ANATOMY OF INSECTS- -EXTERNAL 27 



of higher animals in that they support the tissues of the body, and 

 their structure is characteristic of the different groups. This hard- 

 ening of the skin is found in all arthropods and is due to a sub- 

 stance, called c/iitin, which is formed by the lower layer of cells of 

 the skin, the hypodermis, and which forms an impervious, hard 

 layer over the body of the entire animal, though but slightly devel- 

 oped in the membranous joints between the segments. Chemically, 

 chitin is somewhat akin to silk, or to the spongin of the sponge skele- 

 ton. It is unaffected by ordinary acids and alkalies, though soluble 

 in sodic or potassic hypochlorite. The insolubility of chitin is of 

 importance in the consideration of insecticides, for there is hardly 

 anything that can be applied to any but the most soft-bodied insects 

 which will corrode the skin without injuring the foliage of the 

 plants upon which they feed. The surface of the chitinous skin 

 may be smooth or pitted, wrinkled, striated, granulated, or marked 

 in various characteristic ways. The chitin is not only developed 

 by the outer skin but is formed on the surface of the entire epider- 

 mis, including the lining of the anterior part of the alimentary tract 

 and the respiratory tubes, or trachea, as can be seen by the exam- 

 ination of a cast skin after an insect has molted. 



