Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Input: Passage: Arthropods are the largest phylum in the animal kingdom. Most arthropods are insects. The phylum also includes spiders, centipedes, and crustaceans. The arthropod body consists of three segments with a hard exoskeleton and jointed appendages. Terrestrial arthropods have adaptations for life on land, such as trachea or book lungs for breathing air. The earliest arthropods were trilobites. The earliest land arthropods were millipedes. Answer: insects
Output:
The largest phylum in the animal kingdom, arthropod, is primarily comprised of what?