26 CONNECTICUT GEOL. AND NAT. HIST. SURVEY [BulL 



5. Ocelli 



There are typically three ocelli in the Diptera, although some of 

 them, such as the anisopoclid Mycetobia and the cyrtid Oncodes. lose 

 the median ocellus, and in other Diptera all of the ocelli may be want- 

 ing. The presence or absence of the ocelli is a feature of considerable 

 phylogenetic importance in the Nematocera, but it is of less signifi- 

 cance in the other Diptera. 



The ocelli are wanting in the Tipulidae, and in the psychodoid 

 families Psychodidae, Tanyderidae and Ptychopteridae, and are typ- 

 ically absent in the Orphnephilidae, Simuliidae, Chironomidae, 

 Dixidae and Culicidae, which are apparently descended from psy- 

 chodoid ancestors. They are present, on the other hand, in the Tri- 

 choceridae, and in the Anisopodidae, which lead up to the bibionoid 

 forms ; and the ocelli are also present in the Bibionidae, Mycetophili- 

 dae, etc., which were descended from ancestors related to the Ani- 

 sopodidae. Ocelli are likewise present in the rather isolated family 

 Blepharoceridae, whose closest affinities have not as yet been deter- 

 mined. 



The ocelli are preserved in many of the Orthorrhapha Brachycera 

 and Cyclorrhapha, and their presence in these forms is one of the 

 many features which indicate that they were descended from ances- 

 tors resembling the Anisopodidae. The ocelli become atrophied in 

 some of the Tabanidae, and are vestigial or absent in the Mydaidae. 

 They are also wanting in certain aberrant Phoridae, in some Con- 

 opidae, and in certain Hippoboscidae ; and they are also said to be ab- 

 sent in the Braulidae, Streblidae and Nycteribiidae, but the absence of 

 the ocelli in these forms is a later specialization, and has no connec- 

 tion with the loss of the ocelli in any of the Xematocera. 



6. Antennae 



Although the function of the antennae is ])rimarily tactile, they 

 also contain well-developed olfactory organs, and may likewise be 

 modified for receiving auditory stimuli, etc. The so-called Johnston'ssj 

 organ* is found in the second antennal segment of such diverse Dip- 

 tera as Ghaohorus and GallipJioi'a, and is gxeatly developed in the 

 male mosquito, whose long antennal hairs are thrown into sympathetic 

 vibration by the humming note of the female. The occurrence of this^ 

 organ in the enlarged second segment labelled p in Fig. 5, V, proves- 

 that this segment of the antenna of a mosquito is the true second seg- 

 ment, or pedicel, (and not the first segment or scape as Peterson 

 (1916) maintains), as is also shown by comparing the parts in a series 

 of Diptera leading back through the chironomids to the dixids andl 

 tanyderids, etc. 



The antennae are usually situated rather close together, and they 

 may even be connected basally, as in the cyrtid Pialea. In the diopsid' 

 series represented by the genera Sphyrocephala, Diopsina and Diopsis 

 (Curran (1934), p. 358), they become increasingly more widely sepa- 



* According to Eggers these structures may serve as "tension receptors" or chordo- 

 tonal organs, or being stimulated by displacement of the antenna, may serve to de- 

 tect air currents, etc. 



