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Title: Distribution, composition and water permeability of locust cuticular lipid
Author: Oraha, Victor Shabi
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 1989
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In this work, the distribution, chemical composition and water permeability of the cuticular lipids of the African migratory locust, Locusta migratoria migratoriodes and the desert locust, Schistocerca gregaria have been investigated. Scanning electron microscopy of the wings and terga of the two species showed that the epicuticular lipid formed a liquid coating over the surface. The lipid layer on the fore wing of L.migratoria migratoriodes was thinner and more crystalline than that of S.gregaria. The tergum of L.migratoria migratoriodes had a smooth epicuticular lipid layer while that of S.gregaria had a lipid layer with amorphous lipid masses. Transmission electron microscopy revealed that the wing and tergal cuticle of both species conformed to the basic structural plan of insect cuticle, namely an outer, thin epicuticle and an inner, thick procuticle divided into an outer exocuticle and an inner endocuticle, overlying a single layer of cells, the hypodermis. While no structural differences were observed in the wing cuticle of the two species, their terga differed, particularly in the procuticle. In L.migratoria migratoriodes, the procuticle was about 3 times thicker than that of S.gregaria, while the endocuticle of the latter possessed well developed lipid lamellae which were only poorly developed in the endocuticle of L.migratoria migratoriodes. Chemical analysis of the cuticular lipids of L.migratoria migratoriodes and S.gregaria using thin layer chromatography revealed the presence of five major fractions namely, hydrocarbons, esters, triglycerides, free fatty acids and free primary alcohols. The cuticular lipid of S.gregaria contained higher proportions of hydrocarbons (wings) and triglycerides than the lipid of L.migratoria migratoriodes, while the cuticular lipid of the latter had higher proportions of esters, free fatty acids and alcohols. 47% of the free fatty acids, triglyceride fatty acids and ester fatty acids in L.migratoria migratoriodes were unsaturated, while only 3% were unsaturated in S.gregaria. The hydrocarbons of the two species, which were saturated and comprised n-alkanes and methylalkanes, consisted of the following classes: n-alkanes (class A), terminally branched monomethylalkanes (class C2, 3-methylalkanes), internally branched monomethylalkanes (class D) and dimethylalkanes (class E). S.gregaria possessed one more class, namely, trimethylalkanes (class F). The hydrocarbon mixture of S.gregaria contained a higher proportion of n-alkanes and a lower proportion of methylalkanes than the mixture L.migratoria migratoriodes . The lipid layer on the hind wing of S.gregaria was found to be thicker than that of L.migratoria migratoriodes. The intact hind wings and abdominal terga of S.gregaria were found to have lower water permeabilities at temperatures 25, 35, 45, 55 and 65 C than those of L.migratoria migratoriodes. Extracted hind wings had 8-times the water permeability of intact wings in both species. While the extraction of abdominal terga doubled water permeability in L.migratoria migratoriodes and increased permeability 1.5 times in S.gregaria. The water evaporation from the intact wings and terga of both species was found to rise over a range of 40°C. A gradual rise in water permeability was found to occur between 25 and 45°C and a steep rise between 45 and 65°C. The shape of the water evaporation curves of the two species was sim ilar, though the curve of S.gregari was shifted to the right of that of L.migratoria migratoriodes, indicating a lower water permeability in S.gregaria over a range of temperatures. The effect of amount/unit area, composition and temperature on the water permeability of lipid was investigated by spreading synthetic and natural mixtures of lipids on the extracted hind wings of S.gregaria and L.migratoria migratoriodes. Natural mixtures of lipids were found to be more efficient in reducing water permeability than synthetic mixtures. Synthetic and natural mixtures of n-alkanes, alcohols and esters formed layers which were highly impermeable to water, while synthetic mixtures of alkenes and synthetic and natural mixtures of fatty acids and triglycerides formed layers which were permeable to water. Natural mixtures of methylalkanes were only partially efficient in reducing water permeability, while synthetic mixtures of methylalkanes formed highly permeable layers. Methylalkanes, alkenes and alcohols were found to spread readily over a cuticle surface, while n-alkanes and fatty acids showed only limited spreading. Triglycerides followed by esters showed the lowest spreading characteristics. Compared with L.migratoria migratoriodes, S.gregaria has a thicker lipid layer on the hind wings, a cuticular lipid containing a high proportion of n-alkanes and a low proportion of unsaturated fatty acids and terga with well developed endocuticular lipid lamellae. All of these features may be regarded as adaptations to living in arid desert conditions.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available