OF WASHINGTON. 385 



ot weather and age in bleaching or otherwise changing the ap 

 pearance of the scale is often notable. The characteristic appear 

 ance of the scale varies immensely in proportion as it has free 

 room for growth or is crowded or massed together densely on the 

 bark or leaves. The San Jose scale growing in scattered num 

 bers here and there on the terminal twigs bears no resemblance 

 whatever to the crowded masses on old, badly fested wood. The 

 same is true of almost any other scale insect. 



The covering scale, therefore, cannot be taken as a criterion of 

 very great value in the separation of species, and by itself is almost 

 without value. The specific characters must be found in the insect 

 itself, the scale covering furnishing indications only of a rough 

 sort. The describer of new species who fails to notice the import 

 ance of these sources of error, and sees a species, a sub-species, a 

 physiological species, or a variety, in every such accidental differ 

 ence, greatly retards rather than advances the knowledge of this 

 group of insects. It would be just as legitimate to describe as a 

 new species an insect found on the under side of a leaf as opposed 

 to an insect found on the upper side, as to designate as new a 

 species because a little extraneous matter is adhering to its scale 

 covering, or to describe men as distinct species because they wear 

 different colored coats. 



When the insect itself comes to be examined, other sources of 

 error present themselves. For example, the question of the 

 maturity or adultness of the specimens under study arises, and 

 also the problem of individual variation. In the determination 

 of material it is, as a rule, absolutely necessary to have the adult 

 female insect. In the Diaspinae, for exam'ple, the full grown 

 second stage of the female is often nearly as large as the third or 

 last stage, if not larger in some instances, and yet the difference 

 in the structural characters of the two stages is very great. As 

 an example of a description of a new species from a failure to 

 recognize the maturity of the specimens, Cockerell's so-called 

 variety lateralis of Newstead's diffinis may be cited, lateralis 

 merely representing the immature stage of Comstock's cydonia. 



In the matter of individual variation, this is just as notable in 

 scale insects as in man or other animals. The two halves of the 

 anal plate of a female Diaspine are never exactly alike, and often 

 vary within quite wide limits. In different individuals from 

 the same colony such variations are still greater. Fortunately, 

 however, the characters of real value in this group of insects are 

 much more constant than one who had not studied the sub 

 ject would suppose, even in the case of material representing 

 the same species from widely separated quarters of the world and 

 on totally dissimilar food plants. In the Diaspinae, perhaps 

 more markedly than in most other groups of insects, the specific 

 characters are sharply and satisfactorily defined, and hence the 



