221 MR. W. WESCHE ON THE LABIAL AND 



ligulge. In the Muscidse, Tabanidse, Syrphidte, and other families we find, at the 

 extremity of the labium, two paired organs, the labella aad the transverse chitinous 

 levers that support them. They are in the situation where they might be expected to 

 be found, aod I have little doubt represent — the labella the paraglossse, and the 

 transverse levers the ligulae (PI. 8. figs. 4, 8 ; PI. 9. fig. 17, &c.J. In the paraglossse of 

 Culex (PL 9. fig. 23) 1 have seen two strengthening rods, which coatirm me in the above 

 homology. They only appear on dissection and are not shown in the figure. In species 

 of Empis, such as E. chioptera (PI. 8. fig. 2), and in Sipho/ia geniculata, the ligulae 

 have aborted. 



Teeth. — The large majority of species in the Muscidee, from Tachina down to Scato- 

 phaga, have an armature of teeth at the bases of the false tracheai. These maybe traced 

 through such specialized species as Stomoxys and Glossina, and may even be found in a 

 most minute size in some of the species of the parasitic Kippobosca. They are arranged 

 in symmetrical crescents, and are so close to both the mandibles and the ligulse that 

 they might possibly be parts of those organs ; but the simpler hypothesis that they are 

 outgrowths from the paraglossse is probably the correct one. Some light is thrown on 

 this point by the very exceptional dentition shown in Epjhydra (Pi. 9. fig. 21). Here 

 each of the pseudotracheae is furnished along its whole course with many short chitinous 

 teeth disposed at regular intervals. 



Labrum, Lingua, and Mentum. — All morphologists seem agreed that the lancet-case 

 corresponds with the labrum, and the hypopharynx with the lingua. The homology of 

 the ventral plate on the labium with the mentum is fairly clear (PI. 8. fig. 4). 



Submentum. — The submentum is possibly membranous, but it may be the " fulcrum." 

 This part is described as the chitinized walls of the pharynx, but a long tracheated 

 tube is easily dissected out from the trophi of all the species I know, and seems to be 

 the true pharynx. There is so little to guide that this point must remain doubtful ; 

 allowing this, we thus have the parts complete. 



Summary. — The proboscis in Diptera may be regarded as homologous with the 

 trophi of the typical insect mouth. In the Muscidae it has been modified into a rod-like 

 organ, enclosed in a thin membrane, and capable of extension and retraction. It is 

 mainly formed of the labium, and has the labial palpi well developed and placed near 

 chitinous ridges, which are the palpigers. The palpi have worked round from the under 

 to the upper side — or, taking the usual position of the labium into account, from a 

 posterior to an anterior position, where it is obvious that they would be of greater service 

 to the insect. 



The proboscis has two symmetrical tracheated flaps at the extremity, which represent 

 the paraglossse. The teeth, which are often present, may be considered outgrowths of 

 the paraglossse. The transverse chitinous levers which expand the labella are the ligulce. 

 Mr. Waterhouse shows (' Labium and Submentum ') tliat the paraglossae have a tendency 

 to enlarge at the expense of the ligulae, and this has occurred in the Muscidae. 



The labium has absorbed into its structure the mandibles and maxillae on the dorsal 

 side, and the mentum, and possibly the submentum, on the ventral side. 



