GRASS THRIPS. 63 



and sends a large nerve obliquely backward to each of the legs 

 that are attached to the metathorax. 



The fourth ganglion is the largest of any in the ventral chain, 

 and compared with them it is long and narrow. It lies partly in 

 the metathorax and partly in the abdomen, and is connected with 

 the third thoracic ganglion by a long slender commissure. It 

 gives off nerves to the organs of the somites which it occupies, 

 and is connected with the somites lying posterior to it, by the long 

 nerve cord passing from its posterior end. 



The cord which passes from the fourth ventral ganglion, runs 

 posteriorly to the sixth abdominal somite, where it breaks up into 

 a number of nerve fibers. These supply the reproductive organs, 

 the special muscles of the reproductive organs, and the other 

 organs and muscles in the last two somites of the abdomen. 



All of the ganglia correspond very closely in their minute 

 structure. Each ganglion is enclosed in two delicate membranes. 

 Beneath these membranes there is a layer, of varying thickness, 

 made up of large nerve cells, which stain deeply. This layer of 

 nerve cells is thickest at a point a little way from either end. 

 Between these thickened portions the layer of nerve cells is much 

 thinner, and it is entirely wanting where the ganglion narrows 

 down to form the commissure. The nerve cells are pear shaped 

 and a single nerve fiber passes from the smaller end of each cell. 



The central part of the ganglion is made up of fibers that run, 

 for the most part longitudinally, except where some large nerve 

 is given off. The commissures are composed entirely of nerve 

 fibers, together with the two envelopes that surround them. The 

 cord which passes from the fourth ventral ganglion is composed 

 entirely of nerve fibers. 



EYES. 



The compound eyes, fig. 14, ei, are comparatively small, 

 (about .06 mm. in diameter), placed on the sides of the head 

 just back of the antenme, and nearly circular in outline. Each 

 eye is made up of from one hundred to one hundred and twenty 

 facets. The facets in the two eyes of an individual may differ 

 in number from ten to fifteen. Each facet is irregular in out- 

 line, but approximately circular and is strongly convex on the 

 surface, so that the exterior of the eye has a roughened appear- 

 ance. The facets near the center of the eve are larger than those 

 16 



