EXTERNAL ANATOMY. 17 



B. Descriptive * 

 The Head. 



7he eyes. — With the exception of some of the Epboboscidea 

 (Pupipara), the large compound e^^es so strikingly prominent in 

 most species of Diptera, are invariably present. In the majority of 

 the Nematocera the eyes are sejmrated by a f rons, or front, of equal 

 width in the two sexes, but in most of the families of the Brachy- 

 CERA (except the Asilid.e and Dolichopid^), and in tlie great 

 majority of the higher Calyptrate Muscid.e the eyes in the male 

 toiich each other in front for a considerable distance. Exceptions 

 occur in some Blepharocerid.e, in which the head is lioloptic 

 in one or both sexes, or dichoptic in both ; also in the Cyrtid^, 

 in which the eyes are contiguous in both sexes both above and 

 below. As a rule the eyes are always well separated on the under- 

 side of the head, but in many Tipulid.t they are there contiguous 

 or subcontiguous. In most families they are oval or semicircular ; 

 in some, enormously enlarged, occupying practically all the head 

 (Cyrtiu.?e, PiPUNCTJLiDiE) ; in others, kidney-shaped (Psycho- 

 did^ and many groups of Mycetophilid^). They m:iy be 

 thickly or lightly pubescent, or bare, these features being often 

 not contingent "on sex. In a few families they possess coloured 

 bands in life (Tabanid-E, some Asilidje), which fade after death, 

 but which may generally be caused to reappear by an application 

 of damp sand. In the genus BiUo the male eye is curiously 

 divided into two parts, and in most BLEPHAROCERiDyE the eye, at 

 least in the male, is sharply divided into an upper and lower 

 part, the facets in one being very large, and in the other much 

 smaller, and the two parts being separated by a narrow unfacetted 

 band. In one small family of Acalyptrate Muscid.e, the Diop- 

 siN^, the head is produced on each side into long eye-stalks, at 

 the tips of which are placed the eyes. In some wingless PnoRiDiE 

 the eyes are somewhat aborted. 



The ocelli or simple eyes have been sufficiently described under 

 the heading " Terminology." 



The mouth-2Jarts.—'' No point of insect morphology has given 

 rise to more differences of opinion than the mouth of Diptera.'' 

 (Dr. Sharp, 1899.) This being supported by the existence of so 

 many views as to the homologies of the dipterous mouth-parts, 

 the present references may be judiciously curtailed. 



Of the various authors perused, I venture to select Wiiliston's 

 definition of these parts as being the most concise and recent 

 (1908). This author is therefore quoted verbatim : — 



" The more commonly accepted homologies of the mouth-parts 

 are as follows : labium, maxillae, maxillary palpi, mandibles. 



* The notes comprised under this heading are of the briefest, since a general 

 introduction to the order of Diptera is not attempted in this vokiiue, and the 

 characters of the faniiUes treated of herein are fully detailed in their respective 

 places. 



