Few insects are as misunderstood as the crane fly. Known colloquially in Britain as “daddy-long-legs”, these gangly, long-legged flies provoke alarm out of all proportion to any threat they pose. Belonging to the family Tipulidae (and related families within the infraorder Tipulomorpha), crane flies are completely harmless to humans. They do not bite, they do not sting, and they are not “giant mosquitoes.” So what do they actually do?
Identification
Crane flies are among the easiest insects to recognise:
Key Identification Features
- Body length: 10–40 mm (some tropical species much larger)
- Legs: Extremely long and fragile; readily shed (autotomy) when grabbed by a predator
- Wings: One pair of long, narrow wings; hindwings reduced to halteres (balancing organs), as in all true flies
- V-shaped suture: A distinctive V-shaped groove on top of the thorax
- Flight: Slow, ungainly, and apparently random; they bounce off walls and furniture when indoors
- Antennae: Short, thread-like
Common Myths About Crane Flies
| Myth | Reality |
|---|---|
| “Crane flies are giant mosquitoes” | They are in the same order (Diptera) but a completely different family. They are not closely related to mosquitoes and do not feed on blood. |
| “Crane flies bite or sting” | Adult crane flies have vestigial mouthparts and cannot bite or sting. They are completely harmless. |
| “Crane flies eat mosquitoes” | This myth confuses crane flies with mosquito hawks (a term sometimes applied to dragonflies or robber flies). Most adult crane flies do not feed at all; some drink nectar or water. |
| “Crane flies are the most venomous insects” | This is a conflation with the daddy-long-legs spider myth. Crane flies have no venom whatsoever. |
| “Crane flies serve no purpose” | Both larvae and adults play important roles in ecosystems (see below). |
The Adult Stage
Adult crane flies typically live for only a few days to two weeks. Most species have reduced or vestigial mouthparts and do not feed at all during their adult lives. Their sole purpose as adults is to mate and lay eggs. Some species are able to drink water or dilute nectar, but they do not consume solid food.
The mass emergence of crane flies in late summer and autumn (August–October in the UK) is a familiar sight. Adults are strongly attracted to lights and frequently enter houses through open windows, where their bumbling, ungainly flight causes unwarranted alarm.
Did you know? A crane fly’s legs detach easily when seized by a predator — a survival mechanism called autotomy. The shed leg may continue to twitch, distracting the predator while the crane fly escapes. Unlike some lizards, crane flies cannot regenerate lost legs.
Leatherjackets: The Larval Stage
While adult crane flies are harmless, their larvae — known as leatherjackets — can be significant pests. Leatherjackets are tough-skinned, greyish-brown, legless larvae that live in soil and feed on the roots and base of grass stems. They are the most economically important stage of the crane fly life cycle.
Leatherjacket Damage
Heavy infestations of leatherjackets can cause yellow patches in lawns, where the grass dies from root damage. They also damage cereal crops, vegetables, and ornamental plants. Secondary damage occurs when birds (particularly starlings, rooks, and corvids) tear up turf to feed on the larvae beneath, causing further visual damage to lawns.
- Egg-laying (August–September): Female crane flies lay eggs in soil, typically in damp grassland or lawns.
- Larval development (autumn to spring): Leatherjackets feed on roots through autumn, winter, and spring. They are most damaging in spring when they are largest.
- Pupation (summer): Mature larvae pupate near the soil surface. The pupa is brown and elongated.
- Adult emergence (late summer): Adults emerge from the soil, leaving behind empty pupal cases protruding from the turf.
- Mating and egg-laying: Adults mate within hours of emergence, and females lay eggs in the soil within 24 hours. The cycle repeats.
Ecological Role
Despite their pest status in lawns, crane flies play important ecological roles:
As Prey
Both adults and larvae are a vital food source for many animals. Leatherjackets are a key prey item for starlings, rooks, lapwings, and curlews. Declining crane fly populations (linked to drier autumns affecting egg survival) have been implicated in the decline of starling populations in parts of Britain. Adult crane flies are eaten by bats, swallows, and spiders.
As Decomposers
Many crane fly species have larvae that feed on decaying organic matter in soil, leaf litter, and waterlogged habitats rather than on living plant roots. These detritivorous leatherjackets contribute to nutrient cycling and soil formation. Some species have aquatic larvae that help break down organic matter in streams and ponds.
As Pollinators
Some adult crane flies that do feed on nectar may contribute to pollination, although their role is minor compared to bees, hoverflies, and butterflies.
Managing Leatherjackets in Lawns
- Biological control: The entomopathogenic nematode Steinernema feltiae can be applied to lawns in September–October to infect and kill leatherjackets
- Encourage birds: Starlings and rooks are natural predators — make your garden bird-friendly
- Improve drainage: Leatherjackets prefer damp soil; improving drainage can reduce populations
- Tarpaulin trapping: Cover affected lawn areas with damp hessian or black polythene overnight; leatherjackets come to the surface and can be collected in the morning
- Overseed in spring: Repair damaged lawn areas by overseeding with grass seed in March–April
Key Takeaway
Crane flies are harmless insects that cannot bite, sting, or transmit disease. Adult crane flies serve primarily as food for birds and bats. Their larvae (leatherjackets) can damage lawns and crops by feeding on roots, but they also contribute to nutrient cycling as decomposers. Far from being purposeless, crane flies are an integral part of British garden and grassland ecosystems.