Fly palaeo-evo-devo

We have just published a new paper, titled:  “Fly palaeo-evo-devo: immature stages of bibionomorphan dipterans in Baltic and Bitterfeld amber” and here I will strive to explain why we think that this paper is important, and why we are proud of it.

 Flies and midges (Diptera) are insects with full metamorphosis which means that they have 4 distinct stages in their lifecycle – egg, larvae, pupae and adult [“the fly”]. Eggs are supplying nutrients and protection for the early growth of the larvae, pupae is a stage at which larvae change into a fly (metamorphize); adult flies doing all the flying and breeding (and yes, also biting and transmitting diseases), and therefore most visible to us as humans.

The larva is a stage at which many flies spending more than 90% of their lives, and doing most of the interactions with the environment like feeding, parasitizing, purifying our drinking water, polluting our drinking water, decomposing corpses, capturing CO2, releasing CO2 back into the atmosphere and just being awesome. 

That being said, the majority of what we know about flies, is concerning adults (in particular adult males), as due to societal norms and scientific preconceptions of 18-19th centuries, it was considered that adult males of the insects are the most important and informative of all life stages.

Due to this historic preferences, and several objective reasons, like cryptic habitats, small sizes, slow development of certain types of optics, etc.,  Diptera larvae are relatively poorly studied, and it is especially true for the fossil records. 

While larvae of the aquatic Diptera (see non-biting midges, phantom midges) are relatively abundant in the fossil records (as most of the small animals are preserved by the fine sediments settling in the body of water), terrestrial fly larvae are almost unknown as fossils, only occasionally appearing in amber.

That leaves us when studying the past, mostly with adult flies, which can tell us things about their environment, but to a far lesser degree than larvae, as immature Diptera are the ones living in close association with specific habitat for long periods. In theory research on fossil, Diptera larvae can bring us countless benefits, from the satisfaction of curiosity to a better understanding of the Carbon and water cycle in much warmer periods of the past, probably similar to our climate-change-induced future.

Despite that, conventional wisdom is that Diptera larvae are almost absent from amber, and as rock fossils, you only can get aquatic once. I am mostly in agreement with the second part of the statement, but I did challenge the first- if ammonites can be preserved in amber, why not maggots? Having that in mind, in January of this year, our lab - namely my bosses- Prof. Joachim & Dr. Carolin Haug, bunch  of PhD students (including Mr. Mario Schädel, co-author of the paper) and a class of undergraduate students set out to Hamburg to study the collection of the Centrum Für Naturkünde (CeNak) (Natural History Museum of University of Hamburg). This is a part of the advanced course on the museum collections curating and museum-based research provided by Joachim and Carolin.

While the course has been going, I spent some time scouting private (Carsten Gröhn's collection, Christel and Hans Werner Hoffeins Collection) and public paleontology collections (mainly CeNak collection itself) in the Hamburg area. I ended up with over 100 specimens of the fly larvae from the Baltic and Bitterfeld amber (ca. 38 MYA). That was an amazing treasure trove! I have to note here, that while some studied specimens came from the private collections, all studied specimens were deposited in the public collections before publication, as to prior agreement with owners. Half of all larvae were representatives of the wood gnats genus Mycetobia, while the rest were representatives of some other Bibionomorpha (a large group of the flies with fully developed larval head capsule and terrestrial larvae, including wood gnats, fungus gnats, march flies, etc.) or true maggots – larvae of the short-horned flies (Brachycera). We dealt with Bibionomorphans in the paper in the focus of this blog, while analysis of the maggots will come in the later articles.

So, what're the main takeaways after the study of the Bibionomorpha larvae (and over 15 pupae!)?

Most importantly, Diptera immatures are not rare in amber if you know what to look for, and they can tell fascinating stories about our past. One such story concerns boundaries of how certain groups of animal looks, or a range of possible morphologies. Looking at the animals from the past we see that what we consider to be normal or even possible today not always the case. Enter Dinobibio hoffeinseorum – a brand new march fly, named after Christel and Hans-Werner Hoffeins - prolific, kind and supportive private amber collectors, who are the cornerstone of the European insect paleontology.  Dinobibio, is, in principle, a march fly (Bibionidae), like St. Marks fly you can see in the spring, but it is also very different from the rest of the march flies. Dinobibio's larva is extremely spiky, bearing numerous bowling-pin like, fleshy protrusions on its trunk. The larva also has an amazingly large and stocky "lower lip" (labium) as for a march flies larvae, as well as a strange, spiky protrusion of the palp on the "lower jaws" (maxillae). All that together with an unusual respiratory system arrangement gives Dinobibio a combination of characters that is considered impossible in the extant march flies. The fact, that such combination of characters was possible in the past is reflection of the phenomenon called “push of the past” when most of the group of organisms surviving till now, had to go through the rapid diversification ( origin of multiple related species ) in the past, including “invention” of the new morphologies (see Budd & Mann, 2018).

The older the group is, the more conservative range of morphologies can become, a taxon is experiencing growth from the “rebel youth” to the “respectable elder” state. 

So that is neat, what else we learned? Well, we have got 35 larvae of the Mycetobia sp (wood gnats, Anisopodidae). We thought that’s nice, what can we do with them? We ended up measuring head capsule of all the Mycetobia larvae (all who had had still attached, those are fossils after all).

The head capsule of the insect larvae is only growing in the short interval between the molts of the larval skin, and normally by a predictable factor of increase. That is called “Dyar’s rule” and it is very useful because by measuring head capsule of, say crop-eating caterpillar we can identify how old it is and plan our plant defense campaign, or we can identify how old is the larvae of the protected species of beetle and take stock of the population demographics, and plan for conservation measures.

 So when we did that, we find out, that all available larva of Mycetobia were split into four cohorts of size, corresponding to the 4 larval stages of wood gnats hypothesized by Coombs, Cleworth, and Davies in 1997.

 Unfortunately, besides the work of Coombs, Cleworth, and Davies there are not that much written on the wood gnats development, which is just a shame, since wood gnat larvae are important forensic markers on corpses, and wrong larval age id could lead, in the worst case to the, say conviction of the innocent person. 

Since larvae of the fossil and modern wood gnats having the same life cycle and morphology, we can assume that they have similar habitats (i.e. dead wood, leaves, corpses of animals) which informs us about habitats available in the Baltic amber forests.  Additionally, we have been able to observed several instances of the fossil flies behavior, frozen in time by amber, like this Mycetobia adult, starting to emerge from the pupal skin, as presented in the video below.

Baltic amber (38 MYA), MicroCT by Dr. Marie Hörnig (University of Greiswald), machine time during the MicroCt scanning was funded by the DFG grant DFG INST 292/119-1 FUGG; DFG INST 292/120-1 FUGG; reconstruction and video by Dr. Viktor Baranov, using Inside Explorer Pro (https://interspectral.com/) .

Therefore studying fossil larvae of the gnats and midges informs us about evolutionary dynamics, the biology of forensically important flies and habitats of the times of Eocene climate optimum (which looks quite similar to our possible future under certain emission scenarios). So actually, there ‘s quite a lot to be learned from the long-dead fly larvae, if we will be willing to learn.