HEAD. 21 



times distinctly wider than the thorax, at other times 

 small. In the N)^eteribiidse it may be folded back into a 

 groove on the dorsum of the thorax, but with these ex- 

 ceptions, it is always attached to the thorax by a freely 

 movable neck. The taxonomic characters furnished by 

 the head are second only in importance to those of the 

 wings. 



Eyes. The large compound e5 r es are present in all dip- 

 tera, save seme Pupipara. In the majority of males, 

 especially of the Orthorrhapha, they are contiguous on 

 the upper side of the head, between the insertion of the 

 antennse and the hind margin, for a longer or shorter 

 distance; insects having such contiguous eyes are called 

 holoptic (Osten Sacken). In many males, however (all 

 the Acalypterae and several families of the Orthorrhapha, 

 as well as numerous genera of other families), and in 

 all females, with but few exceptions (certain Cyrtidae, 

 Orphephilidae, Blepharoceridse, Bombylidse, Platype- 

 zidae, etc.), the eyes are separated more or less broadly 

 by the front: such insects are called dichoptic ( Williston ) . 

 Rarely the eyes may be contiguous below the antennae, 

 or both above and below, as in certain cyrtids, etc. In 

 not a few flies, especially those of the aerial, bristleless 

 kinds, the upper facets of the eyes are larger and more 

 conspicuous than the lower ones, sometimes separated by 

 a distinct line, or even entirely divided. This peculiar- 

 ity is rarely seen in the female or even in the dichoptic 

 male, though the dichoptic Asilidae may have the anterior 

 facets somewhat enlarged in both sexes. Those flies hav- 

 ing such enlarged facets usually have the eyes in life 

 brilliantly and beautifully colored with green and purple 

 markings — markings often characteristic of the various 

 species, and the general pattern even of the genera. 

 Unfortunately such markings are obliterated by dessica- 



