ON A SUGGESTED METHOD FOR DETERMINING THE NUMBER OF LARVAL INSTARS IN SITODREPA PANIGEA L.

Abstract

SUMMRY AND CONCLUSIONS A number (887) of larvae of Sitodrepa panicea L. were examined with a view to testing the application of Dyar's law and the possibility estimating the number of larval instars by measuring the head capsule a random population. The measurements were found to fall into two groups, one slightly larger than the other, whose growth ratios approximated to two geometrical series, one with a common ratio of 1–12, the other with a common ratio of 1.11. It is suggested that these two groups represent the sexes. On this hypothesis it would appear that the males undergo a minimum of four larval ecdyses, and the females a minimum of five. Miles (loc. cit,) postulates a similar phenomenon for several species of Tenthredinidae. Furthermore, the sex ratio is not unity. The method of investigating the growth ratio of the larva by the measurement of a large population, is not entirely satisfactory. Owing to diversity of causes such as the periodicity of egg‐laying, insufficient numbers of the young larvae were obtained, and no satisfactory conclusions with regard to the number of these early instars can be reached. The analysis of the data is further complicated by the variation in size and presumed overlapping of the instars in both sexes. Until much larger and more representative populations, or many more samples of small populations can be examined, such a method of determining the number of larval instars in a given insect can be but of debatable value.