IN THE ANTENK^ AND PALPI OF DIPTERA. 33 



(III.) Ge-veral Description of the Sense-Organs 



UNDER REVIEW. 



The sense-organs of the antennse and palps of Dipteia liei-e 

 discussed * are composed of elements of the same general type as 

 those described in many other ii}sects. These elements consist 

 each of a large, modified, hypodermal cell, above which is a verv 

 thin-walled chitinous process, rising from the thicker chitin of 

 the general surface ; and of a nerve-fibre, which runs close up to 

 the base of, if not actually into, the chitinous process. Tlie large 

 cells wei-e taken by some earlier writers to be nervous elements 

 directly connected with the nerve-fibres, but Berlese (1) regards 

 their function as glandular and considers that the nerve-fibres 

 are in close apposition to, but not directly connected with, them. 

 Berlese (1) distinguishes several types of these chitinous sensoxy 

 processes (" sensilli ") including the following : — (i.) ^Trichoid, 

 with the base sunk below the general surface of the chitin 

 but with the apex projecting above; (ii.) Basiconic, arising 

 directly from the general surface ; (iii.) Cceloconic, with the base 

 sunk in a pit and with the apex not reaching the level of the 

 general surface. Wheeler (2) records the occurrence of all these 

 types in ants, From my observations it appears that in man}' 

 Nemocera the sensory processes are scattered singly on the 

 surface and are referable to one or other of the above types. 

 In some Psychodidje a modification is found : the processes are 

 very large and long, either spiral in form or bifid or triradiate, 

 arranged in pairs, a single pair to each joint of the flagellum of the 

 antenna (see text-fig. 3). In Cecidomyidse also, there are peculiar 

 "looped hairs" and other structures which may be modifications 

 of simple sensory processes : see Felt (3), etc. 



In tlie species of Bihio and the Mycetophilida? which I have 

 examined, and in all the other families of Diptera studied, there 

 occur the compound sense-organs or " sense-pits" which are the 

 special subject of this paper. These may be regarded as com- 

 posed of a greater or suialler number of the sense-oi-gans outlined 

 above, united together, sunk in pits of very varying foi-m or size 

 and modified in various other ways (see below). 



In describing a typical sense- pit, Pi. IV. fig. 11 is a good ex- 

 ample to take, being a photomicrograph of a transverse section of a 

 sense-pit in the antenna of Sarco])haga carnaria, magnified al)out 

 600 diameters. There is first a somewhat large opening in the 

 chitin leading down into the pit itself ; in >S'arcopha(/a carnaria 

 this opening is wide and leads abruptly to the sensory processes, 

 but in some species, e.g. certain Muscida?, this opening leads into 

 a long channel' lined by chitin which sometimes exhibits spiral 

 or convolvited folds or even a series of commuuicating lidges like 

 basket-work {Mitsca domestica). The floor of the pit consists of 



* No attempt is made in this paper to deal with the organs known as Chordo- 

 toiial Organ or Johnston's Organ, situated near the hase of the antenna in certain 

 Diptera. 



Proc. Zool. Soc- 1919. l^o. III. 3 



