10 INTRODUCTION. 



emit a volatile and very corrosive fluid, which explodes and vola- 

 tilises as soon as it comes in contact with the air. Almost all 

 insects possess the power of producing sounds, and some are 

 phosphorescent ; but these powers appear to be exerted rather to 

 attract their mates than for defensive purposes. 



At first sight, the enormous number of insects in existence 

 would appear to render it a hopeless task to attempt to learn any- 

 thing about them. The larger animals are comparatively few in 

 number, and when we speak of an elephant, a horse, a lion, a dog, 

 etc., it is sufficiently intelligible for most practical purposes ; but 

 how can we make ourselves understood when we wish to speak of 

 some particular insect among the thousands that inhabit any given 

 country? This can only be effected by an arbitrary system of 

 classification ; and the want of this placed almost insuperable 

 difficulties in the way of those who would have studied natural 

 history; for it made any satisfactory and permanent register of 

 acquired knowledge almost impossible. At length, however, in 

 the course of the last century, and partly guided by the tenta- 

 tive efforts of earlier naturalists, Linn6, after various experiments, 

 devised the binomial nomenclature, and applied it, so far as his 

 knowledge extended, to the whole system of nature. Every 

 animal and plant has now two names, a generic and a specific 

 name, by which it is always spoken of, and which distinguishes it 

 from any other species. The specific name is frequently a Latin 

 adjective, and the generic name is generally of Greek derivation. 

 Every assemblage of individuals which appears sufficiently distinct 

 from its nearest relatives is called a species; and those species 

 which have a distinct general resemblance are said to belong to 

 the same genus. Genera again are assorted into sub-families, 

 families, tribes, and orders ; and thus it becomes possible to dis- 

 criminate between the immense numbers of the insect race. Those 

 specimens of a species which agree best with its characters are 

 called typical, while those which present casual or local variations 

 are called aberrations, varieties, or local forms ; and of course many 

 entomologists differ in their estimate of species and varieties ; but 

 this does not affect the general principle. To avoid confusion, no 

 two species of one genus are allowed to bear the same name ; and 

 no generic name is allowed to be used twice in zoology. The 

 name used by the first describer of a species is always retained, 

 and if the same species has accidentally received two names, the 

 name of later date is called a synonym, and drops out of use. 



