XIV INTRODUCTION. 



from the diverse habits. In some they are adapted for pierc- 

 ing animal or vegetable substances and are, in consequence, 

 firmer and more slender; in others, and by far the greater 

 number, they are adapted only for sucking up juices or 

 such substances as may be dissolved by the aid of the saliva. 

 Grains of pollen have been observed in the digestive organs 

 of the Syrphidae and other flower flies, but, as a rule, fluids 

 only serve as food. Many have the proboscis wholly retrac- 

 tile into the oral cavity, and furnished with one or even two 

 hinges, by which when at rest it may be folded up. In others 

 the proboscis is not retractile, and projects either in front, or 

 downward or backward, beneath the body. While it is usually 

 short, it may be much longer than the body. Finally a few 

 species have the mouth-parts rudimentary and take no nour- 

 ishment in the adult state. 



As regards the different parts of which the mouth-organs 

 are composed, there is a strong contention on the part of Prof. 

 John B. Smith that the homologies previously and generally 

 accepted by entomologists are not the correct ones. I have 

 followed his arguments as carefully and as fully as I am able, 

 but I will not pretend to assert any authoritative opinion in 

 the matter, as that would presuppose a thorough acquaintance 

 with the mouth-structure in other insects than the dipteni, 

 which I do not possess. 



The more commonly accepted homologies are as follows : 

 labium, maxillae, maxillary palpi, mandibles, hypopharynx, 

 and labrum or labrum-epipharynx. The labial palpi are 

 thought to be wholly wanting, or represented by the labella. 

 The labium is always present, more or less fleshy and provided 

 with muscles. It is grooved or channeled on the upper side 

 to receive the other parts, with the exception of the palpi, 

 which are free. This sheath is often nearly complete, the 

 thin margins touching each other above. At its tip are the 

 pair of joints of variable size called the "lips" or labella. 

 The maxillae and mandibles are sometimes absent, the mandi- 



