THE HORNY SKELETON. 241 



connexion of the parts of the skeleton we find chiefly in the thorax, in 

 the sutures by which the several plates are united together. We may 

 distinguish two descriptions of it: 



1. The SUTURE is the connexion of two plates of the skeleton by 

 insertion, a projecting ridge of the one corresponding with a channel in 

 the other, and the connexion is thus made without the intervention of 

 membranes. This mode of connexion is found between the several 

 plates of the thorax. Where both join they bend inwards, and thus 

 form an even suture. All sutures in insects are therefore simple, 

 smooth, without teeth, or interchanging processes. 



2. SYMPHYSIS is a connexion upon the whole resembling a suture, 

 but which is produced chiefly by the intervention of a soft membrane. 

 This admits of a slight separation of the connected parts, which is 

 increased in proportion to the elasticity of that membrane. It is by 

 means of this that the posterior wing of the scapula is connected 

 with the parapleura. This sort of connexion, thus admitting some 

 degree of separation, was the more necessary here, as the second spiracle 

 of the thorax lies between the two plates, and therefore a firm union 

 would have prevented a free respiration. 



A mere variation of this form, which, however, admits of a greater 

 motion of the connected parts, is called by Straus a scaly joint 

 (articulation ecailleuse). It is distinguished chiefly by the lip of the 

 one plate passing over the connecting membrane, and thus covering 

 the lip of the other plate like a scale. This mode of articulation is 

 found in the plates of the abdomen, in which each successive piate 

 is covered by that preceding it. The mobility of parts thus con- 

 nected is but passive, whereby an extension of the body on all 

 sides, but chiefly longitudinally, is made possible, for example, when 

 its contents swell, as is frequently the case in the female after im- 

 pregnation. 



II. CONNEXION WITH MOTION (Diarthrosis). All connexions 

 classed under this head are generally called JOINTS. They are found 

 chiefly in the limbs, in the connexion of their several parts. In 

 insects we distinguish the following different forms of articulation : 



1. The FLAP JOINT (syndesis) . When two parts meet at a suture, 

 and are connected together by membranes at the inner side, but so 

 that they may move in the suture to and from each other. This mode 

 of articulation is found, for example, in the under lip, where the 

 mentum joins the gula. 



R 



