In insects, the head of the adult typically has two attached antennae, each of which contains a total of three segments. For some, the idea that there are but three segments in the insect antenna raises questions. Even a glance at the antenna of a moth, a beetle, or a honey bee reveals a structure that appears to be more than a three-part invention. What’s with that?
The Three Segments
The three segments commonly described are the scape at the base of the antenna, the pedicel in the middle, and the flagellum at the end.
The scape is the segment attached to the head in insects generally. The pedicel, the segment next to the scape, is in the middle. The flagellum, the third segment of the insect antenna, exhibits numerous variations on the theme with quite a range in the number as well as the shape of the parts involved. The parts are distinguished from segments by names that include subsegments, units, sections, flagellomeres—and sometimes “segments” (with the quotes).
In the honey bee, the scape extends outward from the face and rotates freely in its socket by way of muscles that insert inside the honey bee’s head. The pedicel articulates with the scape. And the flagellum moves as a whole by way of muscles that originate in the scape, insert in the pedicel, and insert in the first of the many parts of the flagellum itself. Although the parts do provide a measure of flexibility, they lack the muscles that would enable them to move independent of one another. The honey bee version of parts of the flagellum, which themselves are similar to the second segment, the pedicel, each fit into to preceding part with less-rigid cuticle in between.
The parts number ten in the flagellum of the honey bee worker and queen, whereas they number eleven in the drone. The additional part in the drone’s flagellum may be his version of the more-elaborate antennae found in males of many insect species: perhaps the more, the better in detecting the presence of a queen for mating.
Capacity for Movement
Those who study insects have reason for stopping the count at three. Although a few may suggest that the count stop at two and then note that there’s a related grouping of parts collectively known as the flagellum, the consensus lies in ensuring that movement rules. The defined segments have attached muscles and the capacity for independent movement.
As we stop the count at three in recognizing the interplay of form and function, we might take a moment to marvel at the beauty of both the design and the tremendous range of adaptations based on this trio of segments in insects the world over. The antennae are nothing short of sensational, and that’s another story!