The Chemoreceptors of the Wireworm (Agriotes Spp.) and the Relation of Activity to Chemical Constitution

Abstract

1. This paper continues the work on the responses of wireworms (Agriotes spp.) to chemical stimulation, published in a previous paper (Thorpe et al. 1947), by a further analysis of the relation of activity to chemical constitution and of food finding, and the investigation of the anatomy and physiology of the chemoreceptors. 2. Substances present in food plants elicit two responses in wireworms, orientation and biting. There is a clear distinction between those compounds which cause only orientation and those which cause both biting and orientation. In almost every case the former have high activities (9-11) while the latter have activities of less than 3. All the biting compounds tested for orientation have proved active, the threshold being similar to that for biting. 3. Substances causing orientation fall into two classes, viz. amides and urea derivatives, and dicarboxylic acids. In view of the small number of compounds tested the two classes of active substances should be treated with reserve. In the first, it seems definite that the amide group is responsible for activity. But while all the other compounds found to be active fall into the second class, i.e. are dicarboxylic acids, it is by no means true that all dicarboxylic acids are active and no rule is apparent for predicting whether a given dicarboxylic acid will be active or not. 4. Substances causing both orientation and biting summate when either response is tested. The following pairs of substances summated when tested for the responses indicated: glucose and sucrose for orientation and biting; glucose and peptone, glucose and triolein, peptone and triolein and glucose and tannin for biting. 5. Orientation occurs in a gradient of concentration of which the steps are formed by different substances, as well as when the steps are formed by one substance as was shown in the previous paper. 6. Active substances are secreted in small amounts by the roots of growing potato plants. These would explain the orientation of wireworms to the roots of growing plants (e.g. wheat). It is thus likely that the root system forms an extended trap along which wireworms may be led to the grain or tuber in the centre, where a wound caused by their random biting would release active compounds in sufficient concentration for them to remain and feed. 7. The biting of glucose is inhibited by a number of substances, viz. lead acetate, quinine, allyl-iso-thiocyanate and common salt, as well as by acid and alkaline solutions. Common salt also inhibits orientation in glucose, but not in asparagine. 8. The chemoreceptors which appear to be involved in these responses are of two kinds: peg organs, located on the labial and maxillary palps and galeae, whose stimulation leads to both orientation and biting; and a cup-shaped sensilla on the distal segment of each antenna, whose stimulation leads to orientation only. Both asparagine and glucose stimulate both kinds of receptor.