By Leslie Mertz
That inch-long, gangly-legged insect that sneaks into your house and bounces around the walls and ceiling is a crane fly, and despite rumors to the contrary, it is neither a predator of mosquitoes nor a colossal mosquito. And it’s harmless.
Although the Internet abounds with reports of adult crane flies biting or stinging, they do neither.
“There has yet to be found a predatory adult crane fly,” said Matthew Bertone, PhD, a crane fly specialist and extension associate with the North Carolina State University Department of Entomology. “They just don’t have the mouthparts for it. So no, none are blood-feeding, and none of them attack people.”
In fact, many of the adult crane flies eat very little, if at all, according to Jon Gelhaus, PhD, a fellow crane fly specialist and curator in the Department of Entomology at the Academy of Natural Sciences of Drexel University.
“Some can sponge up liquids, such as dew and honey water, but we don’t see them do that much,” he said. “A number of them have independently evolved long mouthparts, and they’ll visit flowers to take up nectar.”
The slight diet is fine because adult crane flies typically live just a matter of days. Crane flies spend most of their time as larvae living underwater in streams, the edges of ponds, within wet logs, or in other damp places, and then they emerge as adults for a quick mating spree before dying.
The 15,000 or so known true crane flies in the family Tipulidae also share a somewhat similar appearance to mosquitoes. They have a narrow body with two long and slender wings, as well as six stilt-like legs that can be twice as long as the body. Crane flies are diverse in wing pattern, color, and size.
“The smallest crane fly in the world could probably stand on the head of the biggest crane fly in the world,” Bertone said. The tiniest ones have bodies that are mere millimeters in length, while the largest can be more than two inches long with leg spans topping 10 inches. The big differences between species, however, are found among the larvae.
“There are lots of crazy morphologies there,” said Bertone. “Some of them have inflatable rear ends that they use to move through soil more easily, some have fringed setae on the end to break the water tension, and some have these weird creeping structures, sort of like caterpillar prolegs with hooks on them, so it’s extremely variable. We don’t know much about the larvae. In fact, for many species, we have never seen the larvae.”
Although scientists have a greater understanding of adults — thanks in great part to the work of U.S. entomologist Charles Paul Alexander (1889- 1981), who described a whopping 11,000 crane fly species during his illustrious career — a multitude of questions remain. For example, according to Bertone, “Most of the crane flies have big eyes, but we don’t know how good their vision is and how much that’s used to sense where they’re going. The males in some of the groups have antennae with really elongate segments compared to females, but we don’t know what the purpose of that is.”
Scientists are also uncertain about courtship and communication. Some crane fly species engage in all-male swarms that apparently attract females, Gelhaus said. The males of other species will simply flit around their habitat with their forelegs outstretched, presumably using a contact pheromone to seek out females.
Other behaviors are also ripe for study. For instance, Gelhaus has seen both males and females of several tropical species aggregate together in dark areas.
“Whole groups of individuals will all be flying around together, sometimes bouncing at a constant level,” he said. “If you disturb them, they will fly away, but then in a little while they will re-aggregate back into those areas. We really don’t know what they’re doing in those situations.”
Gelhaus has also seen members of another tropical species in Peru that sandwich themselves between the surface of a stream and a suspended spider web.
“They seem to be hanging upside-down from the spider web, holding onto the threads of the web without being caught in it,” he said. “Behaviorally, I’d say crane flies aren’t super complex in comparison to even some other fly families, but there are a lot of these adaptations — including mimicry of ichneumonid wasps and other things — that really need study and will take somebody spending some time in the field and observing to figure out.”
Scientists are also still sorting out the evolutionary tree, particularly whether the large Tipulidae family should be split into several different families. Bertone was part of a research group that used morphology and genetics to try to sort it out. They concluded that the bulk of the species had more in common than not and should remain in the Tipulidae family, while just one smaller family of hairy-eyed crane flies (Pediciidae) should be separated out as a sister group.
Gelhaus appreciates their assiduous tone.
“Instead of splitting up Tipulidae all kinds of ways, which later evidence might not have supported, they said that the weight of the evidence conservatively supports two basic lineages, and I thought this was a pretty good way of approaching it,” he said. “I expect there’ll be some changes as we move along, and as more and more data is put to it, but that’s just part of the nature of classification and the taxonomy. It has to evolve along with our knowledge.”
There are other crane flies that fall outside the Tipulidae and Pediciidae families, but they are not as closely related. These include the phantom crane flies, winter crane flies, and primitive crane flies (Ptychopteridae, Trichoceridae, and Tanyderidae, respectively). The best known of these is the phantom crane fly Bittacomorpha clavipes, a large insect that flies with its inflated tarsi (“feet”) helping to float its long, black-and-white legs in the air.
“Phantom crane flies are one of my favorites,” Bertone said. “They’re really pretty and I just like the way they fly.”
Even the true crane flies alone, however, are deceptively diverse.
“They have weird behaviors and weird morphologies,” Bertone said. “I’m always seeing photos of new ones, and it just blows my mind how they look or how they have all these crazy modifications. There are strange, wingless, spider-like snow crane flies that are thought to live in animal burrows and crawl underneath the snow; there are small, hairy ones; there are larger ones; there are lots of them that suck nectar — it’s a really diverse and pretty amazing group.”
Gelhaus agrees. He took a rather serendipitous path to his study of crane flies, with an internship at the California Academy of Sciences that just so happened to involve these insects, and he has enjoyed every minute.
“I’ve never regretted it,” he said. “It’s a very interesting group for me, and it’s taken me all over the world. Crane flies were definitely the right choice.”
Read more at:
“Phylogenetic synthesis of morphological and molecular data reveals new insights into the higher-level classification of Tipuloidea (Diptera)” by Matthew J. Petersen, Matthew A. Bertone, Brian M. Wiegmann, and Gregory W. Courtney in Systematic Entomology, Volume 35, Issue 3, pages 526–545, July 2010.
Leslie Mertz, PhD, teaches summer field-biology courses, writes about science, and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.