440 DIPTERA CHAP. 



for its mobility, and is connected with the thorax by a slender 

 concealed neck that permits the head to undergo semi-rotation. 

 A large part sometimes nearly the whole of the exposed sur- 

 face of the head is occupied by the faceted eyes. It is usually 

 the case that the eyes are larger in the male than in the female, 

 and the sexual discrepancy in this respect may be very great. 

 When the eyes of the two sides meet in a coadapted line of union 

 the Insect is said to be " holoptic," and when the eyes are well 

 separated " dichoptic." * The holoptic condition is specially char- 

 acteristic of the male, but in some forms occurs in both sexes. 

 There is no definite distinction between holoptic and dichoptic 

 eyes. The eyes may be enormous, Fig. 238, without actually unit- 

 ing, and in the cases where actual contiguity occurs, it takes place 

 in different manners. 2 The eyes are frequently during life of 

 brilliant colours and variegate with stripes or spots ; this con- 

 dition disappears speedily after death, and it is uncertain what 

 the use of this coloration may be. 3 The eyes are frequently 

 densely set with hairs between the almost innumerable facets. 

 These facets frequently differ in size according to their position 

 in the organ. The curious double eye of the male Bilio (cf. 

 Fig. 224) is well worth notice. There are usually three small 

 ocelli placed very near together on the middle of the summit of 

 the head. 



The antennae are of considerable importance, as they offer 

 one of the readiest means of classification. The families placed 

 by systematists at the commencement of the Order have antennae 

 similar to those of the majority of Insects, inasmuch as they 

 consist of a series of segments approximately similar to one 

 another, and arranged in a linear manner (Fig. 213, A). The 

 number of these joints is never very great, but reaches sixteen 

 in certain Tipulidae, and falls as low as eight in some Bibioiiidae. 

 In certain cases where the antennae of the male are densely 

 feathered (Chironomus, e.g.), the number of joints is in that sex 

 greatly augmented, but they are imperfectly separated. This 

 form of antenna gives the name Nemocera to the first series of 

 Diptera. The majority of flies have antennae of another form, 



1 Osten Sacken, Ti: cnt. Soc. London, 1884, p. 501, and Berlin, cut. Zcitschr. 

 xxxvii. 1892, p. 423, etc. 



2 Osten Sacken has recently discussed the intermediate conditions, and proposed 

 the name " pseudholoptic " for some of them, Berlin, ent. Zcitschr. xli. 1896, p. 367. 



3 Girschner, Berlin, ent. Zeitschr. xxxi. 1887, p. 155. 



