"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

March 26, 2015

Emblemasoma erro

During summer the air is filled with the rattling ruckus of cicada songs. Male cicadas produce this summer choir using a pair of noise-making organs located in their abdomen, with the aim of getting attention from any prospective mates. But in some cases, they can also end up with some unwanted attention.

Top: Male Tibicen dorsatus cicada
Bottom: Female Embelmasoma erro fly
Photos from Figure 1 & 2 of this paper
The species we are featuring today is an "acoustically hunting" parasitoid fly - it eavesdrops on the male cicada's flirtatious serenading and uses it to home in on its target. This fly is commonly found on the Great Plains of North America and is a scourge to male cicadas, especially male Tibicen dorsatus.

Most of what is known about such acoustically hunting parasitoids are based on flies from the Tachinidae family - one of which targets crickets (I talked about how crickets on Hawaii evolved to become silent due to the presence of one such parasitoid fly here). But this fly belongs to a different family (Sarcophagidae). Only one species of Emblemasoma is well-studied - E. auditrix- and even though Emblemasoma is widely use in the study of insect hearing, not much is known about how they actually live out in the wild. Until now, the only information available on E. erro are based on two scientific papers - one published in 1981 and the other published in 2009. The paper we are featuring today provides some much-needed update on key aspects of this parasitoid's ecology and life history.

This paper reports on a series of field surveys and laboratory experiments that documented the parasitoid's occurrence, abundance, behaviour, and developmental cycle.

The field surveys were conducted at study sites located across Kansas and Colorado. The surveys found that a bit over a quarter of male cicadas were infected with E. erro larvae, and because of how the flies track down their host, almost all the infected cicadas were male - except for one very unlucky female cicada, which most probably got infected because she was responding to the call of a male, ran into a larvae-ladened E. erro that had the same idea, the latter decided that any cicada will do. Talk about a case of fatal attraction!

And it is indeed the sound of the male cicada's serenade that draws in those flies - a loudspeaker playing the recordings of cicada calls is sufficient to attract the attention of E. erro, but a female fly need more than that to commit to dropping off her precious offspring. In outdoor cage experiments where flies and cicadas were housed together and allowed to mingle freely, the researcher observed that even if an E. erro finds herself perched next to a cicada, she will only attack when the host makes any sudden movements. So E. erro uses two separate signals to track down its prey; an acoustic signal at long range in the form of the cicada's call to guide them in, and a visual signal at close range in the form of cicada movement to confirm the host's identity

Emblemasoma erro larva emerging from a cicada
From Figure 6 of this paper
Once she has confirmed her target, the female fly makes an attack run, and very quickly drops off between 1-6 maggots (usually 3) on the base of the cicada's wings. As soon as the maggots land, they immediately start burrowing between the segments and into the cicada's body. The maggots then start devouring its host alive from the inside. Depending on the temperature and clutch size, they take about 88 hours to reach a large enough size to start pupating. At the end of this period, the maggots use teeth-like "oral hooks" to chew their way to freedom, fall onto the soil below to become pupae, and leaving the cicada an empty husk.

So while the aim of the male cicada's singing is to attract the attention of female cicadas, some of them may instead end up getting attention from females of a very different species, and become reluctant incubators for the broods of some keen-eared, cicada-hunting flies.

Stucky, B. J. (2015). Infection behavior, life history, and host parasitism rates of Emblemasoma erro (Diptera: Sarcophagidae), an acoustically hunting parasitoid of the cicada Tibicen dorsatus (Hemiptera: Cicadidae). Zoological Studies, 54: 30.

March 11, 2015

Crassicauda magna

During this blog's first year back in 2010, we featured a parasitic nematode (roundworm) that lives in the placenta of sperm whales of all places. Today, we're featuring a study on another nematode which lives in the sperm whale's cousin - the much smaller and more enigmatic pygmy sperm whale Kogia breviceps.
Photo of C. magna in whale tissue from Fig. 1 of the paper

Crassicauda magna is a parasites that really gets under the skin of the pygmy sperm whale. While most worms in the Crassicauda genus live in the urogential and renal system of whales, C. magna just had to be different from the rest of the pack. Instead of living in the whale's plumbing system, it had opt for a life being sandwiched between layers of blubber and muscle, living snugly under the whale's subcutaneous tissue.

While it can be a tight fit in there, C. magna can grow quite large -the largest known fragment is 3.7 m (about 12 feet) long, but due to where they are found in the body and the relatively cryptic nature of its host, no fully intact C. magna has ever been retrieved. The original species description for C. magna was published in 1939, and was based upon fragmentary remains from the front half of the worm, as the rest of the parasite not recovered.

Even though this parasite appears to have a global distribution (like its host), very little is actually known about it. Only a few anatomical details have been recorded, pieced together from worm fragments which had been collected over the years, and until the publication of the present study, there were no genetic data for C. magna. This is not too surprising considering that much of what is known about the pygmy sperm whale itself (let alone C. magna) had about from examining stranded individuals - which is not exactly a routine occurrence.

The C. magna specimens which were the subject of this new study were retrieved from a dead pygmy sperm whale which was beached at Moreton Bay, Queensland. Most importantly, from a taxonomist's perspective, the research team involved was able to retrieve parts of the tail from male worms. The reason why this was kind of a big deal is that one of the key features used to identify different species of nematodes are the needle-like structures on the male genitalia call copulatory spicules. The male worms use these spicules to pry apart the female worm's vulva for sperm transfer, and it just so happened that each species have distinctively shaped spicules, which can be used to tell them apart.

The researchers were able to compare the worms collected for this study with other specimens of Crassicauda stored at the South Australian Museum, the Natural History Museum in London, and the Muséum national d'Histoire naturelle in Paris. They noted that the spicules on C. magna are remarkable similar to those found on another species that was described in 1966 call Crassicauda duguyi - which was also collected from the neck muscle of a pygmy sperm whale (in this case, it was stranded on the west coast of France). Their conclusion was the C. duguyi is most likely just C. magna instead of being a different species, but the taxonomist who described it was not able make the match because the original species description of C. magna did not have information on the male genitalia.

Unlike previous studies, the researchers responsible for the current one also managed to extract some genetic material from the worms they collected. They sequence a section of the worm's ribosomal DNA which was used to reassess the classification of C. magna in relation to other parasitic nematodes. With such a genetic marker at hand, it can be used in the future to find out more about this enigmatic parasite and its equally cryptic host.

Jabbar, A., Beveridge, I., & Bryant, M. S. (2015). Morphological and molecular observations on the status of Crassicauda magna, a parasite of the subcutaneous tissues of the pygmy sperm whale, with a re-evaluation of the systematic relationships of the genus Crassicauda. Parasitology Research 114: 835-841