Feature

Flying Long-leggedy Beasties

What strange and unusual lifeforms are living on your walls?

Dr Simon D Pollard

On the inside of a side-light window, next to my front door, is a large cobweb that fans out across the glass and onto the adjacent wall. For flies that have come into the dark hallway and are attracted to the light at the end of the tunnel, this veil of silk is often a deadly filter to perceived freedom.

At the base of the web, which is littered with fly body parts, lives the resident female who "listens" for any telltale vibrations that signal dinner has arrived. When a fly smacks into her web, she rushes up silken lines, stabs the victim with her fangs, and pulls the body down into her lair.

Sometimes, I slip her a blowfly and as she sucks out its insides, her abdomen inflates like a balloon. She lives in prime spider real estate.

Among the various trophies hanging in her graveyard are the dismembered bodies of some very unusual looking flies called craneflies. Over five hundred species of cranefly live in New Zealand, but this species of Leptotarsus is found in Canterbury, and the adults are around for only a few weeks at the end of summer.

Like all adult craneflies, they have a long thin abdomen, a bulbous thorax which supports a pair of huge wings, and a comparatively tiny head, which looks like a caricature of an insect head rather than the real thing; a pair of bulging eyes and a straw-like proboscis make up most of it. Craneflies look anorexic compared to the stocky little bodies of houseflies and blowflies.

Probably the most remarkable feature of these flies is their long spindly legs, which are about three times the length of the fly's body. These appendages and the wings snap off the thorax easily, and after the spider has finished feeding, a jigsaw puzzle of legs, wingless bodies and bodiless wings hang in the web. With such long legs, it is hard to imagine that an assembled and alive cranefly could actually fly.

However, like the aerodynamically incongruous bumblebee, craneflies can fly; although unlike the bumblebee, they make flight look a very precarious pastime. As the wings flap and the legs dangle, images of stunned multi-legged fairies on their maiden flight come to mind.

The larval stage is commonly called a "leather-jacket" because the cuticle (skin) looks like leather. Imagine the sleeve of a pale leather coat moving like a maggot.

Eventually, the larva moults into a pupa, which is the stage where the adult features are formed. When the pupal case is about to hatch, it is close to the soil surface, so the adult will emerge above ground. The case splits open, and a cranefly, its six legs held straight out and parallel to each other, extracts itself from the case. Adults are usually "born" on misty afternoons and once their cuticle has dried, they can move around.

Unlike the males, females are born wingless and cannot fly. They move by flopping around on their long legs, like somebody on stilts who has fallen over and can't get up. Males fly in search of females and often have to negotiate a gauntlet of competitive males to secure a mating. Competition between males even extends to defending pupal cases where virgin females are emerging.

If the female is receptive, and the male is not thwarted by a rival male, the male and female, among a tangle of legs, join the tips of their abdomens together and mate. Females only live for a matter of hours; long enough to mate and lay their eggs.

Even without the life-changing consequences of flying into a well-placed cobweb, males live for only three to four days. And it is during this brief life, having confused street and house lights for moonlight, that they often end up inside our homes, clinging to walls with their splayed-out legs; like the insect equivalent of a temporary tattoo.

Craneflies are often called "daddy long-legs" and share this common name with a couple of other Canterbury household and garden residents: the long-legged spider and the harvestmen.

Sometimes, they are mistaken for huge blood-sucking sandflies, and while their emergence from the ground has some vampiric overtones, the only liquid a cranefly would suck are water or nectar.

I am not sure whether the writer of a well known Cornish prayer was thinking of craneflies when he asked for protection from "ghoulies and ghosties and long-leggedy beasties", but they are harmless visitors.

Next March, when a new generation of males, currently subterranean "leatherjackets", stumble into your home, show them more sympathy than the spider in my hallway did.

Dr Simon D Pollard is Curator of Invertebrate Zoology at Canterbury Museum.