16 MEDICAL ENTOMOLOGY 



referred to above being considered as derived from other parts of the 

 head. This plate corresponds to the anterior arch of the fulcrum, and 

 will be described in more detail further on. At the upper end of the 

 ptilinal area there may be two small plates, the lower one of which sup- 

 ports the antennae ; the upper one is termed the htniile. 



The area behind the ptilinal suture is divided into three lateral por- 

 tions, the extent of which will be seen from the figure. The frontalia 

 are also known as the f rentals, or the mesofrontals, the parafrontals as 

 the geno-vertical plates, and the parafacials as the sides of the face, or 

 the genae. The term cheek is applied to that portion which has already 

 been described as the gena. 



Most of the above terms are taken from Townsend's paper on the 

 taxonomy of the Muscoidean flies, to which the reader who wishes to 

 study systematic dipterology will do well to refer. The difficulty in 

 following descriptions is not so great as the multiplicity of terms in use 

 would suggest, for most papers are illustrated with diagrams, and the 

 meaning of the words can be ascertained by a reference to the figures. 



The eyes in the Diptera do not differ materially from those in other 

 insects, descriptions of which are to be found in most text-books on 

 Zoology. The external surface is made up of a large 

 number of facets, usually equal in size and shape, and 

 generally hexagonal, with rounded corners. (Plate III, fig. 2.) They 

 may be pilose, i.e., covered with short hairs, or naked, the latter being the 

 usual condition. They are of various colours, and the colouration may 

 be of importance in the determination of the species ; this is especially 

 the case in the Tabanidae, in which the eyes are most beautiful objects. 

 Unfortunately the colour fades rapidly on the death of the fly, and both 

 it and the markings should be noted in fresh specimens. 



In addition to the compound eyes there may be two or three ocelli, 

 situated on the vertex of the head, generally in a small raised triangle. 

 (Plate I, fig. 4.) In the fresh state they are seen as small spots of 

 pigment ; in cleared preparations the pigment is readily dissolved out, 

 and the situation of the ocelli shown by the clear spaces which remain. 



The antennae (Plate II.) are sense organs, which probably have for 

 their function the perception of sounds and air currents. They are well 



The Antennae supplied with nervous tissue and air tubes, and have 



peculiar ' sense pits ', in which the olfactory sense 



is believed to be located. They are, as previously stated, the 



appendages of the second segment of the head, and in all cases they 



retain the original form, that is to say, they are composed of a 



