A Method of distinguishing the larval Stages ofAgriotes sputator(L.) (Col., Elaterid.)

Abstract

The different instars ofAgriotes sputatorlarvae cannot be distinguished by measurements of total length or of various parts of the wireworm.The number of teeth on the mesothoracic and abdominal spiracles increases with age, and an examination of 700 wireworms showed that the average numbers of teeth on the two thoracic or on all the abdominal spiracles fell into eight groups. These criteria were valid for populations collected at different times of the year.The number of teeth on the thoracic spiracles of a larva in any particular group approximates to that on the abdominal spiracles of a larva in the next larger group.The eight groups formed by counting either the thoracic or the abdominal spiracle teeth representgrowthstages and not necessarily instars.The larvae may sometimes moult without growth, a phenomenon probably caused by an inadequate supply of food. At such an ecdysis the number of spiracle teeth does not increase.The larvae pupate after attaining an optimum size, and reach this size in seven or eight growth stages.A quick method of determining the growth stage to which a larva belongs is given, whereby more than 80 per cent, of the larvae are placed in their correct growth stages by counting the teeth on one spiracle only ; for less than 1 per cent, of the larvae is it necessary to examine as many as five spiracles ; an accuracy of more than 97 per cent, can be maintained.The possibility of determining the relationship of growth stage and age is discussed.The division of the larvae into those which will pupate after seven, and those which will pupate after eight growth stages is apparent at a very early stage in the life history. One larva was found which appeared to be in its ninth growth stage.The cause of this difference in growth stage number is unknown. It may be due to the size of the egg, to the time of the year at which hatching occurs, or to heredity.