GENERAL CHARACTERS OF THE EXO-SKELETON. loi 



the needs of locomotion. Such considerations led Geoffroy 

 St. Hilaire and his followers to apply the names of various 

 parts of the vertebrate skeleton to those of the insect 

 skeleton which, as they conceived, serve similar functions. 

 Although, perhaps, without exception, such terms are not jus- 

 tified, and in a strictly morphological nomenclature must be 

 rejected, their rejection would be exceedingly inconvenient, and 

 would lead to the adoption of many new and unfamiliar terms. 



Definitions. — In all insects the sclerites of the head, thorax 

 and abdomen, except those of the pre-oral region, form a series 

 of annuli or rings, and correspond more or less closely 

 with the surface of the primitive somites of the embryo; 

 hence their morphological import. As this portion of the 

 skeleton cannot be properly called axial, I shall term it somatic, 

 to distinguish it from the skeleton of the appendages of the 

 metameres, which may be properly termed appendicular. The 

 sclerites of the somatic skeleton are chiefly plates, and the 

 limits of these are marked by seams or sutures. 



The appendages of an insect are also segmented, hence the 

 term Arthropoda, and the several segments of an appendage 

 are termed joints ; this term is therefore used in two distinct 

 senses : it may either mean a segment of an appendage, or the 

 articulation between two sclerites. Ambiguity does not neces- 

 sarily arise from this usage, although it would be better to 

 adopt a more consistent nomenclature. The segments of an 

 appendage are so generally called joints in works on entomo- 

 logy, that it is difficult to avoid this usage. I shall there- 

 fore always speak of the union of two or more sclerites as 

 an articulation, except when the word joint is qualified, as 

 in the expressions ball-and-socket, or hinge-joint, when it may 

 be used without ambiguity in its more usual sense. The 

 different forms of articulation maybe classed under two groups, 

 Sutures and Arthroses. 



Sutures are seams between the sclerites, and are chiefly seen 

 in the somatic skeleton. Arthroses are movable articulations, 

 such as occur in the appendicular parts of the skeleton. A 

 suture may be either a symphysis or a syndesmosis. 



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