THE STUDY OF INSECT LIFE 5 



comprising the Scorpions, Spiders, Mites, and 

 Ticks. 



The Insecta comprise a larger number of 

 species than any other classes of the Arthropoda, 

 numbering according to Dr. Sharp some 2,000,000 

 species, of which only about 250,000 have been 

 described. Although the class is such an exten- 

 sive one, it is characterized by a remarkable 

 degree of anatomical uniformity. If we examine 

 the body of any member of this great class of 

 the true Insects, we shall find that it is segmented, 

 bears a series of pairs of jointed appendages, 

 and that the surface of the body and appendages 

 is covered with a chitinous cuticle, which may 

 be thin and soft as in the body of a caterpillar, 

 or thick and hard as in the body of a beetle. 

 This outward covering or exoskeleton, composed 

 of chitine, is intimately connected with the true 

 skin beneath, the latter forming a deep, soft 

 layer. The minute ducts of numerous small 

 glands which are embedded in this layer of soft 

 true skin, pass outwards to the exoskeleton, 

 entering the base of the external hairs. It is the 

 chitinous outer cuticle, which from time to time 

 is cast, or moulted off during the growth and 

 transformations of an insect. 



The segmented body of an insect is divisible 

 into three well-defined regions, which are quite 

 constant in their disposition, and comprise 

 (i) the head, formed by the union of the head- 

 lobe of the embryo with some five segments, 

 which are so closely united as to be indistin- 

 guishable in the adult. It bears on its front lower 

 (ventral) surface the mouth ; upon its sides two 



