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Volume 77, 1948-49
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The Fatty Acid Composition of Hemideina thoracica White

[Received by the Editor, November 5, 1947; issued separately, May, 1949.]

Complete fatty acid analyses of insect fats have been few, but recently Giral et al (1946) studied in detail the fats of two Mexican Orthoptera. It is interesting to see how a New Zealand Orthopterous insect, the Weta (Hemideina thoracica) compares with these and other insects.

Experimental.

The methods used in this investigation have been described by Hilditch (1940). Fifty insects of both sexes, total weight 88·4 gms., were mixed, dried under vacuum, and extracted with ethyl ether in a Soxhlet apparatus. The extract weighed 5·4 gms., a yield of 6·1% on the original weight. The fat was a soft, pale yellow substance having a strong odour resembling that of tanned leather. A cold acetone–CaCl2 separation yielded 3·3% of phosphatides, but as this amount was too small for further study it was recombined with the glycerides. Unsaponifiable matter amounted to 4·8%, while the yield of fatty acids was 3·66 gms. The acids were separated into solids and liquids by the modified lead salt method of Twitchell, and each fraction was esterified with methyl alcohol (Hilditch, 1940). The weight of solid esters was 0·71 gms. = 22·1% of the total esters, while the liquid esters weighed 2 51 gms. = 77·9%. The solids were. too small in amount, to fractionate, but the liquids were distilled in a Vigreux column of 20 ml. capacity, under a vacuum of approximately 0·05 mm. Although the fractions were small, their separation was indicated by quite distinct increases in the boiling points of the vapour. Tables I and II show the compositions of the liquids and policls, while Table III shows the fatty acid composition of the fat, excluding unsaponifiable matter.

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Table I—Methyl Esters of Liquid Acids.
Fraction No. Weight gms. Temp °C Saponification Equivalent Iodine Value Saturated C10 Unsaturated C10 C18 Unsaponiflable
L 1 036 130–132 275.9 84.8 .042 .212 .106 -
L 2 0.57 152–158 283.8 86.0 .060 .177 .333 -
L 3 0.05 158–172 289.0 91.6 .033 .096 .521 -
L 4 0.52 172 289 9 - - - .510 .010
2.10 gms. Totals .135 .485 1.470 0.010
% Esters 6.43 23.10 70.00 0.47

C18 unsaturated methyl esters calculated as Saponification Equivalent 295.8; Iodine Value 96.5, Mean Unsaturation 2.2H = 1.1 double bond.

Table II–Methyl Esters of Solid Acids.
Weight 0.71 gms.
Saponification Equivalent 301.1
Iodine Value 23.1

The Iodine Value of the fraction was assumed to be due to oleic methyl ester, giving C18 Unsaturated 2 OH = 0.101 gm. Total solids (by difference) = 0.519 gm. Saponification Equivalent of Total Solids (calculated) = 303.1.

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Table III—Fatty Acid Composition of Weta Fat
Esters of Solid Acids. (22.1%) Esters of Liquid Acids (77.9%) Total Esters Total Acids Excluding Unsaponifiable Mols Ratio Per cent
Total Saturated 16.16 5.01 (C16) 21.17 * 21.28 07580 20.9
Unsaturated:
C16 17.99 (2.0H) 17.99(2.0H) 17.99(2.0H) .07084 19.0
C18 5.94 (2.0H) 54.53 (2.0H) 60.47 (2.0H) 60.73 (2.0H) .21570 59.5
Unsaponifiable 0.37 0.37

The figures in brackets indicate the number of hydrogen atoms required to produce the corresponding saturated acids or esters.

Discussion.

Table IV compares the fatty acid compositions of Weta and other Orthopterous insect fats.

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Table IV—Fatty Acid Composition of Some Orthopterous Insect Fats
Hemideina thoracica. (Wgts. %) Sphenarium purpurascens. Melanoplus atlanis. Oxya japonica.
References: Giral (1946) Giral (1946) Tsujimoto (1929)
Saturated:
C11 2.9
C16 14.8 4.3 25.0 (b)
C18 21.3 (a) 11.4 12.2
C20 2.8
Unsaturated:
C16 18.0 (2.0H) 9.6 (2.0H) 4.1 (2.0H)
C18 60.7 (2.2H) 35.5 (2.9H) 29.9 (2.8H) 75.0 (b)
C20 25.8 (2.9H) 38.4 (3.8H)
C22 5.3 (3.7H)

(a) Mainly palmitic and stearic acids.

(b) Saturated acids palmrtic and stearic.

Unsaturated acids oleic, linoleic, linolenic.

Proportions of each acid not given.

Giral (1946) states that the preponderance of stearic over palmitic acid is exceptional in animals, but is found in Melanoplus atlanis. The saponification equivalent of the Weta solid methyl esters indicates that in this insect also stearic acid may predominate. It will be seen that the proportions of total saturated and total unsaturated acids are similar for all the insects. However, C20 and C22 unsaturated acids are absent in the Weta, while the corresponding saturated acids are present in traces only, if indeed they occur at all. This is an important difference from Giral's analyses of the fats of Sphenarium and Melanoplus, but agrees with Tsujimoto's work on the Locust Oxya (1929). It is interesting to note that the Weta also belongs to the Tribe Locustidae.

The C18 unsaturated acids oleic, linoleic and linolenic containing one, two and three double bonds respectively are commonly found in insect fats, the first two being almost invariably major constituents (Hilditch, 1940). Giral (1946) stated that Sphenarium and Melanoplus contained no linolenic acid. The C18 unsaturated acid group of

[Footnote] * Calculated saponification equivalent of total solids = 294.0.

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Hemideina contains an average of 1·1 double bonds per molecule, from which it appears that oleic acid is the main constituent of the group.

The simple fatty acid composition of Weta fat is surprising in view of the fact that the insect is vegetarian, feeding mainly upon leaves (Maskell, 1927). No information is available about the fats of the leaves on which the Wetas feed, but published data show that leaf fats, although complex in composition, have certain characteristics common to all. From the data presented by Hilditch (1940) and Shorland (1945, 1946) the salient features of leaf fats are:

(a)

Total saturated acid content is variable, but according to Hilditch averages 10%. The leaf fats of the Rape (Brassica napus) [Shorland, 1945] and Cocksfoot (Dactylis glomerata) [Shorland, 1946] contain 15·5% and 16·7% respectively of saturated fatty acids. Palmitic acid is the principal saturated fatty acid.

(b)

The C18 unsaturated acids are linolenic and an isomer of linoleic acid, both major constituents. Oleic acid is present in traces only.

The Wetas used in this investigation were collected from many different species of trees and other hiding places, so it is reasonable to suppose that their total dietary leaf fat would have the above characteristics. The main differences between leaf fats and the Weta fat are:

(a)

The C18 unsaturated acids of the Weta have a lower degree of unsaturation.

(b)

The total saturated acid content of Weta fat is higher than that of leaf fats, and is of higher molecular weight.

Two possible explanations for these differences suggest themselves:

(a)

The dietary fat is not modified in composition, but its character is masked by the synthesis of a fat peculiar to the insect.

(b)

The dietary fat is considerably modified either with or with out synthesis of fat from other dietary sources.

The data available are insufficient to enable the matter to be solved, but it is interesting to note that another insect has been proved to have the power to alter dietary fat to a considerable degree. Collin (1933) studied the beetle Pachymerus dactris, which was fed exclusively on the kernel of the nuts of Manicaria saccifera. Lower molecular weight fatty acids were found to occur in the insect fat in only half the quantity found in the kernel fat, while oleic and linoleic acids, which comprised about 40% of the insect fat, comprised only 11% of the kernel fat. These facts suggest that the insect derived its body fat partly from assimilation of the dietary fat and partly by synthesis.

Summary.

The fat of Hemideina thoracica has been shown to have a simple composition, which differs from the probable fatty acid composition of the dietary leaf fats.

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Acknowledgments.

I desire to thank Mr. R. E. R. Grimmett, former Chief Agricultural Chemist, Department of Agriculture Chemistry Section, Wellington, for facilities to undertake this work, and Dr. F. B. Shorland. Scientific and Industrial Research Department, for advice and criticism.

References./

Collin, G., 1933. Biochem. J., 27, 1373.

Giral., J., 1946. J. Biol. Chem., 162, no. 1.

Hilditch, T. P., 1940. The Chemical Constitution of Natural Fata. Chapman and Hall, London.

Maskell, F. G., 1927. Trans, and Proc. N.Z. Inst., 57, 637

Shorland, F. B., 1945. Nature, 156, 269.

—— 1946. lbid., 153. 168.

Tsujimoto, M., 1920. Am. Chem. Abs., 23, 1443.