THE MONTHLY MICROSCOPICAL JOURNAL. JANUARY 1, 1870. I.-The Structure of the Scales of Certain Insects of the Order Thysanura. By S. J. MOINTIRE, F.R.M.S. (Read before the ROYAL MICROSCOPICAL SOCIETY, Nov. 10, 1869.) PLATE XXXVII. SOME months ago I had the honour to submit a few observations to the Royal Microscopical Society respecting some insects belonging to the Thysanura, fam. Poduride-insects which deserve to be better known to microscopists than they hitherto have been, from the single interesting fact, one of many connected with them, that their scales are very beautiful microscopic obejcts. Since then certain views have been put forward relative to the structure of the scales of one of these creatures, Lepidocyrtus Curvicollis, which do not harmonize with my own, nor with those enunciated by the late Mr. Richard Beck, in whose published opinions I entirely concur. By the courtesy of Mr. Joseph Beck I was invited last April to inspect the drawings of Podura scales left by his brother, and on secing them I was agreeably surprised to find that our observations, though quite independent of each other, were upon the same insects. In the drawings, which are exceedingly beautiful, the so-called "bead-like spherules," to quote from the recent papers on the subject, which are forming at the present time a bone of contention, are beautifully depicted, and the conclusion one must come to, therefore, is that they were well known to Mr. Beck. It is strange, then, that so accurate an observer as he is known to have been should not have made public the new views respecting the markings, and corrected his former statements, in which he distinctly asserts his belief that the scale is a corrugated structure, if he believed them to have any real existence as spherules. But, on the contrary, the latest expression of opinion by that authority which I heard him give, was to the effect that no alteration had taken place in his views with regard to the structure of the scale of Lepidocyrtus (Podura scale). In making some notes, as a necessary supplement to the short paper I have already communicated on the scale-bearing Poduræ, J may be permitted, perhaps, to state, with some diffidence, the objections which seem in my mind to militate against the new theories: I say theories because I have heard two urged. The most formidable objection to the "bead-like spherule" theory seems to me to be the fact, that all through the scales of insects, whether Lepidopterous or Coleopterous, so far as my knowledge extends, the markings are due to corrugations or wrinklings of one or both membranes of the scales. Now, if this plan of structure holds good so extensively in the Lepidoptera, Coleoptera, and Diptera, as I believe it does, I cannot help asking, "Why should not the same plan affect the Thysanura? I think I shall presently be able to show, by comparisons, that it does, with certain modifications. The other theory is founded on the optical illusion of rows of beads which is often observable when two Lepidopterous scales of known striated structure lie across each other at any angle. A similar appearance and its cause is alluded to by Mr. Beck in his book on the Microscope, in treating of the scale of Lepisma Saccharina. It also is often seen, but I believe it is equally illusive, in solitary scales of certain foreign Lepidoptera, &c., where the strong longitudinal costæ are crossed at right angles by transverse corrugations. The theory which is advanced is to the effect, that the elongated "beads," or "note of exclamation" markings of the Podura scale, are due to striæ on opposite sides of the scale, crossing each other at an angle of about 17. Against it I would only urge one objection, namely, that supposing this to be the true solution of the question, we ought to see the elongated beads forming patterns with regard to each other either of squares or rhombs; but they do not, as a glance at any of the scales the structure of which is disputed, will, I think, convince most observers. Therefore I am unable to accept this theory as containing the whole truth and its explanation. Now what analogy is there between the scales of the Thysanura, especially Podura scales, and the scales of other insects? If we compare the scales of Lepisma Saccharina, the British species, with the species of Lepisma lately alluded to in 'The Student' by Dr. Mann (as introduced from the East Indies into Natal, where its destructiveness is a great nuisance)—some specimens of which species I believe found their way from Bombay to the Microscopical London Docks in a case of furniture last year, and were kindly given me by R. T. Loy, Esq., [a very pretty insect of speckled appearance, black, brown, and white;] and another of silvery-white lustre from the West Coast of Africa, for which I am indebted to the same kind friend-a strong family likeness is perceptible, and the resemblance to the scales of the Lepidoptera is very striking. (See Figs. 1 and 2.) Passing on to the scale of Petrobius Maritimus, an insect common on the limestone rocks all round our coasts, remarkable for its activity, whether running or leaping, and known as the Bristle-tail, there is a variation to be noticed, in the fact that the corrugations in many of the scales radiate from a line up the centre, and these corrugations are crossed again by striæ at right angles to them. An apparently beaded structure is often seen, but I believe it is illusive, and the explanation of the whole of the appearances is clear to my mind as the result of corrugations (see Fig. 3). So much for the Lepismide. Then, in the second division of the Thysanura, the Poduridæ, the first example I shall allude to is Macrotoma. As this scale is already sufficiently well depicted for the purpose in this Jonrnal, vol. i., page 208, Fig. 5b, I need only call attention to it to show the striking similarity between it, the known scales of Lepismidæ, and Lepidopterous scales. There are strong ribs or costa in a longitudinal direction crossed at right angles by minute wrinklings of the membrane, which under very high powers and certain illumination present the appearance (an illusive one, I believe) of rows of beads, as in the scale of Petrobius just alluded to. Some species of Macrotoma show this feature better than others; Macrotoma major for instance. In the scale of the greenhouse Degeeria† the corrugations are much coarser, fewer in number, slightly curved in direction, and interrupted in their course (more correctly speaking, perhaps, suppressed), so that they to a certain extent overlap each other. Of these short costæ the ends farthest from the tiny shaft of the scale are considerably the highest. In fact, I take these costa to be the representatives of the "note of exclamation" markings in Lepidocyrtus. (See Fig. 4.) In Templetonia Nitida‡ the structure is the same with these modifications;-the ridges are closer, and the suppressions or interruptions are more numerous. The sudden terminations of these little costa moreover are perceptibly broader and higher in proportion to the length of the costa than in the scale I have just These two scales are almost identical. Those from the speckled insect are a little more opaque than from the silvery-white one, but the shape of the scales and the other features are much the same. † 'M. M. J.,' vol i., p. 203, Fig. 3 b. Ibid., Fig. 4 b. markings, and corrected his former statements, in which he distinctly asserts his belief that the scale is a corrugated structure, if he believed them to have any real existence as spherules. But, on the contrary, the latest expression of opinion by that authority which I heard him give, was to the effect that no alteration had taken place in his views with regard to the structure of the scale of Lepidocyrtus (Podura scale). In making some notes, as a necessary supplement to the short paper I have already communicated on the scale-bearing Poduræ, J may be permitted, perhaps, to state, with some diffidence, the objections which seem in my mind to militate against the new theories: I say theories because I have heard two urged. The most formidable objection to the "bead-like spherule" theory seems to me to be the fact, that all through the scales of insects, whether Lepidopterous or Coleopterous, so far as my knowledge extends, the markings are due to corrugations or wrinklings of one or both membranes of the scales. Now, if this plan of structure holds good so extensively in the Lepidoptera, Coleoptera, and Diptera, as I believe it does, I cannot help asking, "Why should not the same plan affect the Thysanura?" I think I shall presently be able to show, by comparisons, that it does, with certain modifications. The other theory is founded on the optical illusion of rows of beads which is often observable when two Lepidopterous scales of known striated structure lie across each other at any angle. A similar appearance and its cause is alluded to by Mr. Beck in his book on the Microscope, in treating of the scale of Lepisma Saccharina. It also is often seen, but I believe it is equally illusive, in solitary scales of certain foreign Lepidoptera, &c., where the strong longitudinal costæ are crossed at right angles by transverse corrugations. The theory which is advanced is to the effect, that the elongated "beads," or "note of exclamation" markings of the Podura scale, are due to striæ on opposite sides of the scale, crossing each other at an angle of about 17. Against it I would only urge one objection, namely, that supposing this to be the true solution of the question, we ought to see the elongated beads forming patterns with regard to each other either of squares or rhombs; but they do not, as a glance at any of the scales the structure of which is disputed, will, I think, convince most observers. Therefore I am unable to accept this theory as containing the whole truth and its explanation. Now what analogy is there between the scales of the Thysanura, especially Podura scales, and the scales of other insects? If we compare the scales of Lepisma Saccharina, the British species, with the species of Lepisma lately alluded to in 'The Student' by Dr. Mann (as introduced from the East Indies into Natal, where its destructiveness is a great nuisance)—some specimens of which species I believe found their way from Bombay to the London Docks in a case of furniture last year, and were kindly given me by R. T. Loy, Esq., [a very pretty insect of speckled appearance, black, brown, and white;] and another of silvery-white lustre from the West Coast of Africa, for which I am indebted to the same kind friend-a strong family likeness is perceptible, and the resemblance to the scales of the Lepidoptera is very striking.* (See Figs. 1 and 2.) Passing on to the scale of Petrobius Maritimus, an insect common on the limestone rocks all round our coasts, remarkable for its activity, whether running or leaping, and known as the Bristle-tail, there is a variation to be noticed, in the fact that the corrugations in many of the scales radiate from a line up the centre, and these corrugations are crossed again by striæ at right angles to them. An apparently beaded structure is often seen, but I believe it is illusive, and the explanation of the whole of the appearances is clear to my mind as the result of corrugations (see Fig. 3). So much for the Lepismida. Then, in the second division of the Thysanura, the Podurida, the first example I shall allude to is Macrotoma. As this scale is already sufficiently well depicted for the purpose in this Jonrnal, vol. i., page 208, Fig. 5b, I need only call attention to it to show the striking similarity between it, the known scales of Lepismidæ, and Lepidopterous scales. There are strong ribs or costa in a longitudinal direction crossed at right angles by minute wrinklings of the membrane, which under very high powers and certain illumination present the appearance (an illusive one, I believe) of rows of beads, as in the scale of Petrobius just alluded to. Some species of Macrotomæ show this feature better than others; Macrotoma major for instance. In the scale of the greenhouse Degeeriat the corrugations are much coarser, fewer in number, slightly curved in direction, and interrupted in their course (more correctly speaking, perhaps, suppressed), so that they to a certain extent overlap each other. Of these short cost the ends farthest from the tiny shaft of the scale are considerably the highest. In fact, I take these costa to be the representatives of the "note of exclamation" markings in Lepidocyrtus. (See Fig. 4.) -- In Templetonia Nitida‡ the structure is the same with these modifications; the ridges are closer, and the suppressions or interruptions are more numerous. The sudden terminations of these little costa moreover are perceptibly broader and higher in proportion to the length of the costa than in the scale I have just * These two scales are almost identical. Those from the speckled insect are a little more opaque than from the silvery-white one, but the shape of the scales and the other features are much the same. †M. M. J.,' vol i., p. 203, Fig. 3b. Ibid., Fig. 4 b. |