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The Wetting of Insect Cuticle*

  • Rajindar Pal (a1)
  • DOI:
  • Published online: 01 July 2009

The best method of assessing the wetting powers of liquids is to measure the contact angle formed with a particular solid surface. In order to study the wetting of insects by spray liquids, it was necessary to measure, as rapidly as possible, the contact angles of very small droplets on restricted surfaces (such as portions of the insects). The apparatus used projected a greatly enlarged image of the drop, the outline of which could be traced very quickly and used for subsequent calculations.

Under practical conditions, spray droplets impact on insects with some relative velocity either due to drift of the spray particles or to flight movements of the insect.Measurements were therefore made of the contact angles formed by droplets of water of known size falling on to insects (or to a beeswax surface) at a known speed. It was found that with rather large drops (5 mm. diameter) the contact angles formed were somewhat lower than the normal advancing contact angle. With small droplets (0·1-0·5 mm. diameter) there was no difference. Biological tests were made with 30 species of insects, differing widely in the nature of their integument and habitat. The resistance to wetting was found to vary greatly, not only from species to species but also on different parts of the body of a single insect. In general, most of the insects were readily wetted by oils and unwetted by water. Insects withhard cuticular lipoids, such as Tenebrio, were more hydrophobic than the Blattidswith greasy cuticular waxes. A few species were both lipophilic and hydrophilic (larvae of Mamestra, Polia, Musca). Apart from the chemical nature of the cuticle, irregularities and the presence or absence of hairs were important. Increased roughnesslowers the contact angles of liquids with good wetting powers, but has the opposite effect with liquids with poor wetting powers. Measurements were made of thecontact angles formed on individual hairs of Arctia caja larvae by spray droplets. The contact angles formed on these hairs by plain liquids were strongly correlated with the angles formed on smooth Tenebrio cuticle or on an artificial beeswax surface; but there was only very rough correlation between the three sets of data when aqueous solutions of wetting agents were tested.

The test liquids included saturated and unsaturated hydrocarbons, mineral oils with varying aromatic contents, organic esters, alcohols and aqueous solutions. Among the hydrocarbons, members of the aliphatic series wet insect cuticle more readily than the aromatic group. The aromatic contents of the oils did not, however, affect their very high wetting powers.

The effects of adding surface active agents to aqueous sprays were investigated. Of the samples tested, the most effective wetting agents were those with neutral un-ionised molecules.

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Y. Ben-Amotz & W. M. Hoskins (1937). Factors concerned in the deposit of sprays. III. Effects of wetting and emulsifying powers of spreaders.—J. econ. Ent., 30, pp. 879886.

G. T. Brown & W. M. Hoskins (1939). Factors concerned in the deposit of sprays. V. The effects of pH upon the deposit of the oil and water phases of oil emulsions.—J. econ. Ent., 32, pp. 5761.

A. B. D. Cassic & S. Baxter (1945). Large contact angles of plant and animal surfaces.—Nature, Lond., 155, pp. 2122.

W. A. L. David (1946 a). The quantity and distribution of spray collected by insects flying through insecticidal mists.—Ann. appl. Biol., 33, pp. 133141.

W. Ebeling (1939). The rôle of surface tension and contact angle in the performance of spray liquids.—Hilgardia, 12, pp. 665698.

G. E. Fogg (1947). Quantitative studies on the wetting of leaves by water.—Proc. roy. Soc., (B) 134, pp. 503522.

C. C. Hamilton (1930). The relation of the surface tension of some spray materials to wetting and the quantity of lead arsenate deposited.—J. econ. Ent., 23, pp. 238251.

G. S. Hensill & W. M. Hoskins (1935). Factors concerned in the deposit of sprays. I. The effect of different concentrations of wetting agents.—J. econ. Ent., 28, pp. 942950.

W. M. Hoskins (1940). Recent contributions of insect physiology to insect toxicology and control.—Hilgardia, 13, pp. 307386.

W. M. Hoskins & E. L. Wampler (1936). Factors concerned in the deposit of sprays. II. Effect of electrostatic charge upon the deposit of lead arsenate.—J. econ. Ent., 29, pp. 134143.

W. R. Lane (1937). A microburette for producing small liquid drops of known size.—J. Sci. Instrum., 24, pp. 98101.

G. L. Mack (1936). The determination of contact angles from measurements of dimension of small bubbles and drops.—J. phys. Chem., 40, pp. 159167, 169175.

F. Stellwaag (1924). Die Benetzungafähigkeit flüssiger Pflanzenschutzmittel und ihre direkte Messbarkeit nach einem neuen Verfahren.—Z. angew. Ent., 10, pp. 163176.

E. L. Wampler & W. M. Hoskins (1939). Factors concerned in the deposit of sprays. VI. The role of electrical charges produced during spraying.— J. econ. Ent., 32, pp. 6169.

R. N. Wenzel (1936). Resistance of solid surfaces to wetting by water.—Industr. Engng Chem., 28, pp. 988994.

V. B. Wigglesworth (1947). The epicuticle in an insect, Rhodnius prolixus (Hemiptera).—Proc. roy. Soc., (B) 134, pp. 163181.

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Bulletin of Entomological Research
  • ISSN: 0007-4853
  • EISSN: 1475-2670
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