INTRODUCTION

In ticks, as in most animals, chemical mediators guide behaviour. These information-bearing compounds, known as semiochemicals, are secreted external to the animal body and, when recognized, direct a specific behavioural response such as food and mate location, escape and other behaviours. Chemical signalling between individuals is clearly one of the earliest types of information exchange to appear in the long history of life on Earth, long before visual or auditory stimuli developed. Indeed, chemical communication via semiochemicals remains the dominant form of communication among many animals. Despite similarities with cell signalling (e.g. cytokines) among the cells of the metazoan animal, or hormones (e.g. ecydsteroids) that stimulate specific physiological responses (e.g. moulting), semiochemicals are fundamentally different in that they are secreted outside of the animal body, are recognized externally and modify the behaviour of the entire individual. With the advances in modern chemistry, biochemistry and molecular biology during the past several decades, a vast literature has accumulated concerning the variety of semiochemicals, their chemical composition, biosynthesis, secretion and perception, and the varying biological roles that these compounds regulate.

Collectively, the repertoire of chemical compounds used within a species or among competing species forms a simple chemical communication system, or chemical language. In many species, this chemical language consists of an ordered hierarchy of specific compounds that are secreted and perceived in a precise, sequential order leading to a desired end result. In others, a single compound (e.g.