26 THE COCCIDAE 



or endoskt'loton of the head and thorax when these are present. 



Setae, Spinulae. — The cuticle bears appendages of different 

 kinds, as setae, spinulae, and eerores, the openings through which 

 the wax is poured from the wax cells. 



Each of tiie hair-like appendages borne by the cuticle is a 

 seta. They are generally designated as hairs or spines although 

 they are entirely different in origin from both of these structures. 

 The setae are hollow, open at the distal end, and are formed by 

 special hyj^odermal cells. Each seta consists of a slender thread- 

 like portion, the shaft, which is articulated in a small cup in the 

 cuticle, the alveolus or calyx. It appears as a depressed circular 

 area when the shaft is in place and when broken off the shaft 

 shows as a minute irregular stub in the bottom of the depressioi' 

 The presence of the calyx is the only certain means of proving 

 whether any given projection is a seta or not. The opening in the 

 shaft of the seta, the lumen, connects with an opening extending 

 through the cuticle, the pore canal or trichopore, which terminates 

 at the large cell secreting the seta, a trichogen cell. This cell 

 forms the seta and in most cases is apparently formed by the 

 fusion of several adjacent cells which have lost their body-wall, 

 since the trichogen cell always contains several independent masses 

 of nuclear matter. The setae are probablj' primarily sensory in 

 function and are connected with the nervous system. The trichogen 

 cells often elaborate an excretion which is poured out through the 

 pore canal and the lumen of the seta. The wax of coccids is never 

 excreted from cells of this type so far as known. The cuticle 

 bearing each seta is sometimes produced pimple-like, as in Icerya. 

 The swollen portion of the cuticle together with the seta which it 

 bears is known as a chalaza. The extent of the cuticle can always 

 be determined from the position of the calyx. The wax excreted 

 by the wax cells is never poured out through setae, the seta-like 

 projections of Orthezia lack calices. The spines of insects are 

 extensions of the body-wall into which the hypodermis extends 

 and frequently bear setae. True hairs are found only in the 

 Vertebrata. 



The surface of the cuticle may be prolonged into numerous 

 minute projections which are frequently seta-like. These projec- 

 tions are known as solid setae, solid hairs, or spinulae. They lack 

 not only the calyx of the seta but also its lumen, are not formed 

 by special hypodermal cells, and are usually inflexible. 



Cerores, Ceratubae. — The setae and spinulae are projections 

 of the cuticle. It usually contains in addition to these in the 



