26 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



is at present, has resulted from a coalescing of what was originally 

 two posterior arms. 



The tentorium of coleopterous larvae, although it may show vari- 

 ous forms, can be homologized with the primitive structure, as has 

 been shown above. 



IV. CONCLUSION 



The muscles of the head capsule of the larvae of Coleoptera con- 

 form, for the most part, with those of a generalized insect such as 

 the roach. There are, of course, various modifications which have 

 taken place in connection with the habits of the particular species. 

 The ligula, when present, was not observed to be divided into glossae 

 or paraglossae in any case, as it is in the orthopteroid insects. There 

 were found no larvae that showed a typical hypognathous head, all 

 the forms having the prognathous type, which has resulted in a con- 

 spicuous elongation of the postmentum or in the addition of a gular 

 region behind the base of the labium. In some cases the postgenae 

 have completely united, and this has caused a lengthening of the 

 ventral wall of the head. 



From the study thus far it does not appear that any particular form 

 of labium is more generalized than another. It seems, however, that 

 the labium of Byrrhus or of Silpha is typical. This type of labium 

 consists of a prementum subdivided into a first and second prementum, 

 and of a postmentum made up of only one division or sclerite. Too 

 much emphasis should not be placed upon sclerites as indices of pri- 

 mary morphological areas, since they are but the result of secondary 

 hardening processes in the integument ; but until some better method 

 is devised for delimiting or describing a part of the insect it is essential 

 that they be used. When they are studied together with muscle origins 

 and insertions, they become fairly safe criteria for separating parts of 

 the external anatomy. 



The tentorium of the larvae studied shows various modifications 

 and specializations, but by means of a comparative study it may be 

 homologized with the conditions as found in the more generalized 

 insects. 



