NO. 8 MORPHOLOGY OF INSECT SENSE ORGANS SNODGRASS 43 



argues that this motion would register the position of the insect in 

 the water. 



SENSILLA SQUAMIFORMA 



Since scales are but modified hairs, it is not surprising to find that 

 some of them should be innervated in the same manner as the tactile 

 hairs. Sense scales have been described on the wings of Lepidoptera 

 by Guenther (1901), Freiling (1909), and Vogel (1911). Vogel 

 says that innervated scales are found on the wings of all Lepidop- 

 tera, even in primitive forms like Hepialis. They occur on both sides 

 of the wings, mostly on the veins, and especially on the marginal 

 veins, but they may be present also on the basal parts of the wings 

 wherever there is an internal space sufficiently large to allow a nerve 

 to penetrate. 



The sense scales are elongate fusiform in shape, with fewer ribs 

 than the other scales, and each has the distal part drawn out into a 

 long, tapering point (fig. 17 F). The spaces between the internal 

 ridges of a sense scale, Freiling says, are so reduced that the scale 

 is almost a solid structure. Each sensory scale is innervated through 

 a single large sense cell, the distal process of which, according to 

 Vogel, ends in a cone-shaped sense rod attached to the base of the 

 scale. The sense scales are evidently tactile in function. 



SENSILLA BASICONICA 



Sensory pegs and cones are undoubtedly to be regarded as hairs 

 reduced in size, and there is no sharply dividing line between sensilla 

 trichodea and sensilla basiconica. The character of the external parts 

 and the structure of the internal parts of the peg sensilla, likewise 

 separate these organs into two groups, there being thick-walled or 

 even solid pegs innervated each through a single sense cell, and thin- 

 walled pegs innervated each through a group of sense cells. The 

 former are regarded as receptive to meclianical stimuli, the latter to 

 chemical influences. 



Sense pegs and cones have been described on all parts of the 

 body and appendages of various insects, on the epipharynx and hy- 

 popharynx, and in the pharyngeal cavity. Many of them are clearly 

 but short hairs of the tactile kind, but the typical pegs, occuring 

 particularly on the antennae and the mouth parts, are of the chemo- 

 receptive variety. In these the walls of the peg or cone are thin and 

 transparent (fig. 18, Pg), some terminating in a membranous cap. 

 The sensillum comprises a large cap cell {CCl), a vacuolated envel- 

 oping cell (ECl), and a compact group of sense cells (SCls). The 



