Having dealt with the genital plates and pectines, let's move further along the opisthosoma. Here we find the remaining segments of the mesosoma contain the respiratory structures. In modern forms these are book lungs, so called because they comprise haemolymph ('blood')-filled lamellae that resemble the pages of a book. Each are separated by chitinous projections and the book lungs are stored in an air filled space (atrium) that communicates with the environment via Spiracles. Today the atrium is joined to the posterior margin of the sternite, but this may not have always been the case.
Spiracles are essentially holes in the sternites ( if you're struggling with the terminology at this point, you may want to check out the scorpion files at
http://www.ub.ntnu.no/scorpion-files/) of the scorpion. The nature of scorpion sternites have changed through time, but just what a scorpion sternite is has been the subject of debate.
Image showing scorpion sternites (sternates) and spiracles. Original image taken from the scorpion files.
STERNITES vs ABDOMINAL PLATES
The difference between and structure of sternites and abdominal plates are historically very important. Kjellesvig-Waering based each of his Infraorders upon the structure of the abdominal plates and constructed his evolutionary history on these. According to Kjellesvig Waering, abdominal plates may have evolved from eurypterid blatfuss and were only sutured to the body at one margin, remaining loose. He felt that sternites were hidden behind these and only became visible when the abdominal plates were lost. Others have thought that sternites are merely fused abdominal plates. One thing that can be said is that the emphasis on their structure was unjustified; no recent study has shown any evolutionary significance of this structure and fossil holosternus and lobosternus forms can be found in almost every current (albeit extinct) Infraorder.
Hypothesised evolution of abdominal/sternal plated as proposed by Kjellesvig-Waering
BOOK GILLS- BOOK LUNGS: The evolution of scorpion respiration.
Evolution can, and has, thrown up many cases of convergence (when two groups evolve the same structure independently), just one of numerous convergences between spiders and scorpions associated with terrestrialisation are book lungs. Both spider and scorpion book lungs are thought to have evolved from very similar gill structures (handily known as book gills), but at different times. This process was probably easier than one might expect, as Stockwell has commented that, "all things being equal, the respiratory books would be far more efficient in air than water.
The timing of the book gill - lung transition is difficult to pin down. Fossil evidence differentiating these structures is equivocal at best, but Stormer identified book gills in Waeringoscorpio hefteri and Jeram claims that marginal stigmata found in mesoscorpions by Kjellesvig Waering were misidentified and only appeared with the Palaeosternina.
A FINAL POINT... HOW MANY STERNITES ARE THERE?
The reason for this final trend in the mesosoma is unknown, but for some reason eurypterids and early scorpions had 5 sternal plates, and the 1st sternal plate was lost by the time of Branchioscorpio and the mesoscorpions (in fact it may have been lost some time before this, but fossils preserving the relevent portion are very hard to come by).
As you can see from the second image in this section, Kjellesvig Waering believed bilobersternous plates gave rise to orthostern forms, and this has actually been supported by embryonic studies of Centruroides vittatus, where bilobostern plates develop into the orthostern sternites. However, only two fossil genera show the bilobostern condition (Branchioscorpio and Dolichophonus) and the holosternous condition may have been better adapted, as all of the relatively derived palaeosterns possess it and scorpions with this condition persisted into the Jurassic (albeit diversity had reduced to two known genera, Liassoscorpionides [a primitive scorpion of uncertain affiliation] and Mesophonus [a mesoscorpion]).
Now, it was once thought that all scorpions possessing abdominal plates had book gills, but fragments of indeterminate lobostern scorpions have revealed that book lungs were enclosed in a stigmate sternite behind the abdominal plates (which would have possessed marginal spiracles) in at least some species. This also indicates that Kjellesvig Waering’s hypothesis that abdominal plates reduced in size and were eventually lost, revealing modern sternites is wrong and that the orthostern condition is more likely to have evolved through the fusing of, and development of stigmata in abdominal plates… a concept supported by embryonic studies of Paruroctonus mesaensis and Centruroides vittatus.
