298 INSECTA. 



junction was sufficient ; for in them the segments were all united by 

 the mere interposition of a thinner coriaceous membrane extending be- 

 tween their contiguous margins ; but in insects several kinds of articu- 

 lation are met with in the construction of the trunk, adapted to the 

 mobility of different regions. 



(774.) The first mode of connexion is effected by suture, or rather 

 by a species of " harmony" as it is technically termed by anatomists ; 

 two plates of the skeleton being accurately and immoveably fitted to each 

 other, but without being decidedly fastened together by serrated edges. 

 This kind of junction is met with in the thorax, and serves an important 

 purpose ; for at the point of union both plates are bent inwards, and 

 prolonged internally, so as to form numerous partitions and processes, 

 from which the muscles moving the wings and legs derive extensive 

 origins. 



(775.) A second means whereby the pieces of the thorax are fastened 

 together is by sympTiysis, in which a somewhat soft membrane is inter- 

 posed between two plates, so as to admit of a slight degree of motion. 



{776.) More extensive movement is required between the pieces 

 which compose the abdomen ; for in this region, that rigidity and firm- 

 ness which are essential in the construction of the thorax would be 

 highly disadvantageous, inasmuch as the abdominal viscera must be 

 subject to constant variations in bulk, caused either by food taken into 

 the intestines, or, in the case of the female, by the development of the 

 eggs after impregnation. The rings of the abdomen are therefore 

 united by a membrane passing from one to another, but so loosely, 

 that the edges of the individual plates wrap over each other to some 

 extent, and thus may be separated by the slightest pressure from 

 within. 



(777.) But in other regions there is an absolute necessity for a mode 

 of communication intermediate in character between the two kinds men- 

 tioned above, having neither the firmness of the one nor the mobility 

 of the other. This is more especially the case in the junction between 

 the head and the anterior segment of the thorax, and also between the 

 last-named segment and the middle piece of the thorax, in those cases 

 where these two parts are not joined by suture. The joint employed 

 in this case is of very beautiful construction, resembling in some respects 

 that formed by a ball and socket : a conical prolongation of one seg- 

 ment is admitted into a smooth cavity excavated in the corresponding 

 margin of the other, and secured in this position by muscles and an ex- 

 ternal ligament. Such an articulation is of course capable of being 

 firmly fixed by muscular action, but at the same time admits of sufficient 

 freedom of motion to allow rotation in all directions. 



(778.) The legs of insects, as we have already stated, are invariably 

 six in number, one pair being attached to each of the three thoracic 

 segments. Considered separately, every leg may be seen to consist of 



