They are called pantaloon bees because, as its name suggests, one of the most distinctive features of this bee is the dense covering of hairs on its legs.
In females, the hairs, also known as scopae (singularly scopa) protect her corbiculae (singularly corbicula), or pollen baskets. These biological characteristics make their hind legs look particularly long and thick. This gives them the appearance of wearing pants or pantaloons. In contrast, the legs of male pantaloon bees appear to be much thinner, as they lack both corbicula and scopae.