A Comparison of O. rimosa to O. bella

A Comparison of O. rimosa to O. bella

Well, folks. Sorry for the delay in updates. Work has been keeping me pretty busy leaving little time for updates. If I had a choice of using my weekends to do website updates or go out in the field, guess which one wins out every time? Yep, you guessed it, going out in the field.

Back on July 14th, I went back to the Montague Plains Wildlife Management Area to see if I could snag more O. rimosa specimens but it seemed that there was little calling and what was there seemed to be greatly diminished. I think that the season for O. rimosa is just about done. Who knows? I may get lucky and find another population somewhere. There just isn't enough time to do a thorough survey of other areas where I have read that O. rimosas are known to be. Be that as it may, as promised, I decided to do a comparison of O. rimosa and O. bella since these two species at first glance seem quite similar.

Okanagana bella and rimosa in Dorsal View.

For best results when viewing these comparisons, click on the thumbnails for a larger view then read the text as you study the larger pictures.

Of the two species, O. bella seems slightly larger in terms of gerth but overall length is about equal at 33mm including the wings. Forewing length of both species seems equal at 26mm. Wing width in O. bella at it's widest point measures 10mm while O. rimosa's wing width at it's widest point is decidedly smaller at 8mm.

O. bella would seem to have a shinier appearance where O. rimosa is flat black in color. O. bella's pronotum is uniformly black with a thin band of flat orange located medially. O. rimosa's pronotum while black contains a pattern of bright orange and black including the orange band medially as found in the latter. The ridge patterns of the prontom in both specimens are decidedly different.

The pronotal collars are similar in that both are black with a thin orange band running posteriorly. O. rimosa's orange band extends laterally left to right into an orange patch located at each end.

The mesothorax of each specimen is unremarkable with both containing the same pattern of orange and black where O. rimosa's orange pattern is brighter than in O. bella. This difference in brightness could also be due to the specimen's age.

The tergites (a total of 10 in each specimen) in O. bella are a uniform black where O. rimosa's tergites each contain a band of orange located on the posterior of each individual tergite segment. The 10th tergite tapers to a fine point in both specimens. Looking at the dorsal view one would assume these specimens were female as they both lack timbal covers so apparent in other species of cicadas including the Tibicens.

O. bella and O. rimosa En-face View.

The heads of both O. bella and O. rimosa are quite narrow and do not extend beyond the pronotum, which is a key to the genus. Width of the head from compound eye to compound eye in each specimen is approx. 6.5 mm. In the ventral view, the compound eyes of O. rimosa are more pronounced, spherical and stand off completely from the the pronotum whereas in O. bella its compound eyes indicate a fusion with its pronotum.

O. bella and O. rimosa Ventral View.

At first glance in the ventral view the opercula are decidedly longer and more pronounced in O. bella than in O. rimosa. The legs of both specimens contain varying patterns of orange and black that are not identical. For instance, the coxa of the front forelegs of O. bella are decidedly black with just a hint of a dull orange, O. rimosa's coxa while containing black still has a brighter color of orange apparent that is more pronounced.

The sternites - for the purpose of this comparison - we'll focus only on those that I can discern with the naked eye and for the purpose of this comparison are numbered one through six. In O. bella, the sternites are noticeably longer than those of O. rimosa giving it the appearance of being longer in the former than that of the latter. O. bella's sternites are more uniformly black with a hint of dull orange where in O. rimosa the opposite would seem apparent with its sternites being mostly orange with just a small coloring of black located laterally. Sternites five and six in O. rimosa seem to lack the black coloration whereas in O. bella black would seem to be the dominant color.

Conclusions and Discussion

It would seem that after comparing these two species that there are indeed morphological differences between them. However, the differences in the grand scheme of things are minor which could probably cause some mis-idenfitications out in the field especially in geographic locations where both species are known to exist.

However, in those areas at this stage of my study of these species, it is unclear if those areas for O. rimosa and O. bella are allopatric (occupying ranges that are adjacent to each other but mutually exclusive), sympatric (occupying the same geographic place) or parapatric (areas that have contiguous but non-overlapping distributions).

In my case I have found no records of O. bella ever being found here in Massachusetts and it would seem that that species is a more north and western species. So, given the conditions described in the previous paragraph, one would have to assume that the calling songs of these two species while similar in appearance would have to be drastically different in order to maintain the cohesion of both species.

Morphological Differences with O. rimosa.

All that being said above, comparing morphological differences within the species of O. rimosa has yielded some subtle differences in the sternites of all four specimens that I have collected from the same area. There are varying degrees of black in all 4 of these specimens even a noted amount of pubescence that is apparent in image number 3. Click the thumbnail to the left to enlarge.

But, this is a study for another time.