106 TJte Preparation of Coechlae 



5. The Jniportanee of Exact Meastircmen/s. 



Finally, I should like to say a few words on the importance of exact 

 measurement in critical comparisons. For this purpose a camera 

 lucida is almost indispensable. Take, for instance, the determination 

 of th.e antennal formula. Direct measurement of such minute parts 

 is extremely difficult ; but, if enlarged camera drawings are made, they 

 can be compared and measured with the greatest facility. Neither the 

 eye alone, nor freehand drawings can be trusted implicitly. Body 

 measurements alone are not of much value between closely allied species, 

 as individuals from a single colony often vary considerably in size, 

 and such variability is still more marked between examples collected 

 on different plants. But the more densely chitinous parts —antennae, 

 limbs, anal lobes, etc. — are much more constant. After the final moult 

 these organs do not increase in size, though the body of the insect may 

 more than double its original dimensions. 



