EXTERNAL ANATOMY OF INSECTS. 399 



to the animal, by means of the connecting membranes, it 

 becomes obvious that particular segments, from the active or 

 peculiar use of the organs which they bear, must occasionally 

 require a greater degree of freedom than others whose organs 

 have no such active or peculiar use ; and when strength, rather 

 than activity in the articulation, is desirable, then a less degree 

 of freedom will be best adapted to the purpose. In the human 

 frame articulations are of three very obvious kinds : these are 

 called, first, diarthrosis, in which the motion permitted is 

 perfectly free, as that of the bones of our arms and legs ; 

 secondly, synarthrosis, in which no motion is permitted, as 

 in the sutures of the bones of the skull ; and, thirdly, amphiar- 

 throsis, which partakes in some degree of both of the others, 

 being a very limited motion, yet plainly distinguishable, — such 

 is the articulation of the vertebral column. These three kinds 

 of articulation, I believe, have been denominated familiarly, 

 moveable, immoveable, and mixed articulation ; terms so easy 

 to be remembered, that I shall not hesitate in adopting them. 

 Besides these, another anatomical term, symphysis, requires 

 an introduction into entomology, or we must have recourse to 

 an English analogue ; by symphysis, in anatomy, is meant the 

 natural union of bones, which in the foetus, or even in early 

 life, have been for a while distinct ; thus, the under-jaw, 

 which in man is one solid and compact bone, has originally 

 been two, which have united in the front. A fifth anatomical 

 term, also applicable to entomology, is anchylosis ; it is em- 

 ployed when the synovial glands are wanting, and the custo- 

 mary articulation becomes ossified : it may generally be 

 contrasted with symphysis by the observation, that symphysis 

 naturally takes place as an approach to strength and perfection ; 

 anchylosis, more frequently as a consequence of accident, or 

 disease. The application, however, of both these terms is 

 very extensive, and often made, even by skilful anatomists, 

 to widely different effects. Now, on the mode of articulation, 

 together with the situation of the locomotive organs above 

 detailed, much will be found to depend. 



In Insecta, the first articulation is moveable ; the second is 

 moveable ; the third itmnoveable ; the fourth varies ; the fifth 

 varies ; the sixth, and all the others, are mixed.'^ 



"^ Subject, however, to exceptions hereafter to be detailed : the variations of 

 articulation generally observable in insects will also be more fully explained. 



