Siphunculi are structures located on the dorsum of the sixth abdominal segment of aphids. They all have an opening or pore from which pheromones are emitted, often as liquid globules. Siphunculi come in a great diversity of forms, from long cylinders to short cones to mere pores, and are absent in some species.
Some siphunculi are extremely long and conspicuous, especially in many species of the tribe Macrosiphini. Note that siphunculi are never perfectly cylindrical, but if they are not swollen (with a distal section thicker than a basal section), or not broadly conical, they are considered cylindrical.
The aphid's left siphunculus is circled. Note also the postsiphuncular sclerite and the flange at the tip.
A siphunculus can be long or short. Note that siphunculi are never perfectly cylindrical, but if they are not swollen (with a distal section thicker than a basal section), or not broadly conical, they are considered cylindrical.
Its length is measured as the shortest straight distance between its base where it is attached to the aphid abomen, and the extreme distal end (black arrows). The white arrows show the measurement of the basal diameter and the diameter at the approximate mid-point of the siphunculus.
Some siphunculi are visibly swollen.
Length measurement is from base to tip (black arrows). The white arrows highlight measuring the diameter of the siphunculus at its sub-swelling constriction and the point of maximum swelling.
Some siphunculi can be extremely swollen.
Length measurement is from base to tip (black arrows). The white arrows highlight measuring the diameter of the siphunculus at its sub-swelling constriction and the point of maximum swelling.
Some siphunculi are broad cones.
These siphunculi are sclerotized cones (black arrows). The white arrows indicate that the basal diameter should be measured from the greatest opposite points, where the cone is at its broadest.
Some siphunculi are short cones, often with setae.
The diameter of this siphunculus is little larger than its pore.
Some siphunculi bear setae.
The siphunculi of most aphids do not bear setae. Those that do are usually conical, although there are other exceptions such as the siphunculus of Greenidea psidii.
The siphunculi of some species bear distinct reticulations towards the distal end. The proportion of the siphunculus covered in reticulations can be used for species identification.
The image of this siphunculus includes the measure of the length and the length of the reticulated regions (black arrows), and the basal and mid-siphunculus diameters (white arrows).
The image of this siphunculus includes the measure of the length and the length of the reticulated regions (black arrows). Note that this siphunculus is also swollen.
In some cases, siphunculi will have a few rows of flattened polygons just under the distal flange. They are highly variable are NOT considered reticulations.
The arrow points to the distal polygons that are not to be considered reticulations.
In some species, the siphunculi are completely absent. It is believed that this is a derived trait. That is, these species are evolved from species that did have siphunculi, but lost them over the course of time, presumably because they no longer served a purpose.