XI 



THE POPULAR INSECT PLAN 



Preview. The insect body plan; the head and its appendages; the 

 thorax and its appendages ; honey manufacture ; digestion ; circulation, 

 respiration, and excretion ; the nervous system • Reproduction and life his- 

 tory • The life in the hive • Suggested readings. 



PREVIEW 



It would seem right in a text on biology that a representative of the 

 largest and most successful group of animals should be described and 

 that more than a passing glance be given to this enormous group, 

 which contains far more than half of all living animals. We are 

 always meeting insects, because they are so plentiful rather than 

 from choice. They annoy us when we are in the woods, they bite 

 us when we are lolling on the beach at the seashore, they get into our 

 foods and render them unfit for use, or they eat our stored clothes. 

 Worse than this, they defoliate trees, and sometimes destroy forests, 

 and take their tithe of the nation's food crops. A good many have 

 been implicated in the transfer of disease and some have actually 

 rendered regions uninhabitable by man. 



Biologists have a good reason for a study of representatives of the 

 great phylum, Arthropoda, because the arthropod plan of structure is 

 the one employed by the majority of the species of the animal king- 

 dom. In its simplest form, it represents an organism made up of 

 segments, each body segment bearing a pair of jointed appendages. 

 The head always bears at least one pair of jointed antennae or feelers, 

 jointed mouth parts, and usually compound eyes. The body is pro- 

 tected by an exoskeleton composed of chitin secreted by the cells 

 beneath. A digestive tract passes straight through the body and 

 there is a nervous system such as we saw in the Annelids, consisting 

 of a ventral nerve cord, a dorsal "brain," and a nerve ring about the 

 esophagus. Dorsal to the food tube is an elongated heart, there 

 being no closed system of blood vessels. Such a simple arthropod 

 would be difficult to find for laboratory purposes, so we have to use 

 other more specialized forms. 



From the strictly biological point of view there is another reason 

 for the study of an insect. It offers an example of a segmented ani- 

 H. w. H. — 14 199 



