186 AN ESSAY ON THE DEVELOPMENT 



cover the (esophagus. 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 pai'aglossa? 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 theii- 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 BlaUuhv the modilication is first 

 from a divided to a single ligula ; next to a disappearance or obsolescence of the para- 

 glosste ; 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 structui'e 

 or function. 



We are now ready to take up the maxill;^, which, though composed of a larger 

 number of sclerites, are usually more easily understood in the ordinary t3'pe of man- 

 dibulate insect. The oi'gan 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. Tiiough the maxillary structures tend to foi-m 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- 

 dage-*. The maxilla? are essentially mechanical structures, and their range of variation is 

 sufficientl}^ great to meet the most diverse possible demands made ujjon them. A dis- 

 tinct and fundamental characteristic is the fact that each set of sclerites has its own 

 l)eculiar possibilities and limitations, and once these are understood the most highly 

 specialized type becomes simply explicable. 



On PI. Ill, Fig. 17, is a copy of Prof Comstock's figures of Hijdropliilus, show- 

 ing the maxilla fiom 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 raaxilUe 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 



