Crane flies are the largest representative of true flies (including robber flies, fruit flies, moth flies and others), with 15,726 recognised species of crane flies, of which 1,475 are found in India. The term ‘true flies’ differentiates them from other insects such as butterflies, dragonflies, mayflies, and others that bear the suffix ‘fly’. True flies are related to mosquitoes, midges and gnats and they all belong to the taxonomical classification, Diptera.

Being food for a variety of species of insects, spiders, reptiles, birds and bats, crane flies are the most diverse fly community, are of ecological importance, and deserve more attention.

Previously grouped under a single family Tipulidae, modern taxonomical revision has now classified them in four different families clubbed under the super family Tipuloidea. Atanu Naskar from Zoological Society of India, who studies Dipterans, told Mongabay India that while representatives of all four families are found in India, the short-palped crane flies from the family Limoniidae are the most abundant. The hairy-eyed ones from the family Pediciidae and the damsel flies from the family Cylindrotomidae are scarcely found.

It is worth mentioning that 11,000 out of the 15,726 crane fly species were described by a single dedicated U.S. entomologist Charles Paul Alexander, including those from India.

Looking at a crane fly

One of the striking features of all Dipterans which is most clearly visible in the crane fly is their wing structure. As the name suggests (Di meaning two, and ptera meaning wings), all true flies have only one pair of wings. The second pair of wings in all true flies is reduced to slender club-like structures called halteres, which do not function as wings, but help them maintain a high level of equilibrium during flight. An insect’s version of a gyroscope, halteres function as high-speed sensors for rotational movements, allowing flies to perform acrobatic feats to frustrate predators. Any wonder why it is so difficult to swat a fly or track a mosquito with your eyes!

Despite varying in colour, size and wing patterns, another prominent feature is their long legs. On a slender body sporting thin transparent wings, the stilt-like legs are even more prominent in the bigger species. Some crane flies with bodies that are five centimetres long, can have legs up to 25 cm long, earning them the moniker crane. In contrast, some of the smallest crane flies are no bigger than a millimetre long.

A male crane fly can be differentiated from a female by its body shape. The abdomen of a male ends bluntly whereas it is pointed in a female, forming an ovipositor, an organ used for laying eggs. Crane flies display a most curious behaviour of swarming to find mates, evoking a lot interest but satisfactory answers are yet to be found. In the tropical regions males and females stay away from light and swarm together in the dark. Researchers have also reported exclusive swarms of just males or females. How does this aid in finding mates? The advantage of such behaviours is yet to be deconstructed.

Read more: Mantispids: secretive insects with lace for wings

An innocuous fly, mistaken for a mosquito

Crane flies of different sizes are often misunderstood. Some call the biggest ones “daddy long legs”, a name generally attributed to arachnids like harvestmen and cellar spiders. On the other hand, the smallest of crane flies resemble mosquitoes. Some medium-sized crane flies can easily be mistaken for predatory insects such as assassin bugs, ichneumon wasps or chironomids. The reputation of crane flies has suffered due to these resemblances.

There is a mistaken, yet strong belief that they sting, bite, and predate on other small insects. ‘Mosquito hawks’ and ‘skeeter-eaters’ are other common names, arising from a mistaken notion that they are giant mosquitoes that hunt and eat mosquitoes. Matthew Bertone, a crane fly expert from North Carolina State University, clarifies that they do not bite or sting humans, nor are they predators.

According to Jon Gelhaus, who studies crane flies in North America and is associated with Drexel University, most adult crane fly species do not feed at all. Their mouth parts – or lack of it in some – bear testimony to this. A few may occasionally sponge up liquids, like dew or water. Others that have developed specialised mouth parts feed on nectar. As adult crane flies live for only a few days, they do not need to feed much.

Some species exhibit mimicry, their wings taking on the colour or shape of the leaves of plants they rest on, while some are mimics of wasps and assassin bugs. White light attracts them; they can be seen resting on the light screens set up to observe moths or on near walls or roofs.

Crane fly larvae and soil health

Being among the least studied insects, many aspects of the crane fly morphology and biology remain relatively unknown and undescribed. They prefer moist, humid environments, and can be seen resting on plants close to water bodies. They are widespread in the forests of Western Ghats and Himalayas. In India, they are not so frequently seen in semi-urban or urban areas, except if the dwellings are close to grasslands or in the vicinity of wooded areas. Interestingly, some crane flies harbour a unique relationship with spider webs. Gelhaus reports seeing some species in Peru residing between the surface of a stream and a spider web. They remained suspended over the stream by holding on to the threads of the web without getting stuck in them.

Crane flies spend more time as larvae than adults. Their larval life can last from a few weeks to some times more than a year. Larvae may be found in a variety of habitats ranging from aquatic, semi-aquatic or terrestrial. Thus, rapid streams, intertidal zones, brackish water, vegetated areas of lakes, water collected in holes in trees, wet beds of moss, decaying wood to all types of soils, especially those rich with humus – are all spaces where larvae flourish. The biology and development stages of the larvae are also not well known. Larvae of different species show diverse features. Some are seen to inflate their rear ends to move through the soil; some have fringed setae (hair- or bristle-like structures) to break the water tension; others have structures to creep, that look like prolegs of a caterpillar but with hooks.

Soil-dwelling Tipuloid larvae play an ecologically important role in maintaining soil health. The microbial communities present in the intestine of these crane fly species influence the soil microbiome and nitrogen fixation. Microbes bring about nitrogen fixation within their digestive tract and enter and persist in the soil for a long time. Studies by researcher N.V. Kostina and her team from Moscow State University demonstrated that a significant acceleration of nitrogen fixation, denitrification, methane production and a stable soil microbial complex was brought about by the larvae of Tipula maxima. Their research showed that nitrogen fixation activity increased in the soil twice during the first month of the larval presence and then eight times during the remaining three months of the larval life. Pupae remain in the same place as the larvae. When adults emerge from it, females are born with a full complement of mature eggs ready to be fertilised by males.

A story for the ages

This description of crane fly diversity would be incomplete without the mention of leatherjackets, a common name given to the larvae of some pests that live beneath the soil. Gardeners fear them for they can munch their way through the flora and leave the soil barren.

In the year 1935, the ground staff at the famous Lord’s cricket ground were keeping their fingers crossed. That May, the leatherjackets had already damaged golf courses and lawn-tennis courts. The spectators and staff at Lord’s sat wondering about the fate of the match on the leatherjacket pitch. The first match went by uneventfully but the second one, Marylebone vs Yorkshire Clubs, was preceded by rain. Out emerged the leatherjackets in full force, munching away to show bare patches on the pitch. Desperate ground staff picking up and burning them, it did very little to alter the outcome of the game. The barren pitch took spin, much to the joy of the captain. It was a massacre of sorts for the batsmen who were bowled, stumped, or out hit wicket. The spinners ruled the match, and the leatherjackets awaited the next rains.

Crane flies are enigmatic Dipterans whose way of life is little known. Why do some males, unlike the females, have elongate antennae? How do they find their mates? What other ecological functions do they serve, apart from influencing soil health? Many unanswered questions linger for which only a handful of researchers are trying to find answers. True flies are not elusive, and their larvae not too difficult to study if one were to go by Gelhaus’s observations. Why are there no takers?

The author is a biology and environment education practitioner.

Citation:

• Oosterbroek, 2024. Catalogue of the Craneflies of the World. https://ccw.naturalis.nl/
• Checklist of Indian Crane Flies https://www.researchgate.net/profile/Dhriti-Banerjee/publication/301216756_CHECKLIST_OF_INDIAN_CRANE_FLIES_INSECTA_DIPTERA_TIPULOIDEA/links/570cf24f08ae2b772e42a586/CHECKLIST-OF-INDIAN-CRANE-FLIES-INSECTA-DIPTERA-TIPULOIDEA.pdf
• Charles P. Alexander (1970), New and little-known Indian craneflies (Diptera: Tipulidae) – III, Oriental Insects, 4:1, 77-88. http://dx.doi.org/10.1080/00305316.1970.10433943
• Mosquito Hawk? Skeeter Eater? Giant Mosquito? No, No, and No https://entomologytoday.org/2015/08/17/mosquito-hawk-skeeter-eater-giant-mosquito-no-no-and-no/
• John K Gelhaus – Tipulidae (Crane Flies, Tipulidos) https://ccw.naturalis.nl/documents/Gelhaus,_2009.pdf
• V. Kostina et al Effect of Vital Activity of Soil-Dwelling Tipulid (Tipula maxima) Larvae on Biological Activity in the Soil. Moscow University Soil Science Bulletin Vol. 74 No. 1 2019. https://link.springer.com/article/10.3103/S0147687419010071
• Andrew Ward: Cricket’s Strangest Matches. Extraordinary but true stories from over a century of cricket. p111. Robson Books, London 2005. ISBN 9781861052933

Banner image: This crane fly Hexatoma (Eriocera) sp., photographed in Wayanad, Kerala, is a wasp mimic. Image by L. Shyamal via Wikimedia Commons (CC BY-SA 3.0).

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Divya Kilikar Editor

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BiodiversityEcosystem servicesInsectsGlobalIndia

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