Residents thought it was a tornado at first sight.

However, as the dense, irregular-shaped dark cloud wafted across the lake and loomed near residences, people quickly realised the bulk was actually a massive swarm of flies.

Some even described the sight as a “doomsday scene”.

This happened last week near south-west Nicaragua’s Lake Cocibolca, one of the largest freshwater lakes in Central America.

Some news sites describe the flies as “mosquitoes”. Others call them “lake flies” of the Chironomidae family, or non-biting midges.

Insect swarms are said to be more common in mid-year, when the male insects seek females to mate with, resulting in a chaotic assembly of flying insects.

The mating frenzy also makes the insects a clustered source of prey for predators such as birds.

These insect swarms are relatively common in Nicaragua’s Rivas and Rio San Juan, and may also be found in countries where large areas of freshwater are available.

But despite the ghoulish appearance, this phenomenon is just a fly in the ointment. According to agro-meteorologist Agustin Moreira, such swarms are common during the rainy season and these winged armies cause no harm to people and crops.