ANTENNAL NERVE-STRUCTURES 33 



smaller than in Mochlonyx. The hollow of the cup is proportionally somewhat 

 deeper and larger. The ganglion cells, instead of lying in one, lie in two layers 

 separated from each other by nerve-fibres. Accordingly the nerve, which corre- 

 sponds to the nerve N' (fig. 1), divides into two parts, of which one passes be- 

 tween the two layers of ganglion cells, and the other between the inner layer and 

 the rods.* The rods are somewhat shorter and more slender than in Mochlonyx. 

 Their length fluctuates from 0.02 to 0.025 mm. They appear shorter in the 

 anterior part of the organ than in the posterior. Their nuclei are richer in 

 chromatin than the ganglion cells." 



2. Female : The following gives the essential parts relating to the organ : 



" It departs in its structure from that of the female Mochlonyx and forms, 

 to a degree, a transition between this and that of the male. 



" The form of the entire segment is that of a very thick-walled cup provided 

 with a very shallow hollow (pi. I, fig. 7) ; in this hollow the shaft is placed, 

 the base of which is formed by a disk perforated by a central opening (fig. 7, 

 B). In connection with this disk and covering its peripheral part, is the con- 

 necting membrane (P), which, as in the male, is furnished with radial thicken- 

 ings and corresponds to the plate. The noteworthy thing is that here the base 

 of the shaft forms a part of the floor of the depression. I consider this part B as 

 belonging to the shaft because it is in direct connection without interruption 

 with the lower part of the shaft and no difference can be detected from the 

 chitinous covering of this. At the same time the demarkation between it and 

 the connecting membrane or plate (P) is very distinct. The latter is nearly 

 transparent, like the plate of the male, and appears to possess a consistency dif- 

 ferent from that of the common chitin. The outer wall of the depression is 

 formed by a thickened portion (fig. 7, ^) of the chitinous covering of the 

 second segment with an annulate projecting margin (fig. 7, V) which projects 

 towards the shaft and reduces the opening of the hollow. Outwardly from this 

 excrescence the chitinous covering forms a zone covered with blunt elevations 

 which carry fine bristles (fig. 7, L). The remainder of the surface of the seg- 

 ment is also covered with short, fine bristles. 



" Close below the chitinous covering lies, as usual, the hypodermis of one 

 layer ; these are succeeded inwardly by the ganglion cells, which here, as in the 

 male, are arranged in two layers, incompletely divided by nerve fibres. Sepa- 

 rated from the inner layer of ganglion cells by a layer of fibres lie the rods, which 

 are directly connected at least with the inner layer of cells by fine fibres. The 

 rods are somewhat longer than in the Culex male, about 0.03 mm., but agreeing 

 with them in form. In longitudinal section they lie in radiate arrangement, 

 radiating from the connecting membrane or plate. In transverse section it is 

 seen that, similarly to the male of Mochlonyx, the rods form groups (compare 

 pi. I, fig. 4), separated by very narrow interspaces, throughout the circumfer- 

 ence of the organ and corresponding to the thickenings of the connecting mem- 

 brane. On the median and lateral margins of the rod-layer more or less modi- 

 fied hypodermal cells represent the transition stage between rods and ordinary 

 hypodermal cells. Two nerve cords and a tracheal trunk pass through the cen- 

 tral hollow space into the shaft of the antenna." 



This organ, in conjunction with the antennal shaft and its whorls of hairs, 



has been generally looked upon as an organ of hearing. The long hairs of the 



whorls vibrate in response to sound waves of a certain pitch and react upon the 



sensory organ of the globose second segment. Johnston, in a paper published 



in 1855, was the first to describe the antenna as a complex sense-organ, and he 



* In fig. 7, plate I, belonging to the female of Culex, this arrangement Is likewise present. 



