Studies on the growth of Pediculus (Anoplura)

Publisher Website
Google Scholar
Abstract
1. The paper discusses the points of difference between the three larval instars in Pediculus and the changes in proportion which take place during growth, not only during larval life but also at the final moult.2. It is easy to separate larvae of the first instar from others, more difficult to distinguish those in the second and third instars. The principal points of difference are as follows:First instar. Setae on dorsum of abdomen, few, long and in longitudinal lines: one median pair on most segments. Paratergal plates (carrying spiracle) absent. Mean measurements as in Table I, the best point of distinction between instars being in length of third tibia, which is 0·197 mm. in first instar. Ratio of head to thorax and abdomen about 1:2·8Second instar. Setae of dorsum of abdomen in one transverse row, containing 2–3 pairs of setae. Paratergal plates present (as also in third instar). Mean length of third tibia, 0·270 mm. Ratio of head to thorax and abdomen about 1: 3·5.Third instar. Setae on dorsum of abdomen, not in one row, but somewhat irregular: a pair of minute supplementary setae, on seventh and eighth segments, close to midline. Mean length of third tibia, 0·363 mm. Ratio of head to thorax and abdomen about 1:4·3.3. During larval life the dimensions of all parts increased as geometrical progressions (Dyar's law), but the rate of increase is different in different parts of the body, so that the proportions, for instance, of head to thorax and abdomen alter with growth; in other words, growth is allometric or heterogonic. But when the insect attains maturity the measurements of several of the parts depart significantly from the geometrical progression which has prevailed during larval life. In most parts of the body the increase at maturity is greater in females than males (no point of distinction having been observed in larvae in the preceding instars); but in the length of the third tibia the increase in the female is much less than in the male. Among other insects in which metamorphosis is slight there are several in which the rate of growth of parts alters at the last moult and becomes different in the two sexes; in some indeed the difference develops much earlier than the last moult. Probably, therefore, it would be true to say that regular allometric growth is generally disturbed by the development of secondary sexual characters.It is a pleasure to acknowledge help received from Dr J. S. Huxley.

This publication has 15 references indexed in Scilit: