284 W. WESCHE ON THE PROBOSCIS OF THE BLOW-FLY, 



in those families which, judging from palaeontological evidence, 

 are the oldest, and it is only when the mouth has been 

 extremely modified for blood-sucking or raptorial purposes, 

 that traces of it are completely lost, as in most Culicidae and 



Asilidae. 



In the older forms the tracheae seem complete, forming perfect 

 rings, and the tubes are much finer and smaller than in the 

 Blow-fly. The lobes of the paraglossae (labella) are much more 

 fleshy, and their under surfaces are usually more chitinised and 

 more thickly haired. But the greatest differences are in the 

 chitinous framework which supports the labium. 



In the Blow-fly this consists of two plates, an upper or dorsal, 

 and a lower or ventral ; these extend from the base of the 

 labrum and hypopharynx (the lancets) to the base of the labella. 

 The upper plate consists of a thin middle plate on which the 

 lancets rest, and two stout lateral rods which on dissection easily 

 separate from the plate (Plate 22, Fig. 2), while the lower part 

 or mentum is a stout homogenous plate of chitin, slightly bent 

 upwards at the sides (Fig. 1). 



In Tipida oleracea, L., most of the species of the same family, 

 and also the Ptychopteridae, all of which might be called Crane- 

 flies (the older families previously alluded to), there is no upper 

 plate with lateral rods, but lying just under the upper surface, 

 and in the median line, is a long flat rod with a bifurcation at 

 its anterior extremity ; to the ends of this bifurcation are 

 attached the maxillary palpi. A microscopical examination of 

 this rod shows that it is composed of two rods fused together, 

 and it can be seen that the entire piece is produced by the 

 fusion of the levers (the cardines and stipes) which work the 

 maxillae, and are aborted in these insects (Fig. 4). They can be 

 clearly recognised in the Blow-fly as two lateral rods in the 

 posterior portion of the proboscis, which end at the base of the 

 lancets (labrum), but are not attached to the palpi, which are 

 labial (Fig. 3). 



