The Clumsy yet Crucial Cranefly

This article looks at the Cranefly, AKA Daddy Long-legs, both generic names for adult members of the Tilupidae family. Its larvae, the scourge of the gardener and lawn aficionado alike, are known as Leatherjackets.

Craneflies are what are called ‘true flies’, meaning they only have a single pair of wings, and their family name Tipulidae derives from the Latin tipula, meaning ‘water spider’. They are not spiders though, this is thought to have come from their spider-like appearance, with extraordinarily long, slender legs, however they are quite often mistaken for giant mosquitoes and some people worry they might even bite or sting, (as you can see in a video later on this article) but this fear is completely unfounded as they are totally harmless to humans or indeed any other creature.

There are over 300 species of Cranefly found in the British Isles, with as many as 15,000 worldwide, and they can be found in many different habitats. However, the two species most often seen in the British isles are Tipula paladosa (yes I don’t know what’s going on with the soundtrack to this video either but I kind of like it!) and Tipula olaracea.

Lifecycle

Like all true flies, crane flies go through a complete metamorphosis cycle with four stages, these being egg>larva>pupa>adult, the adults are famously short-lived though and survive only long enough to mate and reproduce, usually just as long as a week. Most of this time they seem to spend flying around aimlessly and gracelessly, banging into lights and windows, getting stuck in people’s hair, winding up cats and generally being a bumbling, clattering nuisance.

The females of most species deposit their eggs, about 300, either near or in water and when they hatch the larvae may live in the water, in leaf litter or in soil, depending on the species. Those species of Cranefly which are aquatic will usually pupate underwater, but have to emerge from the water to shed their pupal skins, they will do this by night to avoid predation and by the time the sun has risen these new adults are ready to fly and begin searching for mates.

The ‘Leather jackets’ that gardeners, groundskeepers and horticulturists know and hate are the larvae of species of Cranefly that grow up underground, they are very basic in build, basically just grey/brown legless tubes about 3cm long.

The reasons they are so loathed are two-fold; one is the damage caused by them feeding on the roots of vegetables and grasses, making the plants more susceptible to disease and eventually killing them. The other reason is the damage caused by predators such as Badgers, Hedgehogs, Crows, Foxes, Jackdaws, Magpies, Rooks, Woodpeckers and Squirrels, to name a few, which dig up lawns in search of this abundant source of high protein food, which wriggles tantalisingly just beneath the surface.

Daddy Long Legs

Adult crane flies have long, slender bodies and a single pair of long, fragile wings, they are fairly unremarkable in color, although some species do have bands or spots of brown or gray. They have equally long, fragile legs too which can break off, rather disconcertingly, when you are trying to catch them in the middle of the night so you, and the cat, can get some sleep.

They actually shed legs on purpose to escape the grasp of predators and this phenomena is known as autotomy, it’s common with a lot of creatures such as harvestmen (confusingly also known as daddy long legs), stick insects and even lizards such as slow-worms, which will discard their tails to distract would-be predators.

Animals that do this usually have some kind of inbuilt nervous reaction that causes the limb to twitch after being cast off so it will carry on distracting the predator after the would-be prey animal has made its escape. With long-legged insects such as crane flies this is done by means of a special fracture line between the femur and trochanter, so the leg separates cleanly.

Another anatomical feature which is peculiar to true flies like the cranefly is that they have a pair of conspicuous halteres just behind the wings, these look similar to antennae, but extend from the sides of the body. Halteres work like gyroscopes during flight, helping the cranefly stay on course and get their name from a Greek word for weights which are held in the hands to give an impetus in leaping, they evolved from a second pair of wings which older species of insects like dragonflies still possess.

The cranefly hatch and its importance for breeding birds

The Crane fly hatch can be an incredibly important time for many species of birds, with some upland birds, such as Curlew and Golden Plover, timing their peak hatching season to coincide with the hatch of certain species of Cranefly.

Studies of the diets of Red Grouse have found that from May to June, a species of Cranefly; Tipula subnodicornis, made up 8% of their diet and in some of the droppings that were analysed (yes there are biologists who sift through bird droppings!) the remains of this species composed over 50%. This species is aquatic though and so only abundant on moors with a decent coverage of wet blanket bog, droppings of Red grouse from dry moors with little coverage of blanket bog were found to contain very few insect remains.

Although the diet of Red Grouse is mostly made up of the young shoots, flowers, seeds and berries of moorland plant species such as Heather, Bilberry and Cottongrass these studies have shown that insects such as Cranefly are, as well as making up a larger part of their diet than thought, (although gamekeepers have always known that high insect numbers are key to a having a productive shoot) vital for sources of elements and vitamins which are otherwise hard to obtain on the nutrient poor uplands.

Chemical analysis of Tipuls subnodicornis and Calluna vulgaris, the main species of heather that Grouse eat, has shown that compared to heather, the Cranefly contained much higher concentrations of Nitrogen and Phosphorus and the consumption of 8% of insects in the diet increased Nitrogen intake by 67% and Phosphorous by 51%, this is vitally important for the breeding success of Red Grouse and other species of bird that breed on the uplands, as these two elements are key to producing both healthy eggs and chicks.

Research such as this goes to show how beneficial the work currently being done by many landowners, conservation organisations and shoot managers to flood and ‘rewet’ the country’s moorland and blanket bogs is to native bird species, many of which are threatened, that rely upon insects like the Cranefly and how this will combat the effects of increasing temperatures, which may lead to our uplands drying out.

So next time you are annoyed by the clattering and clumsy antics (this video is hilarious btw!) of a Cranefly please try to appreciate how utterly important this seemingly pointless insect is to our wildlife and countryside!

A B-H