Insect Epicuticle a Name Given to the Millimetre of Mercury Pressure As a Unit of Pressure
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No. 3940, MAY 5, 1945 NATURE 545 control of eelworm may be obtained, and this in Units for Degree of Vacuum addition to the already established control of onion fly and onion white rot, it would seem that the use IT has long been considered in this laboratory that of calomel in respect of onion crops may be very when working in the field of medium and high vacua valuable. (pressures of 1 mm. mercury or less), the usual F. 0. MosLEY. methods of referring to and recording the degree of Research Laboratory, vacuum are extremely awkward verbally and little The Nurseries, less so when written. Uxbridge. There are a number of known systems of recording 1 such pressures, the bar and the millimetre of mercury Nature, 155, 241 (1945). pressure (mm. Hg) and their subdivisions being most widely known, the micron being a of mm. and the Torr, found in German practiCe, bemg Insect Epicuticle a name given to the millimetre of mercury pressure as a unit of pressure. The millimetre of mercury pressure Alexander, Kitchene · and Briscoe1• 2 have shown is almost universally used, but when subdivisions of that the desiccation of many insects caused by inert this unit are used, recourse is necessarily made to dust insecticides is due to adsorption of the epicuticle either the negative index method or the decimal wax film, which becomes discontinuous and allows of method of reference or recording. Thus, a pressure increased loss of water through the cuticle. Further, of, say, 2 x J0-5 mm. mercury may be written so, Wigglesworth"·' has demonstrated abrasion of the or as 0 ·00002 mm. Hg ; both methods are lengthy wax film and increased evaporation of water caused to write, but verbally these expressions are even more by the application of an inert dust. It is evident, unwieldy. however, that insects differ conside ably with regard It is proposed, therefore, that a to the epicuticle, for nymphs of Rhodnius are un new unit be adopted, based on the affected by adsorption but are susceptible to abra J0-1 JO-· 55 gg logarithm of the numerical value sion" whereas Tenebrio and other larvro are more 48 46 of the pressure in millimetres of 1 to adsorption than to abras1on • 44 42 mercury. The reading in these Experiments with 'Neosyl' (a proprietary adsorbent units is, in fact, the logarithm of silica dust) and carborundum powder show that w-• 40 38 the pressure in millimetres, reduced Sarcophaga larvro are affected neither by adsorption to its all-negative form and multi nor abrasion, suggesting that they differ from both 36 plied by minus 10, this being Rhodnius and 'l'enebrio larvro with regard to their 34 similar to the method of measur epicuticular waxes. Histological examination ing power ratios, etc., in decibels, that lipoid substances are confined to a very thm the reference-level in this case be layer at the surface of a thin (4[L) protein epicuticle. 32 ing 1 mm. mercury pressure. A This surface layer differs from the remainder of the scale formed on this basis is very cuticle in being insoluble in cold concentrated hydr? simple in use, both for written chloric acid, and may therefore be isolated as a thm 10-· 30 record and verbal reference, result membrane. Prolonged treatment with lipoid solvents 2 ing in pressures of IG-t, 1G- ••• does not ·destroy the ability of this membrane .to 10-s mm. mercury, etc., becoming stain with Sudan Black B, and on account of Its vacua of 10, 20 ... 60 units, etc. 28 "' integrity after these treatments it is to ..., 5 ·s The pressure of 2 X I0- mm . regard it as a constituent layer of the cutwle, the mercury referred to becomes a outer epicuticle consisting apparently of a very "' .. vacuum of 47 units. This system stable lipo-protein complex. The bulk of the protem a 0 also leads to the logical result, that epicuticle does not stain readily with Sudan Black B, 26 "" a 'higher' vacuum has a higher and is almost completely untanned. .s .s numerical value . A similar double epicuticle is present in the cock s Such a system as that suggested roach Periplaneta•, which also possesses an external "' could, of course, be based on layer of labile fatty substance6 • It seems, therefore, ""' p..!!! any other unit of pressure, such that lipoid substances may be associated with the as the bar or the standard atmo 24 insect epicuticle in one or more of a number of ways. sphere ; but it is considered that They may be present as a readily removable surface the basic level of 1 mm. mercury layer ; they may form a complex with the surface pressure gives the working-range of the epicuticle so as to produce a distinct structural fitting best with current practice. layer ; and they may impregnate the .of Also a pressure of 1 mm. mercury epicuticle when this layer is tanned•. VariatiOns m is, in a most marked manner, lipoid distribution in the cuticle must clearly be the border-line between the field taken into account as one of the many factors deter 22 of mediiun- and high-vacua work mining the effectiveness of an inert dust insecticide and the field of low-vacuum work, on different insects. and there is seldom any over R. DENNELL. lap between the two fields. Con Imperial College of Science and Technology, sequently the fact that a pressure London, S.W.7. of greater than 1 mm. mercury 1 Alexander, Kitchener and Briscoe, Ann. Appl. Biol., 31, 143 (1944). would have a negative value on the 'Alexander, Kitchener and Briscoe, Trans. Faraday Soc., 40, 10 (1944). suggested scale is of little disadvant ' Wigglesworth, Nature, 153, 493 (1944). age, since where work is consistently 'Wigglesworth, Nature, 154, 333 (1944). 20 done in the low-vacuum range it is • Richards and Anderson, J. Morph., 71, 135 (1942). Fig. 1. SCALE OF recommended that the scale of • Ramsay, J. Exp. Biol., 12, 373 (1935). TYPICAL MCLEOD ' Pryor, Proc. Rov. Soc., B, 128, 393 (1940). GAUGE. FULL millimetres be adhered to. © 1945 Nature Publishing Group.