Micromethod for Determining Insect Hemolymph Specific Gravity (Periplaneta americana Linn). ∗

Abstract

This is a preliminary report of a falling drop method (based on Stokes's law) that permits the determination of the specific gravity (S.G.) of a single drop of insect hemolymph (4.85 mm.³ or less) within the temperature range of 15°C.-40°C., with an average accuracy of ±0.0021 (1 determination) or ±0.0008 (mean of 10 determinations), and with an average determination time of about 8 minutes. The diameter of the hemolymph drop at a beeswax-mineral oil surface is measured with a calibrated ocular micrometer scale. The hemolymph drop, surrounded by oil, is sucked into a glass tube and thus transferred to the sedimentation cylinder. The time required for the drop to sediment through a given falling distance in mineral oil is measured and converted to falling time at 25°C. by means of conversion factors calculated from the equation

in which n is the viscosity of the oil medium, p_s is the S.G. of the drop, p_m is the S.G. of the oil medium, t is the falling time and the subscripts ₁ and ₂ refer respectively to the experimental temperature and to 25°C. The correction factor, k, for fluidity of the drop, was defined by Hadamard 1 as k = (2n + 3n′)/(3n + 3n′), where n is the viscosity of the medium and n′is that of the drop. This variable factor was found to be k₂ = 0.6692 at 25°C. By means of curves that show falling time-specific gravity difference relationships for drops of various sizes at 25°C., the drop-medium S.G. difference is found and is added to the S.G. of the medium to obtain that of the hemolymph at 25°C. The curves were calculated by means of the equation

which is derived from Stokes's law

In these equations, d is drop diameter, D is diameter of the container of the medium, h is the height of the column of the medium, g is the gravitational constant, a is the radius of the drop, s is the falling distance, V is the falling velocity and n, k, p_s, p_m and t₂ have the meanings already given them. The factors (1 + 2.4 d/D) and (1 + 1.65 d/h) correct for side wall and end effects of the container of the medium. 2