When Mark Bower sent the first pictures of the oleander aphids shown in the previous blog, the picture above stood out with its little brown spheres on the "tail pipes" on one aphid. Mark researched this for me and came up with the description of aphid "cornicles".
|Nymph with dry cornicles|
When muscles at the base of the tubes contract, these hollow tubes secrete droplets of fluid. Originally this was thought to be honeydew, but we now know that the honeydew beloved of ants is actually a digestive byproduct released from the digestive tract.
|Aphid cornicles in action - Wikimedia|
As described by Ira Flatow at Science Friday,
"An aphid only produces E-β-farnesene after it’s been attacked. Once the alarm pheromone is released, any aphid within detection distance will stop feeding and walk or fall off the leaf it’s on. In species with long cornicles, the aphids will flex their abdomens and smear the pheromone onto the predator in the moments before death. This action ensures that wherever the predator goes on the plant, the other aphids know before it even arrives! It’s totally sneaky. This behavior allows the other aphids in the cluster more time to escape predation."The adult aphid is far less likely to release the hormone than the nymphs. The adult has methods of escaping and may not be programed to waste as much energy producing the fluid. Nymphs on the other hand are lined up in a virtual smorgasbord for predators, and the ability to escape in mass helps perpetuate the aphid species.
A lot more detail is at this sciencefriday.com link.