58 ENTOMOLOGY 



chlorite (respectively, Eau de Labarraque and Eau de Javelle) and yields 

 to boiling sulphuric acid. If kept for a year or so under water, however, 

 chitin undergoes a slow dissolution, possibly a putrefaction, which ac- 

 counts in a measure for the rapid disappearance of insect skeletons in the 

 soil (Miall and Denny). By boiling the skin of an insect in potassic hy- 

 droxide it is possible to dissolve away the cuticular framework, leaving 



JO 



FIG. 86. Terminal abdominal appendages of a dragon fly, Plathemis trimaculata. A, 

 male; B, female, i, inferior appendage; s, s, superior appendages. The numbers refer to 

 abdominal segments. 



fairly pure chitin, without destroying the organized form of the integu- 

 ment, though less than half the weight of the integument is due to chitin. 

 The formula of chitin is given as C 9 Hi 5 N0 6 or Ci 8 Hi 5 NOi 2 by Kruken- 

 berg, and Packard adopts the formula Ci5H 2 6N 2 Oio; though no two chem- 

 ists agree as to the exact proportions of these elements, owing probably 

 to variations in the substance itself in different insects or even in the same 



9 10 



FIG. 87. Extremity of the abdomen of a grasshopper, Melanoplus differentialis. A, male; 

 5, female. The terga and sterna are numbered, c, cercus; d, dorsal valves of ovipositor; e, 

 egg guide; p, podical plate; s, spiracle; sp, suranal plate; v, ventral valves of ovipositor. 



species of insect. Iron, manganese and certain pigments also enter into 

 the composition of the integument. 



Chitin is not peculiar to arthropods, for it has been detected in the 

 setae and pharyngeal teeth of annelid worms, the shell of Lingula and the 

 pen of the cuttle fish (Krukenberg) . 



The chitinous integument (Fig. 88) of most insects consists of two 

 layers: (i) an outer layer, homogeneous, dense, without lamellae or pore 



