310 Annals Entomological Society oj America [Vol. X, 



way of a complete lack of anything which might strictly be 

 called mushroom-bodies in Machilis. There are however groups 

 of fibers in the proper region of the brain in specimens of all 

 sizes, but these are hardly more marked than in Campodea. 

 The usual distribution of cells was found in this genus. Some 

 indication of the complex but usual arrangement of fibers 

 in -the optic lobe region is shown in the Figures. Association, 

 projection and commissural, fibers are easily demonstrated. 

 (Figs. 29-31). 



In all the brains examined, cells of the usual and well-known 

 types for insect brains were clearly seen. Especially was this 

 the case with the representatives of Lepisma and Machilis. 

 In these also there were more differences between cells. In all, 

 nerve cells were held in place and otherwise supported by 

 neuroglia cells and neuroglia nets. In all, the fibrils within 

 the central portions of the ganglia formed intricate tangles 

 with the possibility of almost unlimited connections between 

 parts. In the smaller and simpler species few well separated 

 definite tracts were found, fewer than in Lepisma and Machilis. 

 This is in part due to the fact that the cells, fibers and fibrils 

 are not so large, but there may be other reasons. 



The ventral ganglia in the different species differ greatly 

 in complexity. They seem to be the simplest in Campodea and 

 the most complex in Lepisma. In all, the more abundant cells 

 as is usual, are ventrally placed and the dorsal cells are usually 

 limited to the sides and to a small group in the mid-dorsal line. 



If we consider the brain from its three main pairs of nerves 

 or three main regions on each side to consist of three lateral 

 segments, then we must consider the subesophageal ganglion 

 from its nerves and lobings to consist of at least two segments 

 and very possibly more. The first thoracic ganglion in Machilis 

 and Campodea are evidently composed of two segments as 

 shown in the figures. The cell arrangement and fibrous bands 

 were found to be most complex in Lepisma. A few figures 

 are shown of ventral ganglia of several of the species (Figs. 

 22, 27, 28, 33 and 34) and a more detailed examination of 

 the first thoracic ganglion of Lepisma, is given below. 



Beginning at the cephalic end we find the two masses of the 

 connectives distinct for a short distance, then cells are located 

 ventrally and laterally, being thickest on the mid-ventral line. 



