32 



AN INTRODUCTION TO ENTOMOLOGY 



differ from setae in being directly continuous with the cuticula, and 

 not connected with it by a joint; these are termed the fixed hairs, or 

 aculeffi. The mode of origin and development of the fixed hairs has not 

 been studied. 



The spines. — The term spine has been used loosely by writers on 

 entomology. Frequently large setae are termed spines. In this work 

 such setae are called spine-like setae; and the term spine is applied 

 only to outgrowths of the cuticula that are not separated from it by a 

 joint. Spines differ also from spine-like setae in being produced by 

 undifferentiated hypodermal cells and are usually if not always of 

 multicellular origin, while each seta is produced by a single trichogen 

 cell. The accompanying diagram (Fig. 42) illustrates this difference. 



C. THE APPENDAGES OF THE CUTICULA 



Under this head are included those outgrowths of the cuticula that 

 are connected with it by a joint. Of these there are, two quite dis- 

 tinct types represented by the spurs and the setae respectively. 



The spurs. — There exist upon the legs of many insects appendages 

 which on account of their form and position have been termed spurs. 

 Spurs resemble the true spines described above and differ from setae 

 in being of multicellular origin; they differ from spines in being 



appendages, that is, in 

 being connected with the 

 body-wall by a joint. 



The setae. — The seise 

 are what are commonly 

 called the hairs of in- 

 sects. Each seta (Fig. 

 42, s) is an appendage of 

 the body-wall, which 

 arises from a cup-like 

 cavity in the cuticula, 

 the alveolus, situated at 

 the outer end of a per- 

 foration of the cuticula, 

 the inchopore; and each 

 seta is united at its base with the wall of the trichopore by a ring of 

 thin membrane, the articular membrane of the seta. 



The setae are hollow ; each is the product of a single hypodermal 

 cell, a trichogen (Fig. 42), and is an extension of the epidermal 

 layer of the cuticula. 



Fig. 42. — Diagram illustrating the difference be- 

 tween a spine (sp) and a seta (s). 



