Problems of Growth of the African Migratory Locust

Abstract

General Growth.Generally, females have higher rates of growth than males. The phases, however, do not show appreciable differences in the rate. The pronotum has for increase in length the highest values which decrease throughout the instars (whereas the constants for the other parts remain fairly stable up to the fifth instar).Dyar's rule was applied for the growth in length of the middle femur and the width of the head, and it was found that the rule holds good for these parts.Przibram's rule, as modified by Bodenheimer, holds true for the growth in length of the different parts and shows the occurrence of latent cell-divisions varying from one (width of head and of pronotum) to four (length of pronotum). The number of latent cell-divisions keeps fairly constant in both phases.For wet weight Przibram's principle is inapplicable, owing to the large percentage of differences between the actual and calculated values.Gregarious males are heavier than solitary males up to the third stadium ; gregarious females are heavier than solitary females up to the third stadium ; fourth, fifth and adult stadia being characterized by higher values in wet weight for solitary females than for gregarious females. Females have higher rates of increase in wet weight than males. No significant differences exist in the rates of increase between gregarious and solitary individuals. In the fifth-adult stadium all the rates decrease except in gregarious females, which show a rise.Gregarious insects have higher values in dry weight than solitary insects, except solitary females in the adult stadium. The coefficients are higher for females than for males.The rates of increase reach the highest values in the second-third stadium of gregarious insects and solitary females, whereas solitary males have their highest value in the fourth-fifth stadium.With the exception of solitary females, all the rates of growth in dry weight decline in the fifth-adult stadium.The rates of growth of the hind legs obtained from the cube-roots of their wet weights are compared with the rates of linear growth of the hind femora. Their variation throughout the instars seems to be in opposite directions. Therefore it is suggested that the rates of growth in wet weight of the hind legs and the rates of growth in length of the respective hind femora are independent of each other.Growth of the parts.The application of the exponential allometry formula y=bxα to the data on dimensions of the parts of Locusta shows the existence of negative, positive and almost isometric growth.The pronotum has the highest value for the growth in length relatively to the growth in length of the middle femur ; the lowest value pertains to the growth in width of the head.Males have higher values than females ; phase gregaria exhibits higher growth-ratios than phase solitaria.With the exception of the hind femur the growth-ratios decline throughout the instars. The greatest decline pertains to the growth in length of the pronotum.A growth-gradient exists in Locusta with the highest value in the pronotum. The middle femur divides the growth-gradient into two parts : an anterior part with values decreasing with the growth of the insect, and a posterior part whose values increase with its growth.Effects of the amputation of the hind tibiae on crowded locusts.The effects obtained by mutilating both hind tibiae of three hundred first instar hoppers of Locusta migratoria migratorioides and placing them in a crowded condition are compared with the effects obtained by crowding a batch of the same number of first instar unoperated insects.The insects with their hind tibiae cut off did not develop as far as those of the control batch ; the differences in dimensions are greater for the hind femur than for the other parts of the body.In the experimental batch the hind femur, as a consequence of its useless condition, became extremely short as compared with the elytra, bringing the ratio E/F to a high value (over 1·950), thus leading to a false interpretation.The occurrence of the black-orange coloration in both batches suggested that both developed towards phase gregaria. This coloration was stronger and more uniform in the control batch than it was in the experimental batch. Thus the control animals developed into a better gregarious type.