C. L. Metcalf.
A Proposed Nomenclature for the Parts of the Posterior Respiratory Apparatus of Dipterous Larvæ and a Micro-Protractor Useful in Their Description.
Psyche 26:53-57, 1919.

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PSYCHE
VOL. XXVI JUNE, 1919 No. 3
A PROPOSED NOMENCLATURE FOR THE PARTS OF THE POSTERIOR RESPIRATORY APPARATUS OF
DIPTEROUS LARVAE AND A MICRO-PROTRACTOR
USEFUL IN THEIR DESCRIPTION.1
It has been pretty generally recognized that the most serviceable diagnostic character for the specific determination of Dipterous larvae of certain families is the posterior respiratory apparatus, which varies endlessly in the different genera and species, but ap- pears to be very constant within the species. So superior are the characteristics drawn from this part of the larva, that I regard most descriptions which omit consideration of them as practically worth- less. For in most cases the general features of shape, color, etc., may be found to apply almost equally well to other species. The most available, absolutely diagnostic characters are to be found on the posterior stigmata.
This apparatus consists, in many species, of three pairs of spir- acles, generally more or less elongate or linear, and a fourth pair of circular structures (usually present) referred to in literature as "buttons " or "circular plates. "2 This respiratory organ varies in the different families, genera and species in height; in width; in extent of elevation above the surface of the segment which bears it; in the ornamentation of the interspiracular spaces (vide infra); in length and width of the slit-like spiracles, which may be straight, curved, sinuate or denticulate, and variously arranged with respect to the median line and to each other; in the presence or absence of the circular plate, its position and diameter; and in many other ways.
'Contributions from the Entomological Laboratory of the Bussey Institution, Harvard University, No. 160.
2 In those rarer cases (as e. g., CEstris ouis) where the three pairs of spiracles appear to be wanting, only a part of the following characteristics will be found to be applicable. Pachc 2653-57 (1919). hup Yipsychc einclub org/26?26-051 html



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54 Psyche [June
The need of a satisfactory and uniform nomenclature of these parts is evident.
Mr. Nathan Banks/ Mr. M. E. MacGregor,2
and Professor W. B. Herm~,~ especially, have emphasized the value of these structures with reference to the larvae concerned in myiasis; the writer4 has for several years been applying this method to the larvae of Syrphidse; and indeed many investigators in recent years, have recognized the necessity of figuring the stigmal plates of the larvae described in a number of Dipterous families. I have no
doubt that many of the groups of amphineustic larvae can be spe- cifically diagnosed in the same way. In the various publications touching on this subject a variety of terms have been used. It is with the hope of acquiring uniformity of terminology in the various groups and by subsequent investigators that I have herein compiled and described such of these terms as have come to my attention? The spiracles, together with the adjacent surface of the segment which bears them are called the stigmal field. The right and left halves of the respiratory organ, which are especially heavily chiti- nized, are each called a stigmal plate. The diagnostic points of chief importance, so far as my observations have gone are as fol- lows :
(1) Whether the stigmal plates are sessile on the caudal surface of the last segment, or stalked, i. e., elevated at the end of a shorter or longer (sometimes tremendously elongate) tube. (2) Whether the two stigma1 plates are contiguous or separated. (3) If separated, the distance between the plates: the minimum transverse distance intervening. (MacGregor, Herms.) (4) If stalked, or sessile and contiguous, the width of the posterior respiratory organ: the maximum transverse measurement from lateral margin of one stigmal plate to lateral margin of the other. (5) If separated, the width of one stigmal plate may be more satis- 1 Banks, N., The structure of Certain Dipterous Larvae with special reference to those in Human Foods, Bull. U. S. Dept. Agr. Tech. Ser. 22, Jan. 10, 1912. 2 MacGregor, M. E., The Posterior Stigmata of Dipterous Larvae as a Diagnostic Character: with Especial Reference to the Larvae Incriminated in Cases of Myiasis; In Parisitology, Vol. VII, No. 2, June 19, 1914, pp. 176-188, 3 ppl. a Herms, W. B., Medical and Veterinary Entomology, The Macmillan Co., 1915, pp. 259, 260.
4 Metcalf, C. L., The Syrphidae of Ohio, Ohio Biol. Sur. Vol. I, Bul. 1, June, 1913; Me. Agr. Exper. Sta. Bul. 253, Oct. 1916; and do Bul. 263, Oct. 1917. 6 I am indebted to Professor Charles T. Brues for assistance with the literature on this subject.



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factorily used, and in this case the easiest measurement to make appears to be the maximum, strictly transverse diameteral (6) If stalked, the length of the poStenW respiraiory organ or tube: the elevation of the stigmal plates above the general surface of the segment which bears them.
(7) The Jwight of the stvfmal plate or respiratory organ: the maxi- mum dorso-ventral (sometimes apparently cephalo-caudal) meas- urement.
(8) The $We8enee or absence ofthe circuia~ plate m button,. (Banks.) (9) ThÌ pSrtWn. of the button, wkethm on the circumfereiwe of the stigmffl! plate ("ring" of MacGregor) w within it. (MacGregor.)
(10) The dkmeter of the CTrcular plate or button: a strictly trans- verse measurement through its center.
(11) The dzstance between the mead margins of the two circular plates*^
(12) The ,?en& of the slit-like spirachs. (13) The d t h of the spirodes.
(14) The shape qf the spiracles, which are for convenience of further description named dorsal, median and ventral. It is necessary to note that the several spiracles on the same stigma! plate may vary greatly in shape and it is often necessary to refer specifically to them? See figure 1.
(15) The ornamentation of ihÌ stigma2 plate with spines, tubercles, 1 Professor Hem (lot. ck) has used "the diameter of the stigmal plate, the upaca occupied by one etigmal plate on a line drawn through the center of both; (2) Ieqrth, whenelite itte stegnt, the pace -pied by a plate on a line drawn dorsc-ventrally through the aenter at the plate; or when elita art present the spwe occupied by ~t plate along a line drawn from the lower edge of button (or apace if button is abeeatt through the longot dit (middle lit) to the margin of the plate; (3) width, along a line drawn at the middle of the plate at right euglfls to the length line." These are no defined as to bs Ì little difficult to mewuro.~requirine un estimation of such features n$ "'the center of the pit&," and a line at nght angle^ to another. They are to this extent apt to ]pad to error, and will, I Ddie-re, be adequately ~upplasted by the more readily detenninnblc featurce ~iig~~<ted nbom in pragraphn S and 7. MacGregor sugge~ta measuring from center å´t ̤nt of buttong, winah complicates the matter by ueceesitatiqg ftn estimation of the central Doint of each plate. Point 11, in p~~meo- tion with point 10 å´wil give the same result more easily. 8 MaeGrem Vac. cil.) attempte to divide the forma of poBterior stigmata into two mdn types: the uiAimtrww-type eaperiaUy characterizd by poeecsring three paire of aliMike Bpira- elm; and the pfyefliitTeme-type, pogaisgninginstead "a convoluted chain" (0. g., Muw~dtUBfflfc'cli, Stmwxyt ~arCl<mm, Rmatobia serrata, etc.). But he, himself, points out that there are usually two ("three," lot. cit,, p. 1st) breaks in the chain. I -think there is so swcad basis for nuch dietiwtion. Both of thene fonris have three pil-s of spiraclm; the convoluted apirncle snd the !lit-Lke one are fuiKiitm~~?~lly.homologouu, diflering only in dittpe: and all po~~ible intermedlate grades of convolution mid complexity are to be found In cases of wry great complexity, it in true, the tpiradea may run together m that it is difficult to detera-iine thc limits of each, tmt I believe such twins ham the came oriffla.



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56 Psyche [June
carinse, simple, branched or plumose hairs, etc. This ornamenta- tion arises usually between the spiracles, and for descriptive pur- poses, I have numbered these inter-spiracular spaces, l, 2,3, and 4, beginning with the one mediad of the dorsal spiracle. See figure 1. (16) The nature of the delicate chitinous bars crossing the slit-like spiracles (MacGregor) ; a point. undoubtedly of very great impor- tance, but in many species too difficult of determination to be prac- tically useful.
(17) The nature of the chitinous ring surrounding the stigma1 plate; whether heavy or light, complete or incomplete; the last condition usually associated with the absence of the button or circular plate (MacGregor) .
The most casual student could not but note the great diversity in position of the slit-like spiracles with reference to each other and to the median line of the body. Banks uses in his synopsis of the groups (loc. cit., p. 15) such characters as "slits arranged radially," or "sub-parallel to each other" or "rather transverse to body. " MacGregor mentions "the orientation of the stigmata with refer- ence to the . . . longitudinal axis of the larva " and the position of the slits with respect to each other and to the median sagittal plane of the body.
But, so far as I am aware, no attempt has hitherto been made to use for specific description
(18) The accurate1 y -measured angle of divergence between the several spiracles, and
(19) Their divergence from the median line. However, from my observations in the Syrphidse and myiasis- causing species, I am convinced that much more can profitably be made of these points; and in the case of very closely related species, I believe this may at times prove to be the only reliable means of separation. These characters are, of course, available only in those cases where the spiracles are straight, or nearly so, so that their main longitudinal axis is readily determinable. I was much surprised, when I conceived the notion of using this method of description, that I was unable to find anywhere on the market the apparatus by which such angular measurements could readily be made under the microscope. I finally designed a micro-pr~tractor,~ for use in the ocular of the microscope, by means 1 Made by The Bausch & Lomb Optical Co., Rochester, N. Y., who guaranteed its accuracy to within half a degree.




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19191 MetcalfÌÔRespirator Apparatus of Dipterous Larva 57 of which it is very easy to gauge the angle of divergence of these parts. The micro-protractor (see figure 2) is a simple glass disk, 21 mm. in diameter, ruled in angles so designed and arranged that by using different combinations of adjacent larger and smaller angles as indicated by the concentric lines in figure 2, it is possible to measure very accurately any angle from 5' to 360' by 5å inter- '
vals. Even single degrees of divergence may be estimated with considerable accuracy by this method.
In order to use these points it is only necessary to place the quiescent (killed or paralyzed) larva or the puparium under the microscope in such position that the plane of the stigmal plates is perpendicular to the axis of vision. The ocular with the micro- protractor resting on its diaphragm is then revolved until some combination of its measured angles exactly coincides with the angle of divergence of the spiracles to be measured.l The amount of divergence of the dorsal spiracles from each other I have called angle A (See figure 1). In cases where the two stigmal plates are widely separated it may be easier to measure the divergence of either dorsal spiracle from the median line and this may be designated -- age A The amount of divergence between 2
the dorsal and median spiracles on either side is angle B: that between the median and ventral spiracles on either side is angle C; and that of the right and left ventral spiracles from each other is angle D. Here again the divergence of a ventral spiracle from the angle D
median line may be designated --
In all cases where the
2
outer (peripheral) ends of two spiracles are closer together than their inner (central) ends, the fact is indicated by designating the corre- sponding angle a minus one. This is often the case with angles A and D in the Syrphidse.
In addition to its use as described above, I believe the micro- protractor may prove useful to biologists in many lines of work where it is desired to measure the angles of microscopic objects. 1 Whenever it is possible to do so, much better results will be achieved by preparing the stigmal plates with 10 per cent KOH solution, as described by MacGregor and Herms in the papers cited above.




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