Back when I lived in Tucson, Arizona, I would sometimes see strange-looking flies on the the Prickly Pear cacti. The flies were about the size of Housefly – maybe 4-5 mm long. But they had a rounded, almost humpbacked, appearance to them. They didn’t move like flies, they scuttled over the Prickly Pear pads, running between the clusters of spines. They could fly – they would fly away if you got too close. But they seemed to prefer…well, running around on the pads.
I noticed them especially in the Spring time. These photos are from a couple of different locations, but all are from April, 2005. There would often, but not always, be two of the flies near each other on the cactus pads.
Their rounded appearance was partially caused by the way they usually carried their bodies in a ‘c’ shape, with their abdomens bent forward. And, oddly, their wings bent sharply to follow that curve of their abdomen.
I got a few photos of them on young Prickly Pear Cactus buds, as well. They seemed to be feeding on the nectar that was flowing out of the flower buds.
A Puzzle, and a puzzle solved
Back in 2005, I had a hard time determining what these flies were. I came up with all sorts of tentative IDs, none of which really fit. I knew virtually nothing about fly taxonomy, and there are just so very many species of fly. I didn’t even know where to start looking…I gave up and forgot about the flies.
Recently, though, I was browsing through Stephen Marshall’s book, Flies – The Natural History and Diversity of Diptera, as one does. I came across one of his photos and thought – well, that image looks familiar.
It was, finally, my mystery fly: Stictomyia, a “Picture Winged Fly”. Now I had a tentative ID, I could look up more information.
Based upon Elena Kamevena’s 2006 key, there are two species of Stictomyia flies in the US – Stictomyia longicornis and Stictomyia punctata. The two can be distinguished by a couple of characters. One is the length of the antennae – S. longicornis has relatively long antennae, and S. punctata is relatively short. Another is by the black spots below the antennae – are they on raised calluses, or on a flat surface?
To my surprise, I seemed to have both Stictomyia longicornis and Stictomyia punctata in my photos. How cool is that?
I know now that I could have submitted the images to inaturalist.org, bugguide.net, or one of the Facebook groups for an identification. What a marvelous time in which we live.
Biology and natural History
I haven’t found a lot of information on the biology and natural history of these flies.
Stephen Marshall’s book says that S. longicornis frequents and breeds on Opuntia cactus. He further suggests that at least S. longicornis may be a beetle mimic, based upon their movement and the strange ‘bent’ way that the fly holds its wings.
WD Hunter (1915) described S. longicornis as “…an extremely common insect throughout the cactus area.” He mentioned that the larvae occur “…in any part of the cactus plant that may be injured”.
Kameneva and Korneyev (2006) cite a 1913 reference that mentions rearing S. longicornis from rotting cacti. I wasn’t able to get a copy of the cited document, so don’t have more information than that.
On Common Names
Many of the online and in-print images of Stictomyia are labeled with the common name “Picture Winged Fly”. I’m thinking that the source of that common name is Stictomyia’s parent family, the Ulidiidae, whose common name is – you guessed it – the “Picture Winged Flies”. Since there are at least an estimated 700 species of fly in the Ulidiidae in North America, excluding Mexico (Wallace 2021), that common name isn’t terribly specific. Additionally, there are also a number of flies in the Ulidiidae’s parent Superfamily that are called “Picture Winged Flies”.
In his 1915 book “The Principal Cactus Insects of the United States”, WD Hunter suggested that Stictomyia longicornis be called the “droop-winged fly”. I’m not sure why that name hasn’t caught on.
An Interesting Aside – Mites!
An interesting aside. As I was examining one of the photos of the S. punctata, I noticed that it had small pinkish/white mites on its head and thorax (see the two photos below).
I think these are mites are deutonymphs – a non-feeding, dispersing stage of the mites’ life cycle. They are phoretic – they aren’t feeding upon the fly, but are just using the fly as a means of transportation. Given their appearance, I think that they might be in the Histiostomatoidea Superfamily. I’m basing that upon their general appearance and this post on Bugguide.net.
Wikipedia states that Histiomatoid mites feed on organic material and live in wet substrates such as dung, mud…and decaying plant material(!).
What better way for a mite to find decaying plant material in a desert than to hitch a ride with a fly that breeds on decaying cacti?
Sources:
Bugguide.com’s Stictomyia page
Kaitlin Uppstrom’s 2009 comments regarding mites on flies in this 2009 Bugguide.net post
Kameneva, Elena P. and Valery A. Korneyev. 2006. Myennidini, a New Tribe of the Subfamily Otitinae (Diptera: Ulidiidae), with Discussion of the Suprageneric Classification of the Family. Israel Journal of Entomology 35-36:497-586. This is the key I used to determine the species of Stictomyia in my photos.
Hunter, WD, FC Pratt, JD Mitchell. 1912. The Principal Cactus Insects of the United States. ISBN 1120917778.
Marshall, Stephen A. 2012. Flies: The Natural History and Diversity of Diptera. Firefly Books; 1st ed. August 30, 2012. 616 pages. ISBN 1770851003.
Wallace, Charles. 2021. An illustrated identification key to the genera of Ulidiidae (Diptera: Tephritoidea) of the United States and Canada. Canadian Journal of Arthropod Identification No. 45 (April, 2021). doi:10.3752/cjai.2021.45.
Wikipedia’s entry on the Astigmatina mite clade, which mentions the biology of some of the Histiostomatid mites.
