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Thank you for making these final changes. Please make sure a space is added in between "for" and "25" on line 168 during the proofing stage. I would also remove the brackets from ( Figs. 18 A, 18 B) so it reads "Figs. 18A and 18B are falling" on line 991.Looking forward to its publication.
[# PeerJ Staff Note - this decision was reviewed and approved by Andrew Farke, a PeerJ Section Editor covering this Section #]
Thank you for making these revisions - you paper is as good as accepted. I still found some very minor formatting and language issues I would like to take care of before publication (see annotated pdf). Looking forward to seeing it published.
Thank for addressing our suggestions which make the article even more easy to follow. I just found some additional minor issues in the revised manuscript, i would like you to care off before publication. These are mostly formatting issues.
One important thing concerns your figure 1. This figure is nice as it lists the relationships between extant bibionomorpha, but it is not clear on what the phylogeny is based. This should be explicitly mentioned in the figure caption. You mention it is based on Sevcik et al. 2016, but these authors report several phylogenies (based on different data/methods) and yours is not an exact match for any of them. It also does not seem to be a consensus tree. Also these authors do not explicitly list Perissommatidae or Valeseguyidae (which you do) and do mention incertae sedis taxa (which you do not figure), The branch of the listed Bibionidae is also missing in the pdf. Please explicitly mention how you arrrived at these phylogenetic relationships.
I feel these changes can be addressed easily and i look forward to your revisions.
I apologize for the delay in getting back to you but I was out of office for time and needed some time to wrap my head around some of the aspects of the reviews. I feel the manuscript nicely highlights new aspects of ontogenetic development in amber arthropods. There are however some aspects I would like you to address before publication:
Description of a new species: the description of a new species based on a “larva” seem suboptimal but I agree with reviewer 1 that it might represent one of the best options – in this case, it would be worth pointing out potential pitfalls (incomplete knowledge of extant larvae: compare review 2). An alternative would be to describe it as a new larval morphotype as you did for the Mycetobia pupae which might satisfy both reviewers.
Taphonomy: it would be appropriate to back up the possible effect of preservation/shrinking (McCoy et al., 2018) in amber on measurements with some references (see annotated pdf).
Quantitative analysis of head width versus head length: I feel this analysis is appropriately explained in text. Figure 18a would be easier to follow if you would add how many specimens you have per ontogenetic stage (I – IV). Also, it might be worth in text and figure that you only have one specimen of stage IV and so this does not actually represent a mean. In the case of Figure 19, It would be nice to add the standard deviations/errors of the log(length) to get a grasp of the variation around the mean – at least for I-III where these are obtainable.
Offset between character combinations observed in fossil and extant Bibionomorpha larvae: it might be worth stressing that varies aspects might bias our perception of this which could range from the push of the past (Budd and Mann, 2018) to poor knowledge on extant larvae (see comment by reviewer 2).
Selective preservation of trunk and leaf litter fauna: it would be good to back this statement in the abstract recent works on this topic in the text (Sánchez-García et al., 2017; Solórzano Kraemer et al., 2018).
Please address these points in addition to additional suggestions by the reviewers and my own in the annotated pdf.
Budd, G. E., and Mann, R. P., 2018, History is written by the victors: The effect of the push of the past on the fossil record: Evolution, v. 72, no. 11, p. 2276-2291.
McCoy, V. E., Soriano, C., Pegoraro, M., Luo, T., Boom, A., Foxman, B., and Gabbott, S. E., 2018, Unlocking preservation bias in the amber insect fossil record through experimental decay: PLOS ONE, v. 13, no. 4, p. e0195482.
Sánchez-García, A., Nel, A., Arillo, A., and Kraemer, M. M. S., 2017, The semi-aquatic pondweed bugs of a Cretaceous swamp: PeerJ, v. 5, p. e3760.
Solórzano Kraemer, M. M., Delclòs, X., Clapham, M. E., Arillo, A., Peris, D., Jäger, P., Stebner, F., and Peñalver, E., 2018, Arthropods in modern resins reveal if amber accurately recorded forest arthropod communities: Proceedings of the National Academy of Sciences, v. 115, no. 26, p. 6739-6744.
[# PeerJ Staff Note: It is PeerJ policy that additional references suggested during the peer-review process should only be included if the authors are in agreement that they are relevant and useful #]
The paper is an interesting, relevant and novel contribution pointing out an important shortcoming in the research in amber fossils: the lack of attention to fossils of immature stages. It is well written in clear English, though the parts of the structure of the descriptions is overly repetitive, see below. The manuscript makes good use of the relevant literature. The figures are relevant and of good quality.
The descriptions are a bit cumbersome; I will suggest some simplifications to make them easier to read. Features of the head are described in terms of head segments (Line 239 onwards), yet the only visible manifestations of these are the mouthparts. Though head segmentation is an important aspect of basal insect evolution, it does not add any useful information in a description like this. I strongly suggest describing the mouthparts as such, no reference to the non-observable head segments to which they probably belong is necessary in this context.
Body segments are described one by one, but the descriptions are to a large extent identical (line 287 onwards). Here is is possible to just describe the general appearance of each segment, and then note the deviations from this pattern (if any) in individual segments. That will make the descriptions of the body a bit shorter and more readable without reducing the precision.
This point is of limited interest since this is not an experimental study. The technical aspects of the study, and the data analysis (in particular, the use of Dyar plots) are adequately described and discussed.
The systematic placement of the specimens is reasonable. I agree with the authors’ decision to place Dinobibio as Bibionomorpha, incerta sedis (but see comment on the decision to formally name it further down).
The authors have made a choice to formally name a species based on a larval specimen. Generally, it is not adviceable to name new species based on immature stages in insects since it is likely to lead to an inflation in the number of names, and this problem is especially relevant with fossil species since it is very unlikely that this specimen could ever be connected with certainty to an imago specimen, breeding and DNA comparisons not being possible. Most likely this species will stay «incerta sedis», the only likely way it could be associated with an adult insect is if a recent species with similar-looking larvae is discovered. On the other hand, it is also undesirable to have a large number of described but unnamed taxa in the literature. Trying to balance these concerns, I can agree with the authors that the least undesirable option in this case could be to formally name the species, in order to have a tag to attach to it. An alternative could be to use the approach the authors have chosen for the Mycetobia pupae, and describe it as an unnamed morphotype
Line 54: I think «destroying» is uneccessarily negative here, suggest «recycling» instead.
Line 66: Why «» around dystrophic? This is a standard term, delete unless there is reason to believe these lakes were in fact not dystrophic.
Line 116- «A full list of…» - move to line 99
Line 149 – Camera and what? Seems to be a word missing here
Line 183 – I believe that even monotypic genera need a generic diagnosis (though it will maybe be fairly identical to the species diagnosis). I think you have to give a short, generic diagnosis (Line 215), I suggest using those traits from the species diagnosis judged least likely to vary within a genus.
Line 224: «Acalypterate fly» is a fairly inprecise term. Is it possible to indentify further (to superfamily or family?).
Line 312: are there really «dozens» of them? Does not look so to me.
Line 321: should mention that the posterior spiracle is not notably enlarged compared to the others, as this sets this larvae apart from those known in Bibionidae.
Line 366: The body protuberances are quite erect in the specimen, this suggests they may have been more rigid than is the case with modern bibionid larvae.
Line 412 onwards: several of the species of Bibionomorphans described by Skartveit (2008) differ notably from recent counterparts and could possibly be the adult stage of this larvae. In particular, the three species placed in the genus Hesperinus and the enigmatic species Penthetria integroneura, which was tentatively placed in the genus Penthetria.
Line 432 onwards, description of Pachyneura sp. The contracted body suggests the specimen was desiccated before being encased in resin. This should be commented upon. As in the other descriptions, the descriptions of head and abdomen structures are unneccessarily long and repetitive. The same goes for the Mycetobia specimens.
It could be useful to briefly discuss the Mycetobia pupal morphotypes in terms of sexual dimorphism, which is often substantial in bibionomorph pupae. I would guess that morphotypes 1 and 3 are probably males and morphotype 2 probably a female, though this should be checked by comparison with recent material, if any is available.
Line 1009: it is not difficult to erect new taxa, the difficulty lies in trying to relate them to other things afterwards.
Line 1076: body length is difficult to use to identify instars even in recent specimens – this should be døme by measuring rigid structures uch as head capsules.
Line 1149: «pristine» (meaning not affected by human activities) is not a very useful term for Eocene forests. I am not aware of any evidence of extensive logging back then.
It is an interesting and well written manuscript, meeting the high standards of PeerJ. There are numerous minor errors throughout the text, including typos (e.g. “Mycetophylidae”, line 410), grammatical errors and double spaces, which should be corrected. All the manuscript would benefit from shortening. The literature is mostly relevant and properly cited (though several items are absent among the References, e.g. Azar et al. 2018, Paramonov & Salmela 2015, Wojton et al., 2018, 2019a, 2019b). The figures are of high quality, with appropriate legends.
The authors followed standard procedures of systematic paleoentomology. The fossil specimens are preserved in publicly accessible collections.
My only concern is giving a name to a new taxon based just on the larva and without a proper diagnosis. In the groups like Bibionomorpha, usually only male terminalia provide sufficiently distinct species-specific morphological characters. The probability of correct association of the named fossil larva with an adult male or female of the same species in the future is very low or even zero. The authors should ask themselves about pros and cons of such a controversial taxonomic act. Such a name will only feature in the lists of named fossil taxa, without any possibility to recognise the species (or genus) and further work with it. As a taxonomist (neontologist), I would definitely prefer not to give a formal name to a taxon where only larva is known. Interesting discussion related to the taxonomic descriptions without proper diagnosis may be found also in the paper by Jaschhof (2007, Paleontological Journal, 41: 103–106).
A further concern, related to the previous one, is that the immature stages of many genera of Bibionomorpha, especially outside Europe, are still unknown or insufficiently described. This applies also to Penthetria, mentioned as a possible relative to the new genus, where larvae are unknown in the vast majority of extant species. So there is a possibility that the larva described here as a new genus may actually belong to some described but insufficiently known extant genus or species. For me, this would be another reason why to avoid description of a new genus or species based on larva only.
Concerning the morphometric parameters and calculations based on fossil specimens, possible distortions of various body parts of the insect during the fossilisation process should be taken into account. This possibility is not commented on by the authors.
The new taxon may be valid but its practical identification and delimitation in the future will be difficult, see above. However, I see main value of this paper in the general overview of the diversity and abundance of some bibionomorphan immature stages in the Baltic and Bitterfeld ambers and in the contribution to the comparative morphology of dipteran larvae.
Immature stages of Bibionomorpha still represent a little known and challenging topic of study, both in extant forms and in the fossil record. Bibionomorpha really represents a megadiverse insect group but I cannot agree with the sentence of the authors in the Introduction that “Bibionomorpha includes numerous ecologically similar ingroups”. This sentence should be clarified or corrected. Even within a single family of Bibionomorpha, Cecidomyiidae, we can see various ecological adaptations of the larvae, including mycophagy, phytophagy, zoophagy, etc. In my opinion, the authors tend in various parts of the manuscript, especially in the Introduction, to overstatement and unjustified generalisation of the facts. In the Introduction they prefer to cite general textbooks instead of more specialised recent papers.
The exact phylogenetic position and interrelationships of particular subgroups of Bibionomorpha is still much disputed and challenging topic in current dipterology. However, the manuscript under review does not bring new insights to this debate. Actually, both Baltic and Bitterfeld ambers are apparently too young to be essential for the understanding of the origin and diversification of early bibionomorphan taxa. On the other hand, it is definitely useful to realise, in the context of the manuscript under review, that most of the taxa of Bibionomorpha found in the Eocene ambers represent extant genera or higher taxa.
Figure 1 deserves more explanation and improvements. It is actually not based only on the paper by Ševčík et al. (2016), as mentioned in the legend to this figure, because those authors did not include into their analyses taxa like Valeseguyidae or Rangomaramidae. Concerning the latter taxon, its concept and delimitation is especially controversial, so this name should not be used without further explanation.
The description of the new species is rather lengthy, giving the fact that many sentences are repeating. This section (and also several other similar parts of the manuscript) should be shortened. The sentence “The larva is clearly different from any modern representative of Bibionidae …” should be changed to “The larva is clearly different from any modern representative of Bibionidae, where immature stages are known...”.
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