HOMOPTERA 



123 



of other scale insects. Their active stage, however, is much shorter. 

 After crawling about over the twigs of a tree for a few days, the young 

 scale insect settles upon a suitable place and immediately begins to 

 excrete fine threads of wax which soon become compacted into a thin 

 pellicle covering the body. As the insect grows and sheds its skin, this 

 cast skin is joined to the excretion and forms a part of the scale. This is 

 the bright-colored, nipple-like prominence, seen in the center of the San 

 Jose's scale and of the red scale of the orange; and two of them may be 

 seen at the smaller end of the scale of the oyster-shell bark-louse. The 

 position of the cast skins in the scale differs in different genera, and forms 

 a good character for classification. 



JbamM. 



Fig. 205. — Chionaspis pinifoliir: 2, scales on Pinus strobus, natural size, leaves stunted; 2a, leaves 

 not stunted by coccids; 2b, scale of female, usual form, enlarged; 2c, scale of female, wide form, en- 

 larged; 2d, scale of male, enlarged. 



Closely allied species differ but little in the form of the scale. To 

 distinguish these it is necessary to study the insects themselves, which are 

 found beneath the scales. The distinctions between closely allied species 

 are such that it requires very close observation and much skill in this 

 particular line to make the determinations, a careful preparation of the 

 specimens and an excellent microscope being necessary requisites. 



The different species of scale-insects vary as regards their food 

 habits. We find that certain species infest particular plants and will feed 

 upon no others; thus, the red-scale of the orange does not trouble decid- 

 uous fruits. On the other hand, other species have a wide range of 

 food plants. This is true of the San Jose scale, which infests a great 

 variety of both cultivated and wild plants. 



Figure 205, represents the well-known pine-leaf scale, Chionaspis 



