The Crane Fly

by Forrest M. Mims III

Snow has recently fallen in some northern regions of the United States, but spring is in full bloom across the southern States.

Migrating monarch butterflies and birds have been passing through our rural site on Geronimo Creek in South-Central Texas on their way north. Butterflies, moths, aphids, ladybugs paper wasps and many other insects have made their appearance. Among them are large flying insects that perch on porches at night and even enter their homes.

"Are they giant mosquitoes?" South Texas residents have asked.

No, they are crane flies (genus Tipula). They are among the most common of the large flying insects along nearby Geronimo Creek this spring.

So far this has been only a moderate year for crane flies. As these words are typed, several are perched outside my office window.

On cool days they hide in the grass and under leaves in trees. But when the temperature rises above 15 degrees C or so (about 60 degrees F), they begin to fly about.

The crane flies flying around yards, and possibly roosting in kitchens, have only one goal. They are doing their best to find a mate during their relatively brief life as adult insects.

If they survive the sharp eyes of the birds who enjoy dining on them, females that have mated deposit their eggs in moist soil.

Crane fly eggs hatch into grubs that live in the soil. There they eat decayed leaves, roots and even fungi. A year or so after they hatch, the grubs become adult crane flies. The adults do not feed.

Crane flies have a remarkably sophisticated navigation system. A key part of this system is inside the while circle in Fig. 1. The object inside the circle is at the end of a thin stalk that emerges from just behind the wing. There are two of these gadgets, one behind each wing. They are called halteres.

When the crane fly switches on its powerful wing motor, the halteres whip back and forth in time with the wings. Inertia tends to keep the vibrating halteres from moving away from their former position when the fly makes a turn. This causes the stalk of the haltere to activate sensors behind the wing that tell the fly in which direction and exactly how far it has turned.

This gives the crane fly an inertial guidance system similar in principle to the gyroscopic system used in guided rockets and in aircraft. Of course they also came with other fancy equipment, including powerful wings, an array of video cameras, various other sensors, a guidance and control analog computer, and the ability to manufacture tiny pellets that eventually grow up to become new flies.

Other flies have a very similar guidance system, which explains their ability to perform aerobatics no manmade airplane can duplicate. Have you ever seen an airplane land on an upside down runway? Flies do this all the time.

So far the fossil record is silent about the origin of the crane fly and its halteres. Two crane flies are in my fossil collection. One is an impression in shale. The other is frozen in translucent yellowish-orange amber from the Dominican Republic. The almost lifelike amber specimen has clearly visible halteres identical to those of modern crane flies. If the specimen whose impression is preserved in shale has halteres, they cannot be seen.

Crane flies are often accused of being giant mosquitoes. While some species of mosquitoes are indeed rather large, none that I know about match the size of large crane flies. Unlike mosquitoes, there is no need to fear crane flies. Their arrival just mean that spring is here.

Forrest M. Mims III and his science are featured online at www.forrestmims.org .

This feature was originally published in Forrest Mims's weekly science column in the Seguin Gazette-Enterprise, Seguin, Texas. The column is written for a general audience.