Which term refers to the extensible mouthparts of certain flies?

The term that refers to the extensible mouthparts of certain flies is "proboscis." The proboscis is a specialized feeding structure that allows flies, particularly those that feed on liquids like nectar or blood, to reach their food sources. It is typically elongated and can be extended or retracted, which is crucial for accessing fluids from hard-to-reach places.

In contrast, mandibles are primarily used for grasping or chewing food, and while they are important mouthparts, they do not have the same extensibility as a proboscis. Spinnerets are silk-spinning organs found in spiders, not insects, and maxillae are part of the mouthparts in insects that assist in manipulating food but are not extensible like the proboscis. Thus, the term "proboscis" specifically denotes the unique adaptation found in certain flies that enables them to feed effectively on liquid substances.