602 C. E. Osten-Sacken's essay on 



either aerial or pedestrian ; at the same time they are 

 neither holoptic nor ch?etophorous, 



" Insects organised for an exclusively aerial life," says 

 J)t. KYovel {Lihelhda, Tahanns, Bombijlius, &c.), "de- 

 pend on their eyes ; they generally have very little 

 developed antennae, and are absolutely helpless in the 



dark ; they hardly dare to walk In other insects 



(for instance, ants) the eyes play a subordinate part ; 

 these may be called antenna! insects ; they can work 

 by night or underground, as well as by day " (A. Forel, 

 Beitrag z. Kenntniss der Sinnesempfindungen der 

 Insecten ; Mitth. d. Miinchener Entom. Vereins, ii., 

 1878). TipuUdce and some of the Nemocera in general, 

 ChironomidiB, MycctopldUdce, and Cecidomyidce may pro- 

 bably be ranked among the antennal insects. Cidex, 

 for instance, attacks us in the dark, following the scent 

 by means of its antenn£e. In the male Cidcx, according 

 to the observations of A. M. Mayer, the antennae are 

 organs of hearing, and serve to discover at a distance 

 the piping of the female (see Amer. Journ. of Sci., vol. 

 108, pp. 89 — 103). Thus the secondary sexual character 

 of Cidex, consisting in the different structure of the an- 

 tennae, would find its natural explanation in a functional 

 difference. 



It is by no means impossible that the macrochsetae 

 are useful in more than one way ; it will belong to the 

 observer and the anatomist to determine whether, for 

 instance, they may not serve for the transmission of 

 sound (mere perception of the sound-waves of the air, 

 and not specific hearing), like the long antennae of 

 certain Orthoptera or moths. 



The further discussion of this subject would draw me 

 beyond my present aim, which is a merely practical one, 

 the settlement of the terminology of the macrochatae, 

 and, by that means, the utilisation of a set of characters 

 which, it seems to me, have not yet been sufficiently 

 appreciated in descriptive Entomology. 



I. — Terminology of the Parts of the Thorax. 



This terminology refers especially to the Diptera 

 Cyclorhapha ; but the same sutures and regions of the 

 pleurae can be recognised more or less distinctly in the 

 OrthorJiapha. Some of the sutures sometimes become 

 less distinct or obsolete — for instance, in the TipuUdce. 



