Dance Flies: Bigger Abdominal Air Sacs Attract More Males

Dance flies (Rhamphomyia longicauda) have some interesting features. Females have expandable, air-filled sacs under their abdomen as well as hairy legs. A multinational research team has found that males are attracted to females with bigger abdominal air sacs. The flies belong to the family Empididae and are also known as balloon flies and as dagger flies. The latter name is derived from the piercing mouthparts of some species in the family.

A female Ramphomyia longicauda
Katja Shulz, CC BY 2.0 License

What Is a Dance Fly?

The term “dance flies” is sometimes used as a general term for insects in the family Empididae as well as those in the family Hybotidae. The name comes from the behaviour of the insects during their mating ritual. The flies form a swarm flying around in a dance-like motion. The swarms generally develop near sunset and can be enjoyable to watch. Rhamphomyia longicauda is found in the northeastern part of North America. It lives in wooded areas that contain wetlands or bodies of water and swarms near water.

Rhamphomyia longicauda

Rhamphomyia longicauda has a black upper surface. The head is small but has large orange or red eyes. The abdominal air sac of the female is also orange. The insect has a slender body with a long abdomen. The species name of the insect is derived from two Latin words meaning long and tail.

The females have two interesting features that differentiate them from the males. One is their abdominal air sacs. The other is the presence of stiff hairs (technically pinnate scales) forming a fringe on their legs. The fringe is most noticeable on the middle and hind legs. Though the hairs have an advantage, as described below, according to the BugGuide site they make it more likely that the female will become trapped in spider webs.

Swarming and Mating

Researchers from the University of Toronto in Canada and the University of Stirling in Scotland have made some intriguing discoveries about the mating swarm of the species. The females swallow air to fill their abdominal sac just before they take flight. Once they have taken to the air, they pull their legs in and up so that they one leg is folded on each side of the abdomen. This adds to the impression that the females have a huge abdomen relative to their body size. One researcher says that the behaviour makes a female look like a helicopter.

When a male and female have made the decision to mate, they leave the swarm. The researchers have observed that the males prefer females with bigger air sacs. Though we can’t tell what an insect is thinking, researchers believe that the male “thinks” that the females who are faking the appearance of a bigger abdomen contain a larger group of mature eggs that are ready to be fertilized. The female’s behaviour is actually a form of subterfuge. The researchers have also discovered that if two females have abdominal air sacs of the same size, a male choses the one with the hairier legs. The protruding scales that form the hairs likely increase the impression that the female has a large abdomen.

An Exchange of Gifts

The male flies and the larvae feed on smaller insects. The adult females don’t catch their own prey but depend on food brought to them by males. A male gives a female a bit of dead insect prey in exchange for copulation. The female mates with multiple males and gets multiple meals, enabling her to survive and produce offspring.

Some people may not think much about flies, except when the insects become a nuisance. They are interesting animals, though, especially some species, such as dance flies. I think they are worth studying.

References

Male dance flies favour females with bigger abdomens from the phys.org news service

Rhamphomyia longicauda facts from BugGuide

New Species of Snailfish in the Atacama Trench

Snailfish are unusual animals. They have an elongated and scaleless body that is often said to resemble a tadpole in shape. The animals have a widespread distribution in the colder parts of the ocean and are found in shallow to deep water. Despite these facts, they aren't very well known or understood. Scientists have recently found three new and surprising species at a depth of 7,500 metres in the Atacama Trench.

A festive snailfish or Liparis marmoratus (a species not found in the Atacama Trench)
Kitty Mecklenburg, via Wikipedia, public domain license

Snailfish

Snailfish belong to the family Liparidae. The fish have large heads relative to their size but small eyes. They are benthic animals, which means they live on the ocean bottom. In most snailfish, the pelvic fins are modified to form an adhesive disk that attaches the body to a surface. Hadal snailfish are those that live at a depth of greater than 6,000 metres and are still somewhat mysterious. These fish have been found in the Mariana Trench as well as the Atacama one.

The Atacama Trench

The Atacama Trench is also known as the Peru-Chile Trench. It’s located in the Pacific Ocean about a hundred miles off the coast of Peru and Chile. The Nazca Plate is moving under the South American Plate in the area, a process known as subduction. The trench has a maximum depth of just over 8,000 metres and an area of around 590,000 square kilometres. In the depths of the trench the environment is dark, the water cold, and the pressure enormous, yet a surprising number of species thrive there.

The Peru-Chile or Atacama Trench (the dark line)
NOAA, via Wikipedia, public domain license

The Expedition

A recent exploration of the trench by a multinational team of scientists has revealed three new species that they believe are snailfish. The large team consisted of 40 researchers who came from a total of 17 countries. The scientists sent a remote-control lander down to the area that they wanted to explore in a trip that covered four and a half miles. One of the researchers involved in the expedition said that the walls of the vessel were made of steel almost an inch thick and the window was made of “super strong” sapphire glass. This type of glass is almost as hard as diamonds.

The lander took photos and videos and trapped specimens. It returned to the surface as a result of an acoustic signal sent from the surface. It brought one of the new fish species with it, which the researchers examined and classified as a snailfish. Some of the discoveries made by the lander are shown in the video below.

Three New Species

The new fish are being referred to as the pink, the blue, and the purple snailfish for now. Though they are different in colour, they also differ in overall appearance and are believed to be three different species. They are unusual animals because they don’t look like many of the other fish that are found in deep and dark water. They have a translucent and gelatinous appearance instead of a dark, fanged, and menacing one. Their bodies look too delicate to withstand the pressure and cold temperatures at such depths, but the fish seemed healthy and appear to be very successful in their habitat. I think the blue species is particularly interesting. Unlike the pink and purple fish, the blue animal has long, sail-like pectoral fins and a relatively long snout.

One of the purple animals entered the lander’s trap and was carried to the surface. The nine-centimetre long animal was preserved and its body CT scanned. Scans like this are important in case physical specimens of organisms and items are destroyed, as happened in the horrible fire at the National Museum of Brazil recently. Though preserved animals and a detailed scan of the animal’s external and internal features are certainly useful, they aren’t as good as observations of a living animal. Hopefully much will be learned about the new fish in their natural habitat. The remoteness of the area makes it difficult to explore, however. It also makes the trench and its unusual and specially-adapted inhabitants fascinating.

References

Three new species of snailfish discovered from the ScienceDaily news service

Finding snailfish in the deep ocean from The Conversation