178 AN ESSAY ON THE DEVELOPMENT 
Below the mandibles are found a pair of maxillee, 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 somewhat 
deferred. 
Forming the lower lip and closing the mouth inferiorly is the labium, also made 
up of a number of sclerites and usually furnished 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 the cesophacus. 
Attached to the inner surface of the labium is the 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 lower 
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 what modifications these different changes in type have become 
established. 
If we examine the head of a well-developed mandibulate insect from the under 
side—Copris carolina, P|. I, Fig. 7, may serve as type—we find, centrally, the gula or 
throat, bounded laterally by the gene or cheeks, extending to the posterior margin of 
the head and bearing anteriorly the labium. The labium when 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 be occasioned. The structure is lettered m in all the figures. 
