INSECT A. 239 



rings only the organs of locomotion are appended. The remaining 

 segments of the body are much less firm in their texture, especially 

 in insects with hard or horny wing-covers, in which indeed they are 

 almost of a membranous consistence, so as to increase as far as pos- 

 sible the lightness of the animal in parts where strength is not re- 

 quired. Here then is an annulose skeleton adapted to flight ; 

 dense and unyielding where support is required for the attachment 

 of the locomotive organs, but thin and flexible elsewhere. 



(281.) The above conditions being required in the arrangement 

 of the pieces which compose the outward framework of the body in 

 insects, we may easily conceive that the mode of union between 

 the various segments above described is by no means a matter of 

 indifference, inasmuch as very different degrees of motion are re- 

 quired between the individual rings. In the Annelida and My- 

 riapods a very simple kind of junction was sufficient ; for in them 

 the segments were all united by the mere interposition of a 

 thinner coriaceous membrane, extending between their contiguous 

 margins ; but in insects several kinds of articulation are, met 

 with in the construction of the trunk adapted to the mobility 

 of different regions. 



The first mode of connection 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 mov- 

 ing the wings and legs derive extensive origins. 



A second means whereby the pieces of the thorax are fastened 

 together is by syinphysis, in which a somewhat soft membrane 

 is interposed between two plates, so as to admit of a slight degree 

 of motion. 



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 de- 

 velopement of the eggs after impregnation. The rings of the 

 abdomen are, therefore, united by a membrane passing from one 



