Decision of the number of instar and generation by means of measurements on the larval head, of May-beetle

Abstract

This is one of the reports in a series of our work concerning ecology of two species, SHIROSTIZTKOGANE (Granida albolineata MOTSCITUL SKY) and AoDoGANE (Anomala albopilosa HOPE.), by which pine trees planted on the sand dune of the southern coast of Boso Peninsula for a windbreaking forest, have been injured severely.
As these trees suffered from the above two species in their larval stage, it is fundamental to determine the number of moultings, of instar, and of generation for the effective control methods.
Direct observations on these items under field conditions difficult because the larval habitat is confined to under the ground, and the tearing of these species is also difficult under controlled conditions. The head of the insect in the larval stage is definitely chitinized in general, and is so in the two species dealt with here; furthermore, it is believed that the size of the head is almost changeless during a studium. Therefore, it may be proper to measure the head size as an indicator of these items.
The larvae were collected at three different times in 1954, and the length and breadth of parts of the head was measured with a micrometer.
The results of these measurements on a great number of .larvae are as follows, and the observations on the larvae bred in the field confirmed these results.
(1) Larvae of these two species undergo two moults in the larval stage.
(2) The head breadth is most reliable as an indicator showing each instar of SHIRO-SITZIKOGANE and AODOGANE. In the former the head length, frons length, and mandible length; in the latter the head length and the frons length are next to head breadth as indicator.
(3) In SIFEROSUZIKOGANE the total life cycle is completed in two years, whereas in AODOGANE it is one or two years; but the number of individuals extending over the two-year period seems fewer than those of the one-year.