Fly - Phasia fenestrata

Moved

Moved from Phasia.

… Martin Hauser, 22 December, 2011 - 2:21pm

Species possibilities here perplexing

I've recently become fascinated with Phasia and have been studying the reference by Sun & Marshall(1) that Chris mentioned in his comment below, as well the current BugGuide posts and whatever other resources I've been able to muster on the web. Here are some thoughts on this post...

The most distal segment of the longitudinal wing vein R4+5 (seen here reaching the wing edge just a bit behind the wingtip) is called the "petiole (of the apical cell)". A key diagnostic character used for separating species of Phasia is the ratio of the length of this "petiole" to the length of its preceding segment on the vein R4+5.

I carefully measured this "petiole ratio" for the fly here to be between 0.16 and 0.18, using the full-size version of the image posted. That value of the ratio is the equivalent to saying the penultimate segment of R4+5 is 5.5 to 6 times the length of the petiole. Those values don't fall within the ranges of any of the nearctic species treated in Sun & Marshall. So, as a next best strategy, we might consider species with petiole ratios nearest to the 0.16-0.18 value here.

One can check that there are only 4 out of the 14 nearctic species of Phasia treated in Sun & Marshall that have a petiole ratio less than 0.25---P. grandis (0.13-0.15); P. aurulans (0.21-0.24); P. albipennis (0.22-0.24); and P. chilensis (0.24). The other 10 nearctic species have petiole ratios of at least 0.25, meaning their penultimate segment of R4+5 is at most 4 times as long as the petiole.

Of the four candidate species above, it seems that:

1) P. grandis can be eliminated as it's described as having eyes almost touching (unlike the image here, where they're separated by a distance greater that the ocellar triangle); and having the head spherical (unlike the head here, which appears anteriorly compressed).

2) P. aurulans is eliminated because males have a large conspicuous golden spot on the thorax (missing here), while females have hyaline wings.

3) P. albipennis is only recorded from west of the Rocky Mountains.

4) Females of P. chilensis have hyaline wings and eyes almost touching, while males have eyes separated by the width of a single ocellus. Again, this species is described as having head spherical...and the sharp value of 0.24 for the petiole ratio seems significantly distant from the value of 0.16-0.18 here.

So I'm left perplexed, and thinking this may be one of the two formally undescribed species, P. lactuosa or P. occidentis, mentioned in the NADS Catalogue web page...or a potentially new taxon.

… Aaron Schusteff, 22 December, 2011 - 4:06am

Hi Aaron, I asked Jim O'Hara

Hi Aaron, I asked Jim O'Hara about this fly and he answered:

"Phasia fenestrata (Bigot) The wings can be quite variable"

Maybe the wing character is not the best?
Cheers
Martin

… Martin Hauser, 22 December, 2011 - 2:19pm

Hooray!! A second Species Page for genus Phasia!

And many thanks, Martin...to you and Jim O'Hara...for rescuing me from perplexity :-)

I believe the epithet fenestrata means "windowed"...appropriate for the patterned wings here. And I guess this is a male, since females of the species are described as having hyaline wings.

I had noticed earlier that, in many ways, P. fenestrata was a very good fit here (the yellowish tibiae being particularly distinctive). But I was deterred by my focus on the "petiole ratio" character, given as ranging from .25-.30 for fenestrata, which seemed much too large to me. But sadly, I guess you're right about that not being the best character. It's attractive because it's an unambiguously measurable character. The mathematician in me loves such clarity, and endlessly struggles with the wrench that "variability" so often throws into the gears of trying to nicely organize taxa :-) But in hindsight I notice that...while Sun & Marshall list this character in each of their descriptions...they don't really emphasize it in their keys.

BTW, I've been going through a "Phasia phase", trying to see if I can make some significant progress in sorting out BG posts into species...inspired mainly by the fine work of Sun & Marshall(1). Most all I've learned comes from carefully studying that work. The problem is that, without the perspective coming from having viewed lots of specimens, I can make naive errors due to lack of awareness of gestalt and variability. So I really appreciate your help here, and hope you, Jim O'Hara, and others will help keep me from going too astray :-)

I get the impression the Phasia is generally viewed as a difficult genus. But I can't help thinking (hoping?) that some taxa should be distinctive enough to place to species...the present post being an example. Here are some other distinctive looking posts:

It would also be helpful if we could get some representatives of the most common species established on the guide, like P. aldrichii and P. robertsonii.

Possible P. alrichii:

Possible P. robertsonii:

Maybe I'm hoping for too much here, unless we get lucky and Santa Fly drops by and ID's some of these for us :-)

… Aaron Schusteff, 23 December, 2011 - 3:09am

I agree, the genus is awesome

I agree, the genus is awesome... but not easy... and Jim does not like to ID specimens from pics, although I am sure he knows them all... and once in a while I can ask him for a favor...
Cheers
Martin

… Martin Hauser, 23 December, 2011 - 9:29pm

Yes: Neat genus...but not easy! :-)

And after straining over many images to interpret what's visible...and bemoan what isn't...I can understand a dislike of identifying from photos. (I sure have been wishing I could look at the specimens in many of these posts under a microscope!!)

That being said, it's also true that despite the shortcomings and frustrations of trying to ID from photos, it can be particularly satisfying when fortune smiles, and you're able to make out enough distinctive characters to cinch an ID :-) And a big plus with BugGuide is the image will be easily accessible for interested persons to study and appreciate...whereas in-hand specimens are quite difficult to access for most people.

At any rate, I'm extremely grateful for the help Jim, you, and other experts provide on BugGuide. People like me may have enthusiasm, and the patience and ability to read well-written references, and try to decipher them to make a putative ID from a workable photo. But we lack professional training and the time-earned expertise and familiarity coming from studying a multitude of specimens in hand. So best to Jim & you...and Merry Christmas :-)

… Aaron Schusteff, 24 December, 2011 - 2:15pm

Hi Aaron, in my experience th

Hi Aaron, in my experience the genus isn't all that difficult but it does help if you can double-check the genitalia when dealing with areas that have a lot of species ... and of course that's where in-field photos usually let you down. As you quite rightly say, the experts have years of experience confirming identifications using specimens and this means that, once sorted, they can notice trends in morphology that, while they are not stable enough to be worthy of a proper key, they are occasionally indicative of a species and can be seen in photos. The IDs you get from photos are therefore usually best said like "It *looks* like X", which isn't always the same as "It *is* X" ;)

… Chris Raper, 24 December, 2011 - 5:37pm

Well said, Chris!

I guess there are many factors involved in determining whether sub-taxa of a given taxon are amenable to ID from field photos. Like:

Do the sub-taxa have (field photo) visible characters that distinguish them?
Has someone noticed and made known such characters yet?
Is the distinctness of such characters stable over large samples in space, time, and biological context...or does variability blur character boundaries between sub-taxa?
If you're not an expert, is there a reference available with keys or descriptions that don't depend mostly on "photo-cryptic" characters (like genitalia)?
Are the angles and resolution of the photo(s) sufficient to see the character(s)?

For Phasia, a species like P. aurulans (with its distinctive thoracic spot...which I hope is reasonably stable!) is fairly easy to recognize from a photo (and your comments are what first led me to that...thanks!). Others, like P. aldrichii seem more difficult (at least for a non-expert). If distinguishing characters are obscured or enclosed (like genitalia often are) then I guess the best we can do with field photos alone is to put them into the smallest groups that "look alike". And if possible, use info on distribution, host, phenology to help narrow things down, though that can be problematic too (due, as usual, to incomplete knowledge and variability).

… Aaron Schusteff, 28 December, 2011 - 4:15pm

PS...

Here's a field post that shows a bit of detail of the terminalia:

Any guess on species? (I'm thinking P. robertsonii, or P. aldrichii.)
It's indicated as a male in the post...I presume from the terminalia?

… Aaron Schusteff, 28 December, 2011 - 4:20pm

Very nice Phasia sp.

That's a lovely Phasia and not one that I recognise from Europe but there is a world revision & key by Sun & Marshall ("Systematics of Phasia Latreille (Diptera: Tachinidae)", 2003, published in Zootaxa), which is available online and is reasonably easy to use if you have a microscope :)

Let us know what you think it is if you have a chance to key it :)

… Chris Raper, 10 December, 2009 - 6:22pm

Phasia sp.

… Norm Woodley, 15 April, 2009 - 9:28am

It is a Tachinid, very likely

It is a Tachinid, very likely genus Phasia. They are amazingly pretty...

… Martin Hauser, 21 February, 2008 - 9:07am