AND THE SEXUAL DIFFERENCES OF THE EYES IN DIPTERA. 369 



Sexual characters. 



1. The larger size of the eyes of the male and the wider 

 separation of the eyes of the female (the " holoptic "), or the 

 equal separation of the eyes in both sexes (the " dichoptic"). 



2. The division of the male eye into an upper and lower eye. 



3. The variation in size of the facets of the eyes of the male. 



4. The situation of the eyes on lateral projections. 



5. The pubescence of the eyes. 



On the Presence or Absence of Chitinous Structures 

 round the Lenses of the Eyes. 



Xotes and Observations on which the Conclusions are Founded. 



My preparations of the Cecidomyidae and the Mycetophilidae 

 exhibit in the majority of cases circular lenses with a markedly 

 chitinous structure surrounding them (Figs. 1, 3, 4). 



In Bibio marci, L., the chitin has almost disappeared, but there 

 remains a fine line dividing the facets into fairly well marked 

 hexagons. This is much less clearly defined in the female of 

 B. hortidanus, L., though the male shows it clearly (Fig. 13). 



In Dilophus febrilis, L., the hexagons become almost circular, 

 and the chitin is a trifle more evident than in B. marci (Fig. 5). 



A New Zealand insect, D. nigrostigma, Hutton, shows a more 

 chitinous structure (Fig. 12). 



A single specimen of Plecia fulvicollis, Wied., from Ceylon, 

 shows a structure close to that of B. marci. The small Scatopse 

 (two species S. notata, L., <$ $ , and S. minuiissima, Verral, ? <^) 

 show a similar structure to that found in the Mycetophilidae. 



In Ceratopogon the lenses are circular, but the chitin is less 

 than in Scatopse, more like Cecidomyia. In Chironomus dorsalis, 

 Mg., and C. ])luinosus, L., the round lenses in the curiously bent 

 plate are always evident. 



In the four families, Chironomyidae, Psychodidae, Culicidae 

 and Mycetophilidae, the plates bend round the upper part of the 

 head, leaving a space for the insertion of the antennae, and 



Journ. Q. M. C, Series II.— No. 64. 28 



