. Elementary lessons on insects . The hardening of the skin is due to the formation in it of a horny sub- stance called chitin. This chitinous hard layer, forming an outer skele- ton, segmented on body and ap- pendages, is characteristic also of crustaceans, spiders and myriapods. These together with insects comprise that great division of the animal kingdom known as the Arthropoda.* When we look at the stoneflymore closely we observe that its body is divided into three principal parts, head, thorax and abdomen.

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Keywords: ., bookcentury1900, bookcollecti, bookdecade1920, booksubjectinsects