I02 THE INTEGUMENTAL SKELETON OF THE IMAGO. 



Symphysis (Fig. 20, /) signifies the union of two sclerites by 

 an inflected chitinous ridge between them. 



Syndesmosis (Fig. 20, 2) is the union of two sclerites by a 

 soft, flexible portion of the cuticle. Syndesmoses are frequently 

 imbricate ; that is, one sclerite overlaps the other, protecting 

 the soft integument between them from injury. 



Arthrosis. — An arthrosis is a kind of locked syndesmosis : two 

 joints of an appendage are united by syndesmosis, but pro- 

 cesses on the exterior of one fit into hollows on the exterior of 

 the other. These may form a hinge, or peg joint, or, in some 



Fig. 20. — The principal forms of Articulation (Diagrammatic): /, symphysis; 2, 

 syndesmosis ; j, ball-and-socket-joint, from a section of the tarsus of the fly ; ^, 

 gmglymus or hinge-joint ; j", coxa, wiih a hemispherical articular surface, show- 

 ing its relation to the line of attachment of the syndesmotic membrane. 



cases, a ball-and-socket (Fig. 20, j, j). A hinge joint, ginglymus 

 (Fig. 20, /), admits of flexion and extension in one plane only ; 

 a peg joint, of rotation only; and a ball-and-socket, amphi- 

 arthrosis, admits of more or less free movement in any direction. 

 There are other forms of articulation, but these do not need 

 special terms for their description ; they are generally a modi- 

 fication of one or other of the above. 



Sutures may be arranged in three classes, in accordance 

 with their morphological value ; median, primary, and secon- 

 dary sutures. 



Median Sutures are either dorsal or ventral, and are indicative 

 of the bilateral symmetry of the developmental process. They 

 represent the furrow between the two halves of the primitive 



