"Students are literally choking on them," passerby Tim Barrett said.
"I'm not sure where they're coming from, but I experienced clouds of them moving south between the botanic gardens and St David St."
Dr Jenny Jandt, of the university's zoology department, said the insects were aphids, although from the photograph it was difficult to tell which species.
"Aphids can reproduce parthenogenetically throughout the year," Dr Jandt said.
"That means that females can clone themselves and give birth to new females - no need to leave the plant to find a mate.
"In autumn, some species of aphid will produce winged females and males, and these individuals reproduce the old fashioned way."
Dr Jandt believed Mr Barrett and others in Castle St had found themselves in a sort of aphid mating swarm, which appeared to be unusual behaviour for the insects.
