Burmite Cretaceous Soft Tissue: Remarkable preservation of Dipteran Indirect Flight Muscles

[Biotalker]

Here is a Cretaceous Burmite Midge that appears to show preservation of the indirect flight muscles in one hemithorax. Indirect flight muscles distort the thorax cuticle from the inside to indirectly move the insect’s wings and power flight.  While this level of microscopy cannot prove the tissue is muscle, it seems likely that the two sets of power muscles for flight are visible in this fossil: six dorsolongitudinal muscles (DLM) and six dorsoventral muscles (DVM). There are smaller muscles that play a role in guidance and directional movement that may be present but cannot be unambiguously determined without better tools. This midge is on the surface of the amber on its mid-sagittal plane with half of the insect gone.

[Biotalker]

Classic diptera indirect flight muscle:

 

 

 

 

 

[Biotalker]

There is a far more common apparent preservation of soft tissue in the legs of this specimen.

[Biotalker]

And perhaps the most speculative is the interesting tissue preservation with the head of this specimen. However unlikely, it seems to have some order to it. I wouldn't have thought this significant if this fossil didn't already show unusual soft tissue preservation.

[JohnJ]

@FossilDAWG

[FossilDAWG]

Very interesting!  I can't think of anything else that would account for the thoracic structures.  The head structures are less obvious to me.  I will run the photos past a colleague who is more familiar with insect morphology to see what he thinks.

 

Don

[Crusty_Crab]

Do you have a copy of Snodgrass' Principles of Insect Morphology. It was written in 1935 but is still excellent. Your picture is very suggestive of an optic lobe and protocerebrum, (see Figure 248). However, the experts will have the final say.  

[Biotalker]

Here is another example of a burmite dipteran I have acquired with uncommon muscle preservation. In this case, it involves the muscles within the two proximal segments of each leg, the coxa and femur. The two discernible muscles within the femur can be easily seen, the levator of tibia and the depressor of tibia.

[Biotalker]

Fly muscles

[Biotalker]

On 10/9/2021 at 1:39 PM, Crusty_Crab said:

Do you have a copy of Snodgrass' Principles of Insect Morphology. It was written in 1935 but is still excellent. Your picture is very suggestive of an optic lobe and protocerebrum, (see Figure 248). However, the experts will have the final say.  

I don't have a copy of Snodgrass' book, but I will as soon as it's delivered (I already have a soft spot for older biology/fossil books). I do have some experience working with Drosophila brains and the shape of the tissue in the head of this amber is more compelling under the scope than my so-so photography. Thanks for the comment.

[Crusty_Crab]

On 10/11/2021 at 2:22 PM, Biotalker said:

I don't have a copy of Snodgrass' book, but I will as soon as it's delivered (I already have a soft spot for older biology/fossil books). I do have some experience working with Drosophila brains and the shape of the tissue in the head of this amber is more compelling under the scope than my so-so photography. Thanks for the comment.

I think you'll find it very interesting. It seems that drosophilists have their own terminology versus entomologists. For example, the dorsoventral muscles are probably more precisely called the tergosternal muscles (since their attachment points are the sclerotized parts of the dorsum, called tergites, and the sclerotized parts of the venter, called sternites). The dorsolongitudinal muscles could also be described as the oblique lateral dorsals since their attachment points would seem to be the scutum of the mesothorax and the posterior phragma of the mesothorax (my understanding is that true dorsal muscles would attach between the anterior and posterior phragma of the mesothorax). The phragma are large involutions of the exoskeleton that provide interior attachment points.  I know its splitting hairs, but this matters because there are other muscles along the dorsoventral axis that are not involved in flight, for example the tergal promotor and remotor muscles that are involved in leg movement. See Chapter VIII, The Thorax, of Snodgrass' work.

 

I am obviously not a drosophilist and its interesting that the term hemithorax even exists, since that implies there are 2 segments to the thorax. All insects have 3: the prothorax, mesothorax, and metathorax. The prothorax is much reduced in the Diptera, to the point of being indistinguishable from the mesothorax so I guess thats how that term came to be. 

 

The most interesting thing I find is that all of the muscles don't seem to have any clear attachment points. The flight muscles don't contact the scutum or notum and the venter. The muscles in the femur don't contact the trochanter or the tibia. I can think of 2 explanations: 

1. Perhaps the attachment points are there, but are simply not visible

2. The muscles have shrunk through time. In life, they would have been longer and contacted their expected attachment points. 

 

I'm of the opinion that 2 is more likely. If I was writing a paper, this would imply that soft tissue preservation does not always represent the in vivo condition, which has implications for all soft tissue preservation in amber.

 

Anyway, I'm just an amateur with an excessive love of paleoentomology. I am by no means a professional entomologist nor paleontologist. 

Edited October 13, 2021 by Crusty_Crab
Replaced bad example of dorsoventral leg muscle in thorax with better.

[Biotalker]

27 minutes ago, Crusty_Crab said:

The most interesting thing I find is that all of the muscles don't seem to have any clear attachment points.

I am bothered by precisely the same concern. I have seen this in other specimens as well but not all.

I also suspect (with little evidence) that 2) the shrinkage/separation idea is right. 

 

I don't know much about the specific interaction of tree resins with soft tissue, perhaps dehydration occurs. Amber resin chemistry might be the key to understanding the level of preservation that can occur on a molecular level in these inclusions.

 

Ha, the term I used "hemithorax" isn't a drosophila term that I know of, I just used its literal biology meaning "half of the thorax"- to reinforce that we are seeing 6 and 6 IFM and not 12 and 12 (its actually more like 8/6). It was a fortuitous cutting/polishing plane that yields a largely unobstructed view of the left half of the thorax from a medial viewpoint. Incidentally, I can see many smaller muscles in the thorax under the scope, so I think this sample will warrant better analysis.