The Fatty Acid Composition of Hemideina thoracica White

By A. P. Oliver, Dominion Laboratory, Wellington.

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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(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

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:

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:

Two possible explanations for these differences suggest themselves:

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.