584 PHILOSOPHY OF ZOOLOGY. 



or pubescent ; and when they consist of three joints, the 

 proboscis is always scaly. 



Order III. — Diptera. 



The diptera have only two wings; and, in a few instan- 

 ces, none. Many species are furnished with halteres, and 

 squamulae. The mouth is formed for suction. For this 

 purpose, the labrum is more or less produced as a cover. 

 The mandibulae are obliterated, or in the form of threads. 

 The maxillae are produced into threads, and by their union 

 in company with the mandibulae, form the syphon. The 

 labium is either double or single, and forms a sheath des- 

 titute of joints, for the reception of the syphon. In some 

 cases, there are two maxillary and two labial palpi. The 

 larvae are destitute of feet, and pass into pupa, obeda, or co- 

 arctateB. In this numerous division, the labium, or sheath, 

 is univalve, in others it is bivalve. We shall now advert 

 to the first of these. 



Among those with a univalvular sheath to the proboscis, 

 there are some in which this sucking organ is entirely with- 

 drawn, when not in use. Some of these have the sucker 

 consisting of only the produced maxillae. The Muscadae 

 have the eyes sessile, as Musca, Tephritis, Myoda, Macro- 

 cera, Scenopinus. The Achiasidae have the eyes peduncu- 

 lated, as Achias, Diopsis. The genus Oestrus, or Gad- 

 fly, agrees with the muscadae in habit, but the parts of the 

 mouth are imperfect. Others have the sucker of four fila- 

 ments, formed from the mandibulae and maxillae. In the 

 Syrphadae, the front is produced like a beak, as Syrphus, 

 Psarus, Chrysotoxum, Cerea, and Rhingia. In the Aph- 

 ritidae, the front is abbreviated, as Aphritis and Milesia. 

 In the remaining genera of this group, the last joint of the 

 antennae is not simple, as in the preceding families, but an- 

 nulated, and destitute of the lateral hair which they pos- 



