14 



CHRYSOMELItfJE. 



should be absent, thus dividing the whole group of organisms into 

 two sections. From each of these sections a character should be 

 chosen which will again divide the section under review into smaller 

 sections; this process is continued until the species are ultimately 

 separated off. Comparative characters should be avoided as much 

 as possible, for by their use is assumed a previous acquaintance 

 with forms comprising the group for which the key is made. 

 Owing to the nature of the material studied, it is sometimes 

 difficult to find exact contradictory characters, and in such cases 

 the statement of a combination of two or more characters may 

 enable one to separate species or groups of species. Sometimes it 

 happens that a group of organisms is at once divisible into more 

 than two different categories which are convenient to use ; and 

 although in such a case the key is not strictly dichotomous, yet 

 advantage should be taken of the opportunity. 



The characters having been selected, their actual statement in 

 key-form can be made in three ways, which are illustrated by 

 the following examples : namely (1) the spatial form, (2) the 

 numerical, and (3) the double numerical. 



(1) The spatial form. 



A. Anterior coxal cavities open behind. 



a. Antennae passing beyond the base of the pronotum. 



a. Pronotum black, elytra yellow Sp. no. 1 . 



/3. Pronotum and elytra brown Sp. no. 2. 



a'. Antennae not passing beyond the base of the pronotum. 



a. Insect blue, with purple stripes on the elytra Sp. no. 3. 



/3. Insect with no such stripes on the elytra Sp. no. 4. 



A'. Anterior coxal cavities closed behind. 

 a. Eyes emarginate on the inner side. 



a. Insect elongate, parallel-sided, with the upper side 



black and underside yellow Sp. no. 5. 



/3. Insect with no such combination of characters Sp. no. G. 



a'. Eves not emarginate on the inner side. 



a. Insect reddisli-bvown, with black patches on the 



pronotum. Sp. no. 7. 



/3. Insect lighter brown, with no black patches on the 



pronotum Sp. no. 8. 



(2) The numerical form. 



1. Anterior coxal cavities open behind 2. 



Anterior coxal cavities closed behind 5. 



2. Antennae passing beyond the base of the pronotum 3. 



Antennae not passing beyond the base of the pronotum . . 4. 



3. Pronotum black, elytra yellow Sp. no. 1 . 



Pronotum and elytra brown Sp. no. 2. 



4. Insect blue, with purple stripes on the elytra Sp. no. 3. 



Insect with no such stripes on the elytra Sp. no. 4. 



5. Eyes emarginate on the inner side 6. 



Eyes not emarginate on the inner side 7. 



6. Insect elongate, parallel-sided, with the upper side black 



and underside yellow Sp. no. 5. 



Insect with no such combination of characters Sp. no. 6. 



