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Midges keep their distance
New modelling suggests that the swarming insects like to keep a set amount of space between themselves
25 October 2024
Swarming midges are attracted towards the centre of the group and repelled by one another in almost equal measure, according to a new study. Laboratory swarms of the non-biting midge Chironomus riparius were monitored and then modelled mathematically. Male midges form an elliptical swarm to attract females, but there is trade-off between the maximum group size needed to size lure in potential mates, and the physical constraints of swarm size caused by environmental factors such as wind.
The swarm is in constant tension – always on the verge of breaking up. What keeps them together is that the individual midges have evolved to both maintain distance from one another and at the same time form a group.
Flying insects do not circulate around the centre of the swarm in an orderly fashion but instead have more complicated erratic flight patterns. Maximizing the perimeter length increases the likelihood that the swarm is detected by females. The modelling predicts that the trade-off is optimized when, as observed, the swarm aspect ratio is about 1:1.2.
“The midge swarms are poised at the cusp of a stable-unstable phase transition,” said Rothamsted physicist Andy Reynolds. “In other words, the swarm is in constant tension – always on the verge of breaking up. What keeps them together is that the individual midges have evolved to both maintain distance from one another and at the same time form a group.”
This pattern of behaviour maximises breeding success by attracting mates. Even when a female enters the swarm, and is pursued by a male, the swarm keeps its shape. Individuals may be drawn towards her, but this force of attraction is weaker than the negative "impulse" for the males to stay away from each other.
Staying evenly spaced means there is less competition between males. Which means that, as a group, they spend less energy and have more overall mating success.
The mechanisms the midges are using to maintain the swarm and to attract females are currently unknown. It is unlikely that the midges are using sound to “organise” themselves. Although some swarming insects are known to be very sensitive to acoustic communication, it is restricted to close range pair interactions. Moreover, in two specifies of mosquitoes, at least, there is no evidence of long-range sex pheromones involved in swarm detection and recognition by females. If females can detect mating swarms visually from afar then the elongated elliptical shape of the swarm may provide a benefit. In this case the midge swarm is around 30% longer than a purely circular gathering containing the same number of males.
“One remarkable aspect of these studies is the similarity of swarm dynamics with self-gravitating systems like elliptical galaxies,” said Reynolds. “These are filled with old red stars that have random orbits but cohere into an elliptical shape - so there may be some broader natural organisational mathematics in play here.”
Publication
Contacts
Andy Reynolds
Physicist
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