178 AN ESSAY ON THE DEVELOPMENT 



Below the mandibles are found a pair of maxilhe, made up in all cases of a number 

 of sclerites, and nearly always supplied with palpi or jointed tactile organs. The 

 more particular consideration of these organs and their parts may be somcAvhat 

 deferred. 



Forming the lower lip and closing the mouth inferiorly is the labium, also made 

 up of a number of sclerites and usually fuaiished with palpi. It is never entirely 

 paired in existing insects, but is assumed to be made up of two more or less united 

 structures, similar in essential character to the maxilla, as has been well stated by 

 Prof J. H. Comstock. This labium is an exceedingly important structure and forms 

 the oral termination of the digestive tract or the mouth of tlie a^sophagus. 



Attached to the inner surface of the labium is tlie hypopharynx, a variably devel- 

 oped structure, which is supposed to be the remnant of another originally paired organ, 

 the endo-labium. I have never seen the genera in which it is said to be well devel- 

 oped, hence have no well-founded opinion to offer. I find it uniformly a single organ, 

 often highly developed and gustatory in function, sometimes a merely passive structure 

 more or less closely attached to the ligula, usually very near the opening into the 

 digestive tract. 



Briefly recapitulated, the insect mouth, when most fully developed, consists of 

 two pairs of lateral jaws moving in a horizontal plane between an upper and a loAver 

 lip, which are furnished with gustatory structures forming the roof and the floor of the 

 mouth respectively. This mouth is adapted for biting and chewing and varies to types 

 adapted to lapping, to sucking only, and to piercing and sucking. The problem before 

 me is to ascertain by Avhat modifications these different changes in type have become 

 established. 



If we examine the head of a well-developed mandibulate insect from the under 

 side — Cojyris Carolina, PI. I, Fig. 7, may serve as type — we find, centrally, thegula or 

 throat, bounded laterally by the gen;e or cheeks, extending to the posterior margin of 

 the head and bearing anteriorly the laljium. The labium Avhen carefully dissected out 

 is found to consist of a broad basal plate, the submentum, more or less firmly articu- 

 lated to the gula and never, in existing insects, a paired organ. It bears anteriorly 

 another plate, the mentum, also a united organ, though sometimes traces of a division 

 are apparent. It is usually smaller than the submentum, sometimes membranous, 

 often entirely separated and frequently so united with the latter part that the two are 

 not separable. Though the submentum is the most persistent and dominant structure 

 it has been customary to use the term mentum to apply to the united sclerites, and it 

 will become convenient for me to so use the term hereafter when no confusion or mis- 

 understanding can b2 occasioned. The structure is lettered m in all the figures. 



