26 



GENERAL ORISMOLOGY. 



V. MEASURE. 



39. 



A universally known measure, the Paris line, the twelfth part 

 of an inch, has been adopted as unit for the determination of the 

 length of insects. This character is of considerable importance from 

 the very constant uniformity of size, not only of the parts of the same 

 individual, but also of all the individuals of the same species* ; and 

 thus the length of every possible part can be as precisely ascertained 

 as the purpose in view may require. This mode of measuring has by 

 far the advantage, and must consequently never be omitted when a 

 s pecies is named and published. The difference of size which imme- 

 diately catches the eye is frequently the first best character whereby 

 we are enabled, at the very first glimpse, to separate two, or more, 

 closely related species. 



40. 



Besides this universally applicable, absolute measure, there is 

 another relative one. A portion of the insect is adopted as the unit, 

 and by means of it, the length of the remainder is determined, or two 

 or more parts are compared together, and thereby a proportional rela- 

 tion formed. This plan is also useful particularly when given in con- 

 junction with its absolute length. The folloAving is the mode of 

 proceeding to the precise determination of the longitudinal pro- 

 portions. 



We must commence by measuring the whole length of the body 

 and giving it, and then the length and breadth of the different di- 

 visions must be placed as in the following table : 



Such a table immediately gives the relative proportions of each 



' This is liable to innumerable exceptions, but a familiarity with insects soon gives an 

 idea of the range that it may be allowed, as it varies considerably in different species. It 

 can never be permitted alone to determine a difference, unless supported by other cha- 

 racters which, in themselves, sometimes (particularly in colour) would scarcely suffice 

 for a separation. Its use is consequently of importance for identification, exclusive of its 

 value in determining the effects of climate and temperature. TR. 



