Blastodacna hellerella vs atra
For many years, I have struggled to find any convincing means of separating these species. What is written here reflects my current understanding and may be wrong! It should also be noted that I have very limited personal experience of specimens I have been happy to call B.atra.
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External features:
Base of forewing dorsum: in B.hellerella largely white with some scattered fuscous scales; in B.atra heavily scattered with fuscous scales
This typically results in B.hellerella having a pied appearance and B.atra appearing dark overall.
Both species have a white head, a white thorax with a dark median line, 2 black forewing scale-tufts, extensive "solid" fuscous colouring along the costa and 2 (somewhat variable) chestnut streaks. The basal chestnut streak is longer and separates the base of the dorsum (described above) from the base of the costa; it extends from the forewing base to just distal to the proximal scale-tuft (it is possible that in B.atra the scale-tuft interrupts the chestnut line while in B.hellerella the chestnut line runs on the costal side of the scale-tuft). The distal chestnut streak is short: in B.atra it extends from just proximal to mid-wing to the distal scale tuft and has a few white scales around it; in B.hellerella it is in a similar position but usually poorly marked and may be absent. In B.hellerella the distal scale-tuft has a white line on its costal and proximal sides separating it from the fuscous costa and the distal chestnut line; in B.atra the black scale tuft is "continuous with" the fuscous costa with at most a few white scales between it and the distal chestnut line.
Judging from web images it seems that intermediate forms and variations occur, that many specimens are not "typical" and wear will reduce the degree of difference between the species with respect to these external features. And it is entirely possible that specimens showing the "typical" features of one species may prove to be the other.
MBGBI4.1 distinguishes B.hellerella as having the white parts of the forewing pure white and B.atra having these areas creamy white with additional yellow-ochreous scales at the forewing base. In comparison of material I have examined and web images, these described differences are wholly unconvincing.
Sterling & Parsons illustrate B.atra as having the "white parts" heavily speckled with fuscous scales (including the thorax), without chestnut lines and with a white patch proximal to each scale tuft. One image of this species at Moth Dissection is similar to this illustration (in neither case is the speckling yellow-ochreous). Other images of B.atra at Moth Dissection are indistinguishable from those of B.hellerella.
UK Moths shows a dark form of B.hellerella which could not be identified from the above description.
Base of forewing dorsum: in B.hellerella largely white with some scattered fuscous scales; in B.atra heavily scattered with fuscous scales
This typically results in B.hellerella having a pied appearance and B.atra appearing dark overall.
Both species have a white head, a white thorax with a dark median line, 2 black forewing scale-tufts, extensive "solid" fuscous colouring along the costa and 2 (somewhat variable) chestnut streaks. The basal chestnut streak is longer and separates the base of the dorsum (described above) from the base of the costa; it extends from the forewing base to just distal to the proximal scale-tuft (it is possible that in B.atra the scale-tuft interrupts the chestnut line while in B.hellerella the chestnut line runs on the costal side of the scale-tuft). The distal chestnut streak is short: in B.atra it extends from just proximal to mid-wing to the distal scale tuft and has a few white scales around it; in B.hellerella it is in a similar position but usually poorly marked and may be absent. In B.hellerella the distal scale-tuft has a white line on its costal and proximal sides separating it from the fuscous costa and the distal chestnut line; in B.atra the black scale tuft is "continuous with" the fuscous costa with at most a few white scales between it and the distal chestnut line.
Judging from web images it seems that intermediate forms and variations occur, that many specimens are not "typical" and wear will reduce the degree of difference between the species with respect to these external features. And it is entirely possible that specimens showing the "typical" features of one species may prove to be the other.
MBGBI4.1 distinguishes B.hellerella as having the white parts of the forewing pure white and B.atra having these areas creamy white with additional yellow-ochreous scales at the forewing base. In comparison of material I have examined and web images, these described differences are wholly unconvincing.
Sterling & Parsons illustrate B.atra as having the "white parts" heavily speckled with fuscous scales (including the thorax), without chestnut lines and with a white patch proximal to each scale tuft. One image of this species at Moth Dissection is similar to this illustration (in neither case is the speckling yellow-ochreous). Other images of B.atra at Moth Dissection are indistinguishable from those of B.hellerella.
UK Moths shows a dark form of B.hellerella which could not be identified from the above description.
Male genitalia:
The critical feature is the shape of the apical ½ of the juxta lobes: in B.hellerella this is rounded and narrower than the basal ½ (due to concavity of its posterodorsal edge); in B.atra it is angular (in the set position this gives a sharp obtuse anterolateral angle, a straight lateral edge, and a bluntly rounded, and very finely dentate acute posterolateral angle).
There is a substantial difficulty in getting the juxta lobes to set flat for a slide prep - partly because they naturally lie in the dorso-ventral plane, partly because they are very small and fiddly to manipulate and partly because their attachment to each other means that manipulating one lobe into the desired position tends to pull the other out of position (it may be that the identifying features can be best/most easily observed in a lateral view).
To date I have not discerned any consistent difference in the aedeagal cornuti.
Comparative images are shown at Moth Dissection and several dissections are shown of each species. Images are also available of both species at Lepiforum. Both these sources show some images similar to those shown below, matching the description above. They also show another form with a rounded apex and a spur at a little distance from the apex - this latter form being labelled as B.atra and it is this form that features on the comparative image at Moth Dissection - is there a 3rd species involved?
The critical feature is the shape of the apical ½ of the juxta lobes: in B.hellerella this is rounded and narrower than the basal ½ (due to concavity of its posterodorsal edge); in B.atra it is angular (in the set position this gives a sharp obtuse anterolateral angle, a straight lateral edge, and a bluntly rounded, and very finely dentate acute posterolateral angle).
There is a substantial difficulty in getting the juxta lobes to set flat for a slide prep - partly because they naturally lie in the dorso-ventral plane, partly because they are very small and fiddly to manipulate and partly because their attachment to each other means that manipulating one lobe into the desired position tends to pull the other out of position (it may be that the identifying features can be best/most easily observed in a lateral view).
To date I have not discerned any consistent difference in the aedeagal cornuti.
Comparative images are shown at Moth Dissection and several dissections are shown of each species. Images are also available of both species at Lepiforum. Both these sources show some images similar to those shown below, matching the description above. They also show another form with a rounded apex and a spur at a little distance from the apex - this latter form being labelled as B.atra and it is this form that features on the comparative image at Moth Dissection - is there a 3rd species involved?
Blastodacna hellerella, male genitalia with tegumen and aedaegus removed
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Blastodacna atra, male genitalia, aedeagus removed
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Female genitalia:
The situation with females is even more unsatisfactory. Moth Dissection shows a single image of B.atra which does not seem to provide any clear means of distinguishing it from B.hellerella. Lepiforum shows no female images of B.atra. and I have no personal experience of any convincing female B.atra.
The situation with females is even more unsatisfactory. Moth Dissection shows a single image of B.atra which does not seem to provide any clear means of distinguishing it from B.hellerella. Lepiforum shows no female images of B.atra. and I have no personal experience of any convincing female B.atra.
Page published 20/07/2022
