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Toxorhynchites: the mosquitoes that hunt other mosquitoes

Mosquitoes enjoy a cosmopolitan distribution and equally cosmopolitan infamy. They are a core element of both terrestrial and aquatic food chains, serving as prey for a variety of animals, but they are also dangerous vectors of disease-causing agents. According to some sources, like the Gates Foundation, mosquitoes represent the most significant threat to humans outside of humans themselves. In the year 2015 alone, they are believed to have been responsible for more than 200 million new cases of malaria and an estimated 429,000 malaria-related deaths (Source: WHO).

But not all mosquitoes are destined to be the sanguivores we’ve come to resent when we step outside on a warm evening. Toxorhynchites mosquitoes, for instance, aren’t blood-feeders as adults and are actually powerful and highly-effective predators of other mosquitoes during their larval stage.

They are also gigantic. Adults of the species Toxorhynchites speciosus are believed to hold the title of “world’s largest mosquito”, reaching a length of more than 3.4 cm (approx 1.3 inches). That massive size is already enough to make them conspicuous but just to make things interesting they also possess spectacular, flashy coloration. “Beautiful” and “mosquito” probably isn’t a common word association but that connection is definitely applicable here. Adult Toxorhynchites mosquitoes often sport gorgeous patterns of iridescent blues, golds, greens, and whites along their legs and bodies. They almost shimmer in sunlight.

Toxorhynchites brevipalpis. (Image: Piotr Naskrecki)

Their charismatic appearance is complimented by a very agreeable lifestyle (from the standpoint of humans). Adults are believed to be universally obligate exudativores and nectarivores (they feed on carbohydrate rich nectars and plant sap). It’s seems like a peaceful and pleasant life. No need to worry about sneaking up on unsuspecting humans and trying to steal a blood-meal under the constant threat of being crushed under a swatting hand. No need to constantly look over their shoulder for lurking blood-loving jumping spiders like Evarcha culcivora. As long as they can avoid predators, the rest of their time can be spent just bouncing between flowers, eating really sugary food, and reproducing.

Toxorhynchites speciosus in New South Wales, Australia. (Image: John Tann/Wikimedia Commons)
Close-up view of the head of Toxorhynchites speciosus. (Image: Graham Wise/Wikimedia Commons)

Their larvae, on the other hand, are the voracious stuff of nightmares for other mosquito larvae and small aquatic organisms. Hatched from eggs scattered in ponds, rock pools, hollow logs, tires, or just about anywhere there is a decent amount of standing water, the larvae emerge with well-developed mouthparts, a fierce appetite, and a violent disposition.

Whereas other mosquito larvae are busy munching on detritus or filter feeding microorganisms from the water column, young Toxorhynchites are big, hungry hunters during all instar stages (the periods between larval molts). They will go for just about anything that moves and is within reach. Often, that means pursuing other mosquito larvae (like those of the disease-causing Aedes aegypti) which are unfortunate enough to find themselves in the same pool as Toxorhynchites.

To say they have a healthy appetite for other mosquitoes would be an understatement. A single Toxorhynchites  will consume around 5,300 other mosquitoes before it pupates. Get a group of these larvae together in the same pool and (assuming they don’t quickly cannibalize each other, which is a possibility) they can substantially reduce the number of other mosquito species in the same body of water. One paper published in 1979 in the Journal of Animal Ecology noted that the presence of Toxorhynchites in the study’s sample areas reduced the available prey population by a median of 71%!

These larvae also have a habit of just outright killing other mosquito larvae without eating them (including members of their own species) once they reach the final part of their 4th instar. Researchers theorize that this practice of killing other mosquitoes and not consuming them may have evolved as a means of eliminating any potential predators before the Toxorhynchites larvae transform into their vulnerable pupal stage.

The ability to substantially reduce the number of other mosquito species making it out of the water and into adulthood has made Toxorhynchites an obvious subject of interest to researchers exploring ways to reduce the spread of mosquito-borne diseases like malaria and several attempts have been made, particularly in Pacific islands, to use them as biological control agents.

For instance, Toxorhynchites  amboinensis was released in Hawaii in 1955, but the effort had relatively little success. However, in that same year in American Samoa, the introduction of Toxorhynchites  amboinensis and Toxorhynchites  brevipalpis to control the filariasis vector Aedes polynesiensis was effective by at least 1978 when it was noted that Toxorhynchites  amboinensis had become fully established and was effectively controlling the target mosquito population.

So there you have it. A mosquito that hunts other mosquitoes! While I wouldn’t blame you for taking a swat at any mosquito that happens to land on you, I would say that should you find yourself being visited by a gigantic shimmering mozzie during your next outdoor excursion, you might want to think twice. It could very well be that you and that little beast are on the same team.

This post was inspired by reading The Secret Life of Flies by Erica McAlister.

 

References and Further Reading
  • Abbitt, B., & Abbitt, L. G. (1981). Fatal exsanguination of cattle attributed to an attack of salt marsh mosquitoes (Aedes sollicitans). Journal of the American Veterinary Medical Association.
  • Bonnet, D. D., & Hu, S. M. (1951). The introduction of Toxorhynchites brevipalpis Theobald into the Territory of Hawaii. (PDF)
  • Collins, L. E., & Blackwell, A. (2000). The biology of Toxorhynchites mosquitoes and their potential as biocontrol’agents. Biocontrol News and Information, 21(4), 105N-116N. (PDF)
  • Engber, B., Sone, P. F., & Pillai, J. S. (1978). The occurrence of Toxorhynchites amboinensis in Western Samoa. Mosquito News (USA).
  • Lounibos, L. P. (1979). Temporal and spatial distribution, growth and predatory behaviour of Toxorhynchites brevipalpis (Diptera: Culicidae) on the Kenya coast. The Journal of Animal Ecology, 213-236. (PDF)
  • Matheson, R. (1944). Handbook of the mosquitoes of North America. Handbook of the Mosquitoes of North America., (2nd).
  • McAlister, E. (2017). The Secret Life of Flies.http://www.youtube.com/watch?v=tYOMbTizqKA
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