You found an arthropod in the garden that has many segments, short antennae, and two pairs of legs per segment. What is it?

The organism described in the question has several specific characteristics: it has many segments, short antennae, and two pairs of legs per segment. These traits are indicative of millipedes.

Millipedes are a class of arthropods known for their elongated, cylindrical bodies that are divided into many segments. Each body segment typically bears two pairs of legs, which distinguishes them from centipedes, who have only one pair of legs per segment. The presence of short antennae is also a common feature in millipedes. They tend to have a more rounded body than centipedes, which usually possess a more flattened and slender appearance.

In contrast, centipedes have one pair of legs per segment and are generally more agile hunters, while spiders are characterized by having eight legs and two main body segments (cephalothorax and abdomen). Beetles, on the other hand, belong to a different class of insects, which typically exhibit a hard exoskeleton, a pair of wings, and do not have the segmented body structure described.

Overall, the combination of many segments, short antennae, and two pairs of legs per segment clearly identifies the organism as a millipede.