PHYLUM ARTHROPODA — CLASS INSECTA 301 



"The exoskeleton has made possible very definite advances in the 

 evolution of insects, but at the same time has limited their evolution 

 in fully as many other ways." 



Aside from the ehitinous exoskeleton, other distinctive character- 

 istics, such as power of flight, which is possessed by no other in- 

 vertebrate animal ; a tracheal system, which keeps the hemolymph 

 or blood from becoming impure ; and finally their great variability 

 and power to reproduce, have made the insects, no doubt, the suc- 

 cessful creatures they are today. This leads us to wonder how 

 successful man will be in his evolution during the next fifty million 

 years. Will he be able to meet the demands of a changing environ- 

 ment as the insects have? 



INSECT CHARACTERISTICS 



Insects are Arthropoda in which the body is divided into three 

 regions, the head, thorax, and abdomen. The head, which consists 

 of six segments, bears a single pair of antennae, the eyes, and the 

 mouth parts; the thorax consists of three segments and is the re- 

 gion which bears three pairs of legs and two pairs of wings, when 

 they are present in the nymphs and adults; the abdomen bears a 

 variable number of segments in the various groups of insects, also 

 the genital apertures which are situated near the anus at the pos- 

 terior end of the body. 



Head 



The head consists of a number of immovable plates or sclerites 

 forming the head capsule, to which are attached the paired append- 

 ages. In many insects the sutures separating the sclerites are 

 visible, and these plates and depressions have been given definite 

 names. The paired appendages furnish much evidence that the 

 head has resulted from the fusion of several segments. The eyes, 

 antennae, mandibles, maxillae, and labium are considered as de- 

 veloping on distinct somites. Evidence concerning the fusion of the 

 six anterior segments of the body in the formation of the head 

 comes through an embryological study of the insect. 



There are two kinds of eyes : ocelli, or simple eyes, and compound 

 eyes. The simple eye is a small area consisting of a single cornea. 

 Simple eyes are generally found in varying numbers along with 

 the compound eyes in adult insects; they are, however, usually lack- 



