|Checklist of the Collembola: Note on the identification of the 'Podura' of Victorian instrument makers|
Department of Biology, University of Antwerp, Antwerp, B-2020, Belgium
David Walker, 'Micscape' e-zine, UK
Taken into account the complex synonmy of 'Podura plumbea',
the scales of 'Podura plumbea'
and Lepidocyrtus curvicollis Bourlet may look very different,
even using a well corrected microscope objective of modest resolution.
The question arises as to how far back this inaccurate identification of
the springtail with 'test' scales occurred and what were the underlying causes.
This note reviews the published accounts of the test scales from a modern
taxonomic viewpoint to see if they reveal clues as to what species
early instrument makers were really studying.
Pritchard (1832) did add 'lead coloured springtail' to his Podura plumbea description, but variability in common names may serve to add further uncertainty. This author as do earlier workers, refers to dry cellars being the collecting source habitat. Carpenter's (in Gill 1828) and Pritchard's (1832) observations and drawings of the test scale are two of the earliest detailed illustrated descriptions. Other studies predating 1832 invariably added a brief note mentioning Podura or Podura plumbea but there are no discussions or drawings of whole insect species collected. Chevalier (1839) also published a description with scale illustrations, but none of these early reports illustrated the arthropod itself.
In addition to the above earlier work, the ???likely most useful???but not convinced yet??? papers for an insight into the naming confusion, are by McIntire (1867, 1868) and Joseph Beck's Appendix on scales (in Lubbock 1873). Both these later workers combined species discussions with microscopical studies of the test scales.
After Richard Beck's correct assignment of the genus in 1862, McIntire in 1868 was one of the first workers to refer unambiguously to the test species Lepidocytus curvicollis (he remarks: 'after consulting Lubbock's papers on the Thysanura'). Adding the authority clause seems rare. A full specification of the species is Baker and McCrae's ultrastructure TEM studies in 1960s where they collect fresh 'Lepidocyrtus curvicollis Bourlet'. They also give an extensively referenced historical overview which the present authors found most valuable.
Query - how important is it to fully state the authority if the species
is unambiguous without it?
Microscopists then and now rarely seem to add the authority to most
organisms studied unless it is a taxonomic study.
Answer: the authority clause is mandatory only in the case of synonymy. The more complex the synonymy, the more important it is to add the authority clause to solve the ambiguity caused by the synonymy.
To the author's knowledge,
there are no recent ultrastructure SEM studies of the scales.
Given the observation of wavy lines on the scale surface, the scales are not of Tomocerus but of Lepidocyrtus (compare with Fig.9,10 of Pritchard 1832).
Carpenter in Gill 1828:326 :
On using small heaps of wheaten flour upon dark coloured paper in cellars to attract the 'Podura'.
Carpenter in Gill 1828:327 :
On viewing 'Poduras(sic)' as beautiful opaque irridiscent objects in "Mr. Tully's new combination of two sets of triple achromatic object glasses combined", "a proof that their transparent scales or feathers are delicately furrowed".
Carpenter is the first to conclude that the surface of the scales is provided with 'furrows'.
Carpenter in Gill 1829:137-139 :
In searching for objects
for this purpose, I met with a very minute species
of podura in my wine-cellar, its natural size was less
than that of a flea; and on placing one of these insects
under my microscope, I discovered that this minute
speck was covered over with a profusion of scales, and
which you have already mentioned in your former numbers.
These scales were found, by the eminent opticians to
whom I have presented them, to be much more difficult,
as test objects, to discover the fine markings in them,
than those of the Menelaus or brassica, and every exertion
was used by these gentlemen, in the working of their metals
and glasses, in order to bring out distinctly the characters
in their scales.
Fig.6 is one of the scales of the podura[sic]. The markings in this are still more difficult to discover than those of the cabbage butterfly, and require the same precautions to be used, in order to render them visible. They are, indeed, a severe test of the goodness of microscopes; notwithstand which, the Editor has frequently opened them with the lenses of his single Varley's microscope, of the twentieth, the thirtieth, and the sixtieth of an inch focus, a convincing proof of the perfection of those lenses, and of the peculiar and excellent method of mounting them. And, indeed, Mr. Carpenter himself has recently confessed to the Editor, that he has never seen the markings upon these scales more perfectly defined in any of the compound, reflecting, or achromatic microscopes he has hitherto tried, than in this single microscope of Varley's construction. "
Plate IV Fig.6 to be found... This is possibly the first published image of the scale surface markings...
Wollaston 1829:12 :
"With this microscopic doublet I have seen the finest striae and serratures upon the scales of the Lepisma and Podura, and the scales upon a gnat's wing, with a degree of delicate perspicuity which I have in vain sought in any other microscope with which I am acquainted."
Apparently Wollaston used Pritchard's lenses ???ref???.
???Did Wollaston use Pritchard's Podura scale test slides???
Brewster 1830:322 :
" ... The effect of this instrument, so constituted as to bring out proof objects, which, so far as we know, are invisible by any other engiscope or microscope whatever, viz. a set of longitudinal lines on the scales of the Podura, in addition to the two sets of diagonal ones already discovered, ... "
???Who discovered the two sets of diagonal lines???Was it Thomas Carpenter???
The diagonal and longitudinal lines are typical for Lepisma and Tomocerus scales.
Brewster 1831:344-345 :
Dr. Goring assures us that this microscope exhibited
a set of longitudinal
lines on the scales of the podura(sic) in addition to the two sets
of diagonal ones previously discovered, and ...*
... Dr. Goring has the merit of having introduced the use of test objects, or objects whose texture or markings required a certain excellence in the microscope to be well seen.
... The most difficult of all the test objects are those of the podura(sic) and the cabbage butterfly...
* See Edinburgh Journal of Science, No. IV. new series, p.321."
Brewster refers to Pritchard for some examples of test objects.
Goring 1831:246 :
About a hypothetic achromatic object lens to exhibit
the lines on the scales of Podura plumbea.
Pritchard 1832:149-153 : (See note 2)
"(7.) The Podura Plumbea(sic)-(Lead-colour Springtail.)- As before noticed, these insects belong to the same order as the Lepisma. They are about the tenth of an inch long, and leap about like fleas (Pulex irritans), though not so high. They are found among saw-dust and damp wood, abounding also in wine-cellars; they are very active, and consequently difficult to catch.
The body and limbs of these insects are covered with scales, which, from their extreme delicacy, require great care in removing. They are also very soft, and easily wounded. The fluid which exudes from the injury so completely adheres to the scales as to obliterate all their markings. Hence they must be cautiously handled.
I have remarked, that, with the discovery of any more difficult object than what is already known, an improvement in the microscope has soon followed. This was strikingly exemplified in the discovery of the lines on the scales of this insect; they were observed accidentally by the late Thomas Carpenter, Esq., of Tottenham, while making some experiments with a plano-convex jewel lens, employed as the objective of an engiscope, having an Huyghean eyepiece. They were then submitted to various instruments, and, from the difficulty with which they exhibited the lines, even on the larger dark specimens, this object became of great consequence to the microscopist, and some of them were immediately transported, that our neighbours the French might try their microscopes on them.
I have never been able to see the lines on them with a power much below 350 (that is, one thirty-fifth of an inch focus), and therefore microscopes of a lower power cannot be expected to show them, except of very superior quality; for it must constantly be kept in mind, that that instrument is the best which exhibits an object with the least amplification, all other things being equal.
It is also proper to notice, that single magnifiers will resolve them, but not without considerable attention is paid to their illumination; good doublets, of sufficient power, show them readily with Dr. Wollaston's illumination; but they are most easily made out, by the simple light of a candle, in the aplanatic engiscope, if it possess an angle of aperture of about fifty degrees, exhibiting all their delicate minutiae with precision.
It is affirmed, by a very acute experimentor, of these scales, that "all are difficult, and some seem to defy all power of definition." The latter part of this quotation is perfectly accurate; but I differ in the former, because many specimens, especially the French ones, are very easy, and unworthy the title of proofs; and, as they might be substituted for those I am describing, and thus a common instrument might pass for one of superior excellence, I feel justified in giving this caution.
To be completed. "
Pritchard gives a list of test subjects in order of difficulty. Petrobius maritimus then Lepisma saccharina then way down as most difficult Podura plumbea.
To be completed.
Pritchard 1832:161 :
Mimicking poor optics by postprocessing (blurring) the digital image of the drawing of the scale of Lepidocyrtus curvicollis of Beck in Lubbock 1873 plate 73 produces the wavy lines (fig.P3) as described and figured by Pritchard (fig.P2), and is the proof that the curved lines are indeed virtual lines not real lines.
To be completed.
Chevalier 1839 :
To be completed.
Beck 1862 (second paper):
makes the most telling assessment of the earlier work.
To be completed.
Wollaston (published 1829, read 1828) is cited by Nelson 1907 as the earliest published report of Podura scales as test object. However, Thomas Gill, Editor of the 'Technological Repository', in his regular 'On the Microscope' series gives a more detailed account in 1828, so Gill could be credited with the earliest published report. Gill's scales were supplied by Thomas Carpenter, the person regarded by Nelson (1907) to be the discoverer of the scales as test in ca. 1827. Gill comments on the importance of 'using a 'good instrument' and 'considerable skill in the peculiar management of the light, otherwise their delicate wavy lines [on the scales] are entirely invisible'. Scales of Lepidocyrtus curvicollis when studied with low resolution modern optics can look like wavy lines whereas Podura plumbea when considered as tomocerid has separate scales do not seem to show this effect, at least under modern corrected optics of similar aperture; see accompanying notes. This could be evidence that Gill was studying L. curvillcollis, but is not conclusive, because there is a possibility of overlapping scales showing a Moiré effect of wavy lines (this has been shown for Lepisma, yet to be checked with P. plumbea, pending a sample). Also artifacts created by their less than perfectly corrected optics showing the strong longitudinal ridges as wavy lines cannot be ruled out???IMHO longitudinal ridges cannot produce wavy line effects???. 'Podura plumbea' illustrated by Pritchard 1832 shows distinct wavy lines (see below).
Gill states that he uses an example of Varley's microscope with single lens of one-thirtieth of an inch focal length. Carpenter successfully used a Tulley microscope to show the markings. It would be interesting to know the typical performance at the time of these optics and thus how they may have shown Podura plumbea and Lepidocyrtus curvicollis. [Will try and find out its specification.]
Carpenter in Gill (1829) may be the earliest published description of the Podura scales as test which was also accompanied by an illustration of the scale (but see Footnote 1). Gill includes an extensive entry entitled, 'Mr. T. Carpenter on the Scales of the Cabbage Butterfly, and the Menelaus Butterfly, and those of the Podura, as difficult test objects for the Microscope'. Carpenter remarks that he first found the 'podura' in his wine-cellar when specifically looking for difficult test objects for opticians 'improving their microscopes'.
Thomas Carpenter features regularly in Gill's 'Repository' from 1827 and was referred to as a source of test objects by other workers such as Goring???ref???. It is clear that Carpenter was very experienced in entomology, carried out insect dissections and wrote on his own account in the 'Repository' on aspects of insects.
Pertinent remarks from Gill's and Carpenter's entries on the springtail,
which both referred to as just 'podura' include:
p.138. 'a very minute species, .... natural size less than a flea'
p.139. 'covered over with a profusion of scales'
p.139. the scale markings are 'still more difficult to discover than those of the cabbage butterfly', and 'severe test of the goodness of microscopes'
p.139. Gill used single lenses on his Varley microscope of focal lengths 1/20th, 1/30th and 1/60th inch focus to show the markings. Carpenter remarked that the markings with the Varley were the most 'perfectly defined' of microscopes he had tried either 'compound, reflecting or achromatic'.
Pending access to the correct figure referred to (Footnote 1), comments are limited as the scale markings are not described in the accompanying text to the figure. However, later in the same volume p.203, Carpenter writes to the Editor stating that 'the curious zig-zag lines shown in our figure .... as viewed in the Editor's Varley's single microscope, have likewise been seen by Mr. Tully, in a microscope fitted with one of his achromatic object glasses.' If the figure shows none overlapping scales, thus ruling out any Moiré effect, and if Tulley's / Varley's optics were capable of the resolution required, then Gill and Carpenter may have been studying L. curvicollis rather than P. plumbea. Further support of this, is that the workers recognised markings on the Podura scales finer than those being seen on either the Menelaus or Cabbage White scales, both of which have strongly ridged markings as does P. plumbea. [Provisional comment. Will check resolution of these other scales cf P. plumbea]
The serratures of Wollaston (1829) are equivalent to serrations (serrate=saw-tooth-like). Presumably they may have been the toothed edges of the coarser Lepisma and Tomocerus scale. (L. curvicollis would be fine lines or ridges usually in parallel. Wollaston's striae would correspond to the longitudinal parallel Lepisma ridges and/or Tomocerus minor ridges.
Given Wollaston refers to the Podura scales as test objects, and Gill in 1827 refers to the 'difficult test objects', this implies that microscopists have been investigating the test objects and possibly a species of Podura before 1827. Hooke in his 'Micrographia' (1666) describes and illustrates a 'small Silver colour'd Book'worm' i.e. Lepisma saccharina down to scale level under his microscope ???ref???.
Goring (1827) is generally cited (e.g. Schickore 2003, Bradbury 2002) with the first report of test subjects for the microscope. But this is challenged by Chevalier (1839) who claims that it was Le Baillif (1823); Bracegirdle (1986) cites Chevalier's work and remarks on this claim. Chevalier discusses scales of insects including from certain butterflies and beetle are mentioned and drawn in some detail including Podura.
Given Wollaston mentions the scales of Podura with those of Lepisma this suggests the scales look alike, using the equipment (microscopes, illumination) of that time. Especially the scales of tomocerids compare well with those of Lepisma in lower powers (e.g. at 350x). Both show then the characteristic longitudinal ridges. So we could assume that Wollaston's Podura is the tomocerid Podura plumbea (now Tomocerus minor). At that time, the scales of the tomocerid Podura plumbea and Lepisma could serve as test. Later when the quality of the optics improved they could not serve anymore as test given there surface markings are too easily resolved compared to the markings on scales of other species. [???does this only hold if Carpenter didn't supply Wollaston with scales, will check if any mention of source???]
Lepidocyrtus curvicollis has been described for the first time by
Bourlet in 1839 from French specimens.
We have now 2 hypotheses:
1. All 'Podura test scales' described by microscopists before 1839 are then from the tomocerid Tomocerus minor and not from the entomobryid Lepidocyrtus curvicollis.
2. The microscopists studied the scales from the not yet described entomobryid while using the name Podura plumbea (name of the then already known tomocerid). Taken into account what is said above, we assume hypothesis 1 applies to Wollaston 1829.
The tomocerid Podura plumbea has relative gross ribbing, so Pritchard (1832) seems to be aware by 1832 that there was something difficult to resolve about his 'P. plumbea'. This suggest that by 1832 Pritchard at least was studying the then not yet described L. curvicollis. As remarked earlier, Gill / Carpenter also describe and illustrate wavy lines / zig zag marks on their examples of 'podura' scale. [pending confirmation of seeing correct plate in Gill].
So for Pritchard (1832) hypothesis 2 is applicable.
But hypothesis 1 is also occurring as well by other workers who may have been less rigorous with the vague collecting methods described. Also, Pritchard was part of the new optics development so was making and had access to the best optics with potential to spot unusual tough subjects. Others may have been using poorly corrected low magnification single lens or compound microscopes that were not able to resolve the surface details on the L. curvicollis scales.
???To be added: figs of scale surface as seen in poor optics or mis-setup microscope to compare them with those of Tomocerus minor???
To be completed.
Early workers up to 1831 used just the higher taxon name Podura to identify their specimens. Apparently they used the scales of 2 different not at that time described species. Carpenter (and Gill) used the scales of the entomobryid Lepidocyrtus curvicollis Bourlet 1939, while Wollaston and Brewster used those of the tomocerid Tomocerus minor (Lubbock 1862). It was Goring (1831) who introduced the name Podura plumbea and as such unfortunately initiated the confusion about the used species due to the complex synonymy of that name. Originally Goring used that name for specimens of Tomocerus minor. But Pritchard applied the same name for specimens of Lepidocyrtus curvicollis.
To be completed.
To be completed.
To be completed.