What do we call the extendable mouthparts found in certain flies, including mosquitoes?

The extendable mouthparts found in certain flies, including mosquitoes, are referred to as a proboscis. This specialized structure is an elongated feeding tube that allows these insects to pierce the skin of their hosts and draw blood or nectar. The proboscis is highly adaptable and plays a crucial role in the feeding habits of various species, particularly in the context of blood-feeding behavior in mosquitoes.

Mandibles are the jaw-like structures used primarily for biting and chewing, which are not the primary function of the mouthparts in flies. Antennae are sensory appendages that help insects detect environmental cues but are not involved in feeding. Maxillae are also mouthparts, functioning alongside mandibles for manipulating food, but they do not possess the extensible capability that defines a proboscis.

Thus, the term "proboscis" specifically denotes the functional adaptation that enables mosquitoes and similar flies to feed efficiently, making it the correct answer to the question.