ON THE TERMINOLOGY OF DIPTEEA. Xlll 



The compound eyes are sometimes encompassed in a larger or 

 smaller part of their circumference by a ring, somewhat swollen, 

 and separated more or less distinctly from the remainder of the 

 surface of the head ; it is called the orbit (orbita), the successive 

 parts of which may be called the anterior (orbita anterior sive fa- 

 cialis), inferior (inferior s. gejialis), posterior (posterior s. occipi- 

 talis), superior (superior s. verticalis), and frontal (frontalis) orbits. 

 An orbit is also often spoken of, where no ring is distinctly set off 

 from the rest of the surface of the head ; in this case a distinct 

 color or some peculiar structure mark the nearest surroundings of 

 the eyes. 



The oral parts of Diptera, destined for sucking, are called the 

 sucker or proboscis (proboscis). They are either inserted at the 

 end of a more or less cylindrical prolongation of the head, called 

 the snout (rostrum), or project from a wide aperture often occupy- 

 ing a great part of the under surface of the head, called the mouth 

 hole (cavitas oris). The common, fleshy root of the oral parts is 

 connected by a membrane with the border of the mouth. This 

 membrane often has a fold, sometimes of a quite horny substance, 

 and is then called the clypeus (clypeus s. prcelabrum) ; it is either 

 entirely concealed by the anterior border of the mouth and is then 

 usually movable, or it projects over it as a ridge and is then usually 

 immovable. The largest of the oral parts in most Diptera is the 

 fleshy under lip (labium), consisting of the stem (stipes) and the 

 knob (capitulum labii) formed by the two suctorial flaps {lobelia). 

 Besides the under lip, the palpi (palpi) are most perceptible and 

 must be noticed in the description of the species. The remaining 

 oral parts are generally rather small and stunted, having the form 

 of bristles or horny lancets; they are considered as being the tongue 

 (lingua), under jaws (maxillce), upper jaws (mandibulce) , and up- 

 per lip (labrum), the latter shutting the under lip from above. 

 These parts are not easily applicable in distinguishing species. 



The thorax of Diptera as well as that of other insects consists 

 of three segments, the prothorax, the mesothorax, and the meta- 

 thorax. But in the order of Diptera the development of the meso- 

 thorax exceeds so much that of the two other portions, that it forms 

 by far the largest part of the whole thorax, and in the description 

 of Diptera is exclusively designated with the latter name, while 

 other names are given to the prothorax and metathorax, when 

 some particular part of them is to be characterized. The protho- 



