More Fun With Scuttle Flies

The first days of April have brought January weather to my region and the January issue of Proceedings of the Entomological Society of Washington to my mailbox. The latter includes descriptions of a new species of gall wasp that has recently become abundant on oaks along the New England coast, as well as a new species of midge that causes flowerbud galls on joe pye weed. It also includes my second collaboration with Emily Hartop (this time with Maria Wong as well) involving scuttle flies (Phoridae)*.

It all started when I was at work one day (July 18, 2013, to be precise) in southeastern Massachusetts and came across this little scene on the underside of a dangleberry leaf (Ericaceae: Gaylussacia frondosa):


The large, furry object is the remains of a white-marked tussock moth caterpillar (Erebidae: Orgyia leucostigma) that had begun to construct its cocoon (made of white silk mixed with black, clubbed hairs plucked from its own body) when it succumbed. To what had it succumbed? This was answered by a dozen tiny black objects that were scattered around the caterpillar’s remains, mostly on the leaf beneath the caterpillar, but one was stuck to a few of the caterpillar’s long hairs and is plainly visible in the above photo. Here’s a closer look at that one:


These black objects are the pupae of eulophid wasps that fed as larvae inside the living caterpillar. To find out what kind, I picked the leaf and stuck it in one of the vials with which my backpack is always well supplied. Along with the leaf, dead caterpillar, and wasp pupae came a little scuttle fly who can be seen just to the left of the caterpillar in the first photo. She was sufficiently interested in the caterpillar that she didn’t try to escape as I picked the leaf. I assumed she was laying eggs on the carcass, and that her offspring would feed on it.

Sure enough, eight days later I noticed several little fly larvae in the vial.


The next day, all twelve of the eulophid wasps emerged from their pupae.


I sent them off to Christer Hansson, who reported that they were all females of Elachertus cidariae. This species was already known to parasitize caterpillars of Orgyia leucostigma as well as ten other moth species.

Four days after the wasps emerged, the fly larvae began to pupate. Here is one with its body beginning to contract:


…and here is a finished puparium, showing the spiracular horns characteristic of phorid flies. (I didn’t crop this one more closely because I liked the detail of the feathery club at the tip of the black caterpillar hair.)


Between August 17 and 19, six adult scuttle flies emerged—three males and three females. The females were the same species as the original female I had collected, and in all probability were her offspring and were siblings of the males… but as we’ll see in a moment, extreme caution is warranted when trying to identify female scuttle flies.


I sent the flies off to the Los Angeles Natural History Museum, along with the one that Emily and I later described as Megaselia nantucketensis. In July of last year, Emily informed me that they appeared to be another new species, and the same as a couple of specimens that had recently been collected in Los Angeles. At the end of August, we had a paper about ready to submit that would have given this species a name, when Emily came across the suspiciously similar holotype of Megaselia globipyga, which had been collected in Idaho in 1919. A holotype is the type specimen, on which the original description of a species is based. The type series can also include any number of paratypes, which are other examples of the species examined by the species author while preparing the description. Emily had previously ruled out M. globipyga on the basis of existing keys to Megaselia species and (I believe) comparison with one or more paratypes. After slide mounting the holotype, however, she discovered that my flies (along with five from Los Angeles) matched it. Further investigation revealed that none of the paratypes were actually the same species as the holotype (with the possible exception of one that is in Brazil  and another that is missing from its pin and presumed lost). One paratype from Montana was in poor condition but appeared to be M. atrox, and the remaining two (from Washington and Idaho) belonged to an undescribed species.

The rest of this story has nothing to do with my Massachusetts flies, but it further illustrates how much work is left to do in insect taxonomy, and why you may not always get a straight answer if you send a specimen to a specialist for identification. On further investigation, Emily found that one of the paratypes of Megaselia postcrinata belonged to the same undescribed species as the two M. globipyga paratypes. The rest of the M. postcrinata paratypes belonged to three other species, and any of the four might be true M. postcrinatabut the holotype is a female, and modern taxonomy of Megaselia is largely based on male genitalia. So the status of M. postcrinata will be uncertain until someone is able to definitively associate a male with a female that matches the holotype. In the meantime, our paper designates the first paratype of M. postcrinata as the holotype of a new species, M. risoria (risor is Latin for “one who laughs”; so named because a little line on this fly’s thorax—the notopleural cleft—looks like a smile). The paratypes of M. risoria include the two aforementioned paratypes of M. globipygaas well as a specimen recently collected in Los Angeles. Although I am technically a coauthor of M. risoria, I can take absolutely no credit for this taxonomic wizardry.

This week I’m sending Emily another set of scuttle flies, which emerged last month from larvae I collected last May. What can of worms am I opening this time?  We shall see…

* Hartop, Emily A., Maria A. Wong, and Charles S. Eiseman. 2016. A new species of Megaselia Rondani (Diptera: Phoridae) from the BioSCAN Project in Los Angeles, California, with clarification of confused type series for two other species. Proceedings of the Entomological Society of Washington 118(1):93-100.

About Charley Eiseman

I am a freelance naturalist, endlessly fascinated by the interconnections of all the living and nonliving things around me. I am the lead author of Tracks & Sign of Insects and Other Invertebrates (Stackpole Books, 2010), and continue to collect photographs and information on this subject. These days I am especially drawn to galls, leaf mines, and other plant-insect interactions.
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10 Responses to More Fun With Scuttle Flies

  1. Fay Cotton says:

    Just love reading this fascinating story, with the incredible photos and useful links. I am interested in moths as an amateur photographer of them, plus I have a row of dingleberries growing in my ditch, plus a white spotted tussock moth came in on a shoe of mine last summer, now I am going to look much more closely at them. Just amazing how the more you look in nature, the more you see, especially with a digital camera; my $69.99 Canon Powershot 1400 opened up a whole new dimension in my naturalist sleuthing. Thank you so much for sharing this discovery in plain English for the likes of me. The wonderment never ceases.

  2. stubirdnb says:

    As Fay has already said this is a fascinating story and I am so glad that you conduct these explorations and share your findings with us. Thanks so much, Charley!

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  4. Wow, it’s an a great story and not to sound weird or anything, but it’s an adorable creature. I had never heard about Scuttle Flies but you got me with the first pic! Thank you for the sharing. Now I’m pumped to learn more about these flies.

  5. neil dear says:

    I used to have these in my house and was also suffering from clothes moths.I was thinking about these little flies(which I thought were wasps) and how they sustained themselves.I guessed that maybe they were laying eggs in the moth larvae.Your essay is interesting and from what you say it may be possible.I like these little fellows for the seemingly mad way they act.
    I also have never known what they were called and spent time trawling images of wasps,I eventually found out via reading about mice and then to here.

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