of water.