The Growth of the Head Capsule in Individual Milkweed Bugs

Dyar (1890) reported that the width of the insect head capsule in one instar seemed to have a constant ratio to that in the preceding instar throughout larval development. Since then, many investigators have reported data bearing on this rule. In an extended review, Gaines and Campbell (1935) plotted the logarithm of the mean head width against the instar number. The points deviated convexly from the straight line predicted by Dyar's rule but could be fitted with a parabola. Their observations, those of the many investigators they quote, and also those of Yasumatsu (1946) were based upon means computed from a different lot of insects in each instar. This technique requires relatively large numbers for validity. An alternative approach is to measure the same individuals repeatedly through their larva or nymphal development. Such measurements have been made on nymphs of the milkweed bug, Oncopeltus fasciatus , and these form the basis of the present note.