2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 82 



found in studying the anatomy of insects that a muscle has no an- 

 tagonist. Moreover, while most insect muscles are muscles of motion, 

 some are tensors inasmuch as they appear merely to maintain rigidity 

 between parts that are subject to strain from other muscles. 



The ectodermal cell layer of the body wall, or epidermis (commonly 

 known as the '' hypodermis "), is covered externally by a cuticula of 

 which the general constituent is probably chitin, but in which other 

 substances are deposited to form hardened areas called sclerites. The 

 nature of the sclerotizing substances in insects is not yet known, but 

 it appears to be definitely established that the sclerotic areas of the 

 cuticula are not places where the chitin is thicker or denser (Campbell, 

 1929). Sclerites are secondary formations in the body wall, and it 

 would be both interesting and important to know the physiological 

 processes that produce them, for we should then be better able to 

 evaluate sclerites as morphological units. 



The major plates of the body-wall of an insect are very definite 

 structures that are consistently reproduced by the deposit of sclerotiz- 

 ing substances throughout the whole series of insect forms, and some 

 of them appear to be homologous with corresponding plates in other 

 arthropod groups. On the other hand, all parts of the insect skeleton 

 called " sclerites " in descriptive entomology are not of equal value. 

 Many of them are simply areas of larger plates which have l^ecome 

 secondarily demarked by lines of inflection in the cuticula that have 

 formed internal strengthening ridges. The so-called sclerites in such 

 cases are in themselves of no significance. The important morpho- 

 logical features are the endoskeletal structures ; these are the rafters, 

 the joists, and the upright supports that give strength to the edifice 

 and enable it to withstand the strain of the muscles pulling on its walls. 



It frequently happens, however, that a primary region of sclerotiza- 

 tion becomes broken up by a discontinuity in the hardening substance 

 of the cuticula, thus producing true secondary sclerites. The inter- 

 vening " membrane " may take the form of a narrow line of flexibility 

 (" suture "), or it may cover a large part of the original hard surface 

 and reduce the primary sclerotization to two or more widely separated 

 plates. Or again, an original sclerotic area may be contracted to a 

 relatively small sclerite, or it may be obliterated. It then becomes a 

 question, if the primary plate has been given a name, whether we are 

 to apply this name to the area originally occupied by the plate, or 

 restrict it to the sclerotic remnants or remnant. It is the usual prac- 

 tice to apply the name only to the sclerite, whatever its extent, and, 

 if the sclerite is obliterated, to say that the part in question is obsolete 



