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Photo#964084
Obese acrocera - Acrocera obsoleta - male

Obese acrocera - Acrocera obsoleta - Male
Scott's Valley, Santa Cruz County, California, USA


The acrocerid imaged in this series is from Randy Morgan's synoptic insect collection at UC Santa Cruz. (I misplaced my record of the collection date, but will try to get it next time I visit the collection.)

From the virtually imperceptible mouthparts; the thread-like antennae attached near the top of the head; and the bare eyes...one can see that the genus here is Acrocera (see "Identification" on the BugGuide Acroceridae info page).

To get to species I cross-referenced using a number of sources: Johnson (1915); Cole (1919); and Sabrosky (1944, 1948). See "Print References" on the Acrocera info page for bibliographic details. From the keys and discussions in these references, I've confidently concluded this is a male of A. obsoleta. The details of how all this panned out appear below.
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Starting with the earliest reference, Johnson (1915), this specimen goes to A. obsoleta via the following characters, all visible among the 4 images in this post:

Dorsum of thorax black; 2nd longitudinal vein (=R2+3) lacking; both branches of 3rd longitudinal vein (i.e. R4 and R5), as well as the anterior cross vein (r-m), are present; abdomen yellow except at very base; veins extremely light.
Johnson's description of A. obsoleta states:

"The abdomen is bright yellow except for a narrow basal margin, a dorsal triangle at the base of the third segment, and a small anal spot of black."
That, and his description of the venter and wings, all fit the specimen here perfectly.
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In the key in Cole (1919), things go about the same as in Johnson's key...except at the end, where Cole separates A. obsoleta from his newly described species, A. convexa. These two are distinguished by Cole as follows:

A. obsoleta: Veins, except first, with an obsolete (i.e. weak) appearance.
A. convexa: Veins black, or at least distinct; legs pale yellow; the abdomen with basal black spots on 2nd, 3rd and 4th segments.
In Cole's description of A. obsoleta he mentions specimens from Maine, California, and Washington with the following characters:

The humeri and postalar callosities are white; thorax and pleura black; abdomen bright yellow except narrow basal margin, a dorsal triangle at base of third segment and a small anal spot of black. Males with legs honey yellow, claws and last tarsal joint black. Scutellum jet black.
All the above characters for A. obsoleta fit the specimen in the images here exactly. In particular, note the above description does not mention a black triangle at the base of the 4th tergite...even though one appears in Cole's Fig. 36c for the abdomen of a male A. obsoleta. I presume this means the absence of the medial black triangle on the 4th tergite is within variation for A. obsoleta. This was reassuring, as the specimen here does not have such a marking on the 4th tergite.

Cole's description of A. convexa indicates that it is very similar to A. obsoleta, but that A. convexa differs in having scutellum mostly yellow; an irregular black spot at the base of the 4th tergite; and (from the key break) legs pale yellow. For reference, A. convexa is dipicted in Figs. 29 and 29a of Cole. Note that Cole's descriptions of the abdominal venters of both A. convexa and A. obsoleta are similar, and that the portion of the venter visible in the 3rd image of this post is consistent with those descriptions, and with what is visible in Cole's Fig. 29.

So...according to Cole's circumscription of Acrocera, this specimen again goes to A. obsoleta.
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Now we get to the somewhat more complicated and messy part, namely, the most recent treatments of Acrocera in Sabrosky's 1944 and 1948 papers.

One of the complicating issues here is that Sabrosky had encountered a 1926 paper by E. A. Brunetti (Ann. Mag. Nat. Hist., series 9, 18:561-606) where a number of type specimens for Acrocera had been redescribed in more detail than in the original descriptions given by J. O. Westwood in 1848. (Some of Westwood's descriptions were too terse and inadequate, and his actual type specimens were apparently not seen by Johnson, Cole, or Sabrosky.) From reading the comments under A. unguiculata on pg. 407 of Sabrosky (1948), and comparing Sabrosky's keys with those of Johnson and Cole...it seems that Brunetti's notes indicated (among other things) that the venation characters relating to the "presence or absence of R4" in the type specimens of A. unguiculata and A. bulla were the opposite of what had been presumed by Johnson and Cole. This introduced a messy taxonomic/nomenclatural problem which ended up affecting Sabrosky's circumsciption of A. obsoleta.

Sabrosky also emphasized that, in addition to what already appeared to be significant variation in color patterns among multiple specimens of previously named taxa, the presence of substantial sexual dimorphism was a significant confounding factor...and that some males and females of the same species had been assigned different names based on lack of adequate attention to this issue. All this highlighted the difficulties arising from having too few specimens to work with...more complete series for both males and females were needed to resolve issues of "variation within a putative species" vs. "differences between distinct species" vs. "differences due to sexual dimorphism".

As a consequence of this, some of Sabrosky's circumsciptions (in particular for females of A. obsoleta and A. unguiculata) depart significantly from those of Cole and Johnson. Sabrosky emphasized that his revised circumscription of Acrocera was provisional, and hoped further study might resolve the uncertainties. I'm not aware of any more recent publications on the nearctic Acrocera that may have resolved these issues. (I'd love to know if such do exist!). But if the actual type specimens are still available, Brunetti's notes on the types (and Cole's treatment) could be checked for accuracy. Also, I'd guess there are a lot more Acrocera specimens available for study now. So the difficulties encountered by Sabrosky in 1948 may be amenable to resolution at this time.

But, thankfully, all this doesn't happen to impact the outcome of the species ID for our particular individual in this post (though it would have if it had been a female instead of a male!).

So...below are the details from the couplets encountered in running the specimen here through Sabrosky's key:

1) 2nd longitudinal wing vein (i.e. R2+3) absent; 2) 3rd longitudinal vein forked in apical portion of wing (i.e. R4 and R5 both present); 3) Abdominal sternites II-V mostly yellow, with the basal black bands narrowed mesad and broadened laterad to nearly the length of the segment; 4) Abdominal tergites mostly yellow, only the 2nd tergite with a complete anterior black band; 5) male; 6) the 3rd (and sometimes 4th) tergite with a small black triangular spot antero-medially; 7) Third and fourth tergites with only a small area of black in the anterolateral corners (Cole, 1919, fig. 36b) ; proleuron black; fifth abdominal segment not telescoped but characteristically indented, its entire dorsum visible in caudal aspect (Cole, 1919, fig. 36b, appearance in profile) .............. obsoleta Van der Wulp ( ? )
In the discussion of A. obsoleta on pg. 406 of Sabrosky (1948), it's stated that:

"The males of this species have a distinctive appearance, though it is still possible that a good series of specimens would show intergradation with A. unguiculata on the one hand and A. convexa on the other."
On pg. 404, Sabrosky indicated he was able to examine only 5 specimens of A. convexa: 2 males (including the holotype) and 3 females. He stated that the two male A. convexa shared the distinctive "indented" 5th tergite character of A. obsoleta. However they lacked the black medial triangles on tergites 4 or 5 which were present in the females, having only a single medial black triangle on the 2nd tergite, well-separated from the black antero-lateral patches on that tergite. If these 5 specimens were characteristic of the species, then Cole's Fig. 29a presumably shows the abdomen of a female A. convexa. At any rate, in contrast to our specimen, both male and female A. convexa are apparently understood to have an isolated black medial triangle on tergite 2, as opposed to the complete basal band present in A. obsoleta.

For females, Sabrosky was not able to give consistent characters for separating A. obsoleta and A. unguiculata, due to mainly to the complications resulting from his acceptance of Brunetti's description of the type of A. unguiculata, which stated it had both R4 and R5 present (as opposed to just R5), which invalidated the assumptions on which Cole's treatment was based.

So despite all the complications...even if A. obsoleta, A. convexa, and A. unguiculata may ultimately be understood as "clusterings" within the continuum of a single variable taxon...the upshot here is that this specimen lies squarely within the species (or "clustering") corresponding to the name A. obsoleta!!
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In summary, a careful study of the treatments in each of the references cited above strongly supports the determination of A. obsoleta for the specimen here. Figures 36b and 36c of Cole (see here) show lateral and dorsal views of the male abdomen, and Fig. 29 of Cole (here) shows a profile habitus of the very similar species A. convexa. It's worth emphasizing that the specimen in this post can be seen to be a male, due to the distinctive trait in male A. obsoleta (and A. convexa) of having the 5th tergite dorsally "indented" or flattened (as seen in Fig. 36b of Cole).

Images of this individual: tag all
Obese acrocera - Acrocera obsoleta - male Obese acrocera - Acrocera obsoleta - male Obese acrocera - Acrocera obsoleta - male Obese acrocera - Acrocera obsoleta - male