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Genus Physocephala

Fly - Physocephala marginata 6012792 Physocephala - Physocephala tibialis Wasp on yarrow - Physocephala sagittaria Conopid being attacked by an ant - Physocephala Fly - Physocephala tibialis Physocephala sp. - Physocephala tibialis - male Thick-Headed Fly - Physocephala tibialis Physocephala 460A 8209 & 8214 & 8219 - Physocephala
Classification
Kingdom Animalia (Animals)
Phylum Arthropoda (Arthropods)
Subphylum Hexapoda (Hexapods)
Class Insecta (Insects)
Order Diptera (Flies)
No Taxon ("Acalyptratae")
Superfamily Sciomyzoidea
Family Conopidae (Thick-headed Flies)
Subfamily Conopinae
Genus Physocephala
Explanation of Names
Physocephala Schiner 1861
Numbers
7 spp. in our area(1)
Size
5-14 mm(1) (guide photos)
Identification
Adults resemble solitary wasps in general shape and color. They have a petiolate abdomen (i.e. with a long, narrow second segment...followed by broader, shorter ones). The proboscis is long, straight and needle-like, projecting forward well beyond the face. The antennae are also long and straight, positioned close together, and point forwardly-upward (i.e. are porrect).
Physocephala are often confused with Physoconops, which they closely resemble...for details on distinguishing the two, consult the "See Also" section below.
Key to New World Physocephala species in Camras (1996)...available online here.
Terms commonly used to describe and identify Physocephala species are illustrated and defined in the following two labelled image posts:
       
A summary of key characters, with their various states and associated species, are listed below. Note that some species exhibit variable states for certain characters, and so may be listed more than once per line:
discal cell: hyaline = clear (furcillata, marginata, texana, burgessi); infuscate = darkened (tibialis, sagittaria, floridana). cheeks: solidly dark black (tibialis, furcillata, floridana); reddish (burgessi); yellow-to-whitish (texana); or dark brown on fore and aft edges, with a yellow spot in middle (sagittaria, marginata, texana). facial grooves: black (tibialis, floridana); dark brown (sagittaria, marginata); or pale white to yellowish (furcillata, burgessi, texana). facial keel: black, at least near top (tibialis, floridana); pale brown to yellow (sagittaria, marginata); or pale white to yellowish (texana, burgessi, furcillata). humeral spots: white to yellowish pollinose (="waxy-dusted") and widely dash-shaped (marginata, sagittaria, texana); narrower, inset from lateral edges, and faintly white pollinose...or absent (tibialis, furcillata); red...not pollinose (burgessi, floridana). scutellum: black or dark brown (tibialis, furcillata, sagittaria); reddish (marginata, burgessi, texana, floridana, sagittaria). vertex: black or very dark brownish (tibialis, furcillata, marginata, sagittaria); reddish (burgessi, texana, floridana, sagittaria); or yellowish-brown (sagittaria). 1st antennal segment: less than half as long as 3rd antennal segment (furcillata, burgessi); roughly similar in length to 3rd segment (tibialis, sagittaria, marginata, floridana, texana). costal cell: hyaline or lighter colored than adjacent cells (furcillata, burgessi); dark and concolorous with adjacent cells (marginata, floridana); not consistent...usually concolorous with adjacent darkened cells, but often lighter, or even hyaline (tibialis, sagittaria, texana). "T"-shaped marking on frons: black and fairly wide (tibialis, furcillata, marginata, sagittaria, floridana); reddish brown and fairly wide (sagittaria); dark to light reddish brown, and relatively thinner (texana, burgessi); or entirely yellowish (texana).
Range
Worldwide, incl. most of our area
Habitat
Adults visit flowers during the day.
Season
Adults fly from late spring to late summer.
Food
Larvae parasitize various Apoidea/Sphecoidea & Vespidae; adults feed on nectar.
Females usually oviposit on hosts in flight.
Life Cycle
Adults alight on and inject an egg into the abdomen of their hymenopteran hosts while in flight, or after the pair tumble to the ground following aerial impact of the fly with the host.
P. tibialis has been recorded parasitizing workers of the bumble bee Bombus bimaculatus.
A study in Alberta showed that bumble bees parasitized by P. texana had the same lifespan as unparasitized individuals (Otterstatter et al. 2002)(2)
Remarks
The most commonly seen genus of Conopidae(3)
Males and females are both dichoptic (i.e. have eyes widely separated). Females can be distinguished by the conspicuous bump below their abdomen called the theca, which males lack. The theca is attached to the 5th sternite, but is often flap-like and folded against the 4th sternite. It is used to help grasp and pry open the tergites of the the host insect while inserting an egg into the hosts abdomen.
See Also
A small group of genera in Syrphidae (tribe Cerioidini) are often mistaken for Conopinae.
Within the subfamily Conopinae, Physoconops closely resemble Physocephala and the two genera are often confused. They can usually be distinguished by examining the wing venation and the shape of the hind femora.
Wing Venation In Physocephala, the anterior crossvein (denoted "r-m" in the diagram below), is located well beyond the middle of discal cell (denoted "dm"); and the discal cell is strongly "indented" just basad of the crossvien r-m. Note that the tiny crossvein bounding the discal cell at its base is usually hard to see, but it is always just beyond where the vein on the lower edge of the discal cell meets cell "cup" (cf. 2nd diagram below):

In Physoconops, the crossvein r-m is near or before the middle of cell dm; and the discal cell is typically less strongly indented basad of crossvein r-m, and often more-or-less straight along its upper edge:
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Hind femora In Physocephala the the hind femur is irregularly thickened and widest in its basal portion:

In Physoconops, the hind femur has a more uniform thickness and is widest near the middle:

Note that the hind femur character is best employed with a profile view. From some angles the basal swelling in Physocephala may not be clearly apparent.
To be confident of an identification, it's best to have a number of characters available for verification, since some may be difficult to interpret (or even "not work", due to variation), or they may not be visible. While the above characters are the primary ones used to separate Physocephala and Physoconops in most keys, and are presumably the most consistent and dependable ones, the following additional characters are also useful in distinguishing these two genera:
1) Physocephala do not have ocelli, whereas Physoconops have very small ocelli located on a swelling at the vertex(4) [see comment here by Jeff Skevington]. 2) Most Physocephala have a sharply defined "T"-shaped marking on the frons, most Physoconops do not. 3) In Physoconops the last few tarsi on all legs are typically dark black and the pulvilli are large and yellow; whereas in Physocephala the tarsi are typically narrower, lighter reddish or yellowish, and the pulvilli and darker (with little or no yellow) and typically smaller. 4) In some (but not all!) Physoconops the 3rd antennal segment is noticeably much longer than it is in Physocephala. 5) In some (but not all!) Physoconops the female's theca is noticeably much longer than it is in Physocephala.
Print References
Camras S. (1957) A review of the New World Physocephala (Diptera: Conopidae)". Ann. Ent. Soc. Am. 50:213-218.
Camras S. (1996) New information on the New World Physocephala (Diptera: Conopidae). Ent. News 107: 104-112 (Full text)
Otterstatter M.C., Whidden T.L., Owen R.E. (2002) Contrasting frequencies of parasitism and host mortality among phorid and conopid parasitoids of bumble-bees. Ecol. Entomol. 27(2): 229-237.
Internet References
Revista Brasileira de Entomologia. Larval development of Physocephala (Diptera, Conopidae) in the bumble bee Bombus morio (Hymenoptera, Apidae)
Works Cited
1.American Insects: A Handbook of the Insects of America North of Mexico
Ross H. Arnett. 2000. CRC Press.
2.The Conopid Flies of California
Sidney Camras and Paul D. Hurd, Jr. 1957. University of California Press.
3.Insects: Their Natural History And Diversity: With a Photographic Guide to Insects of Eastern North America
Stephen A. Marshall. 2006. Firefly Books Ltd.
4.Manual of Nearctic Diptera Volume 2
Varies for each chapter; edited by J.F. McAlpine, B.V. Petersen, G.E. Shewell, H.J. Teskey, J.R. Vockeroth, D.M. Wood. 1987. Research Branch Agriculture Canada.