8 DURHAM DIPTERA. 



DESCRIPTION OF THE FLY CHART. 



The terms used in the analytical tables will be clearly under- 

 stood by the following description of the Fly Chart, or 

 Diagram. Of course it will be understood that all of 

 these parts are not to be found in a?iy single species. 



A. THE HEAD. — The front portion bearing the sense 

 organs. It may be divided into nine principal parts. 

 I. The Vertex. — On the top between the eyes. It 

 contains : — 

 (i) The O cellar Triangle. — A triangular patch, often 

 raised, in the middle. 



(2) The Ocelli. — Or simple eyes, generally three, 



arranged in a triangle within the ocellar triangle. 



(3) The Vertical Triangle. — A coloured triangular 



patch surrounding the ocellar triangle. 



(4) The Vertical Bristles. — Close to, and rather behind, 



the upper inner corner of the eye. Two, an inner 

 and an outer, on each side. The inner, rather 

 nearer to one another in front ; the outer, a little 

 behind and farther apart. 



(5) The Post- Vertical Bristles. — One pair directly 



behind the ocelli. 



(6) The Ocellar Bristles. — One pair among the ocelli. 



(7) The Vertical Orbit. — The sides of the vertex, close 



to the eyes. 

 II. The Compound Eyes. — On each side of the head, 



large and often hairy. 

 III. The Frons. — The forehead, between the eyes, above 

 the antennae, and below or in front of the vertex. 

 The latter boundary is indefinite, but may be generally 

 indicated as an imaginary line from eye to eye, 

 across the forefront of the ocellar triangle. The 

 frons contains : — 

 1) The Frontal Stripe. — A dark stripe in the middle, 

 often bifid or forked. 



