Aphid antennas are composed of three muscularized articulated true segments. The scape is the first, the pedicel second, and the remainder of the antenna is the flagellum. Most features used for aphid identification are those of the four (rarely three) flagellomeres. In common parlance, each of the antennomeres is referred to as a segment.
The flagellum is composed of the four (or three) terminal segments of the antenna.
It measured as a straight line from the base of third antennal segment to the tip of the sixth (or fifth, when there are only five). In cases where the antenna is strongly curved on the slide, it may be best to measure the length of the flagellum as the sum of its individual segments.
The terminal antennal segment, usually the sixth but sometimes the fifth, is divided into the base and the processus terminalis (PT). The PT is sometimes called the unguis, which is not to be confused with tarsal claw of other insects.
The base of the terminal antennal segment is measured from its base to the distal margin of the primary sensorium. The PT is measured from the distal margin of the primary sensorium to the tip of the antenna.
The PT can be longer or shorter than the base of the terminal antennal segment.
The terminal antennal segment always bears a primary sensorium, sometimes with a cluster of accessory sensoria assoicated with it. The fifth antennal segment also usually bears a primary sensorium, and sometimes the fourth antennal segment does as well. Primary sensoria on the fourth and fifth segments are located towards the distal end of the segment and usually at a slight angle to the main axis of the segment (like the primary sensorium of the terminal antennal segment). When present, seondary sensoria, also known as rhinaria, may be found on the third, fourth, or fifth antennal segments. They are usually smaller than the primary sensoria and generally in the same plane as the main axis of the segment.
When counting sensoria, AphID does not consider primary sensoria. Count only rhinaria (=secondary sensoria).
These are standard circular secondary sensoria arrayed as a line along the full length of the third antennal segment.
These are standard circular secondary sensoria clustered at the base of the third antennal segment.
These are annular secondary sensoria arrayed along the full length of the third, fourth, and fifth antennal segments. Each one almost completely encircles the antenna.