MAXILLARY PALPI IN DIPTEEA. 221 



observations I have made mostly confirm the ideas of Savigny. Speaking of the mouth- 

 parts in Insecta he says : " the organ is the same, the use aloae is modified or 

 changed " *. His nomenclature of the palpi in the Muscidse must be excepted from this 

 statement. Of all the morphologists known to me, K,obineau-Desvoidy alone calls these 

 labial. Dr. Lovrne f thus quotes him : — The base of the labrum " is enveloped by the 

 base of the labium, of which the palpi are always present." 



Before proceeding further, it may be as well to say that I use the terminology, much 

 the same as that of Kirby and Spence, given in C. O. Waterhouse's + essay on the 

 " Submentiim," to which I wish to express my obligations. 



In this investigation I have been guided by the following facts : — 



1. Mandibles. — In the Tabanidae, which have all the mouth-parts present except the 

 labial palpi, the proboscis or labium has tracheated labella, and is so like in appearance 

 and position that there can be no doubt of its homology with the proboscis in the 

 Muscidae. If a labium of Samatopota pliivialls has the soft parts dissolved away and 

 is mounted Avith pressure (and I may remark, for the sake of brevity, that all the parts 

 referred to later have been treated in this manner, unless expressly stated to the contrary), 

 and is examined with the higher powers of the microscope, it will be seen that the dorsal 

 surface has no chitinous structure, the necessary stiffening being afforded by a chitinous 

 plate on the ventral side, the mentum (PL 8. fig. 1). I have examined Tabaims sudeticus 

 and T. bromius, and have found the labium identical in this respect with S. pluclalis. It 

 is, however, as well to mention that Chri/sops cceciitiens, which has a very elongated labium, 

 has chitinous structure on the dorsal side, wliile still retaining the mandibles. But this 

 species, judging from the length of the labium, is a later and more specialized type 

 than Tabanus, and the chitinous plate followed as a consequence of increased leagth 

 requiring more " stiffening." It may generally be assumed, though not on conclusive 

 evidence, that the archaic types in Diptera were furnished with a short labium and 

 that extreme length is a case of specialization. A fairly obvious example is found 

 in Siphona. 



We may then say that Tabanus, having the mandibles present, has no chitinous 

 structure on the dorsal side of the labium. 



When we compare the corresponding parts in the Syrphidoe and the Empidre, families 

 in which the mandibles have disappeared, though the maxillye are present, we find well- 

 marked chitinous rods on the dorsal side. We may reasonably presume that tlie 

 mandibles are soldered into the dorsal surface of the labium. This view is held by 

 Professor Packard, and has general acceptance with little or no opposition (PI. 8. 

 figs. 2, 3). 



2. If we examine CalUphora we find a similar chitinous structure on the dorsal side 

 of the labium. Dissection shows that the parts readily separate into two lateral rods 

 with a broad central plate (PI. 8. fig. 7). 



In Eristalis tenax, E. arbustorum, and Syrphiis balteatus these rods are difTerent ip. 



* ' Memoires sur les Animiiux sans Vertobres," Boiicbo, p. 11. 



t ' Anatomy and Physiology of the Blowrty,' p. 130. 



X ' The Labium and Subraentum in certain Mandibulate Insects ' (London, 1895, 8vo, pp. 12, 4 col. pis.). 



30* 



