I. WHAT INSECTS ARE, AND HOW 

 THEY ARE CLASSIFIED 



What is an Insect ? 



Most people, when they speak of an 'insect', have in mind any small, 

 creeping thing, which may possibly fly, and which probably bites or 

 stings. Zoologists use the name in a much more restricted sense, and 

 apply it to one of the Classes, or major subdivisions of the Animal 

 Kingdom : the Class I N s E c T A . 



The living world is divided into two great Kingdoms, the Animals 

 and the Plants. It is true that some organisms, such as bacteria and 

 viruses, are difficult to classify, and some others seem to be part-plant, 

 part-animal. Nevertheless all the hving things that we normally see, if 

 they are not plants, are animals. People who speak of ' animals and birds ', 

 or 'animals and insects' are confusing 'animal' with 'mammal', a 

 warm-blooded animal with hair or fur. 



Insects, then, are animals. Each major group of animals is called a 

 Phylum, and consists of animals that have the same general construction 

 of the body, and the same arrangement of the principal organs. The 

 Phylum Arthropoda gets its name from the Greek for 'jointed legs', 

 and groups together the animals that have the outer surface of the body 

 hardened, so that it forms a protective shell, as well as a support for the 

 soft internal organs. This exoskeleton, as it is called, will bend a little 

 in the living animal, but it is so stiff that it has to be jointed, hke a suit 

 of armour, in order to allow the animal to move about. 



Arthropods, therefore, are segmented animals, with the body-surface 

 divided into plates or sclerites, and the body as a whole planned as a 

 series of compartments, or segments. Each segment may have append- 

 ages, or limbs of various kinds, which in one part of the body may be 

 used as feelers (antennae), and in another as mouthparts, or as legs, or as 

 sexual organs. The appendages, too, are segmented, with flexible joints. 



Obviously an animal cannot grow while enclosed in such an un- 

 yielding case, and so the hardened outer integument or cuticle is shed at 

 intervals in the process of ecdysis, or moulting. The Arthropod then 

 grows rapidly as long as the new integument is soft and elastic : as soon 

 as this has hardened, usually a matter of hours, the size remains fixed 

 until the next moult. If the integument remains soft, as, for example, 

 in many larval insects, growth may go on between moults. 



