186 AN ESSAY ON THE DEVELOPMENT 
cover the cesophagus. They may be united so as to form a single organ, and their 
tendency is to become internal head structures. The ligula has at its base the opening 
into the alimentary canal; it is rarely paired, may be rigid or flexible, and has closely 
associated with it the hypopharynx, recognizable by the salivary duct which it shel- 
ters. ‘The paraglossze arise on each side of the ligula or glossa, and may be chitinous 
or membranous. They are never jointed, never developed for any specific mechanical 
purpose, and their tendency is to become obsolete. The labial palpi are essentially 
tactile and never become mechanical save as they may form a covering or sheath for 
the ligula. 
From the most generalized type found in the Glattide the modification is first 
from a divided to a single ligula; next to a disappearance or obsolescence of the para- 
glossze ; later the labial palpi also disappear, and finally the hypopharynx is also dis- 
pensed with. There is no break, and nowhere is there any violent change of structure 
or function. 
We are now ready to take up the maxilla, which, though composed of a larger 
number of sclerites, are usually more easily understood in the ordinary type of man- 
dibulate insect. The organ is usually paired and never so completely united as the 
labial structures. The two parts are always external to the labium, which it is their 
tendency to enfold, and they never have any direct connection with the alimentary 
canal. Though the maxillary structures tend to form a covering or sheath for the 
labium and its appendages, there is never any intimate connection between them. No 
part of the maxilla ever unites with any part of the labium or with any of its appen- 
dages. The maxillz are essentially mechanical structures, and their range of variation is 
sufficiently great to meet the most diverse possible demands made upon them. A dis- 
tinct and fundamental characteristic is the fact that each set of sclerites has its own 
peculiar possibilities and limitations, and once these are understood the most highly 
specialized type becomes simply explicable. 
On PI. III, Fig. 17, is a copy of Prof. Comstock’s figures of Hydrophilus, show- 
ing the maxilla from both surfaces, and these may conveniently serve as a text to. 
explain the sclerites composing it. At the base is the cardo or hinge, giving attach- 
ment to muscles and tendons articulating it to the head. It is to be noted that there 
is no firm or chitinous articulation to any head sclerite, and except by muscles or ten- 
dons no direct attachment. This we found the case also in the labium in the more 
specialized forms, and in the Hymenoptera, for instance, labium and maxille together 
are easily dissected out without cutting any but muscular tissue, and without breaking 
any chitinous connections or joints. This is in marked contrast with the mandibles 
which, when functional, are always firmly articulated by chitinous joints to the external 
